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6
Search for Life
Outside the Solar System
INTRODUCTION
Life as we know it is a planetary phenomenon: its origin appears to have
required interactions among liquid water, a gaseous atmosphere, and miner-
als provided by a solid planetary surface. The energy required to produce
the appropriate chemical reactions was available from solar ultraviolet light,
bombardment by charged particles, meteoritic impacts, local volcanism,
hydrothermal vents, lightning discharges, coronal discharge, and even acous-
tic shocks.
A nearly circular orbit about a stable star promotes fairly uniform condi-
tions for the billions of years required (at least on Earth) for life to evolve
from single cells with no nuclei to multicelled intelligent organisms. The
evolution and dispersion of life on Earth have radically altered the surface,
oceans, and atmosphere of this planet in ways that are discernible from a
remote observational vantage point. During the past 1.5 to 2 billion years, a
distant observer would have found presumptive evidence for life on Earth in
the oxygen-rich nonequilibrium chemistry of the Earth's atmosphere. More
recently, the microwave signals generated by human technology could pro-
vide that same remote observer with circumstantial evidence for the exis-
tence of some form of intelligence on the planet Earth.
These two examples of life detection should apply to other planets as
well. Isolating the light or thermal radiation of a distant planet from the
brilliance of its star would enable us to examine the spectrum of the planet
and search for chemical evidence of the existence of life. This evidence
would take the form of some massive departure from chemical equilibrium,
a large amount of free oxygen being the most obvious example and proba-
bly the most observationally traceable. However, the detection of trace
gases such as CH4, NH3, N2O (nitrous oxide), and CS2 (carbon disulfide) in
105
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106
THE SEARCH FOR LIFE'S ORIGINS
excess of the amounts predicted by chemical equilibrium and plausible
nonequilibrium sources might also provide presumptive evidence for life if
the 4- to 11-,um region of the spectrum could be accessed. Detection of
signals generated by an extraterrestrial technology would be even more
compelling evidence that life is not uniquely confined to Earth. (Although
searches for such signals have become known as SETI [the search for
extraterrestrial intelligence], in reality such searches could detect only those
intelligent forms that utilize an electromagnetic technology.) In this chap-
ter, both of these approaches to the detection of life outside the solar system
are discussed.
GOAL: To understand the nature and distribution of life in the uni
verse.
The underlying scientific goal in searching for life beyond the solar
system is the same as the goal of the Viking biology experiments on Mars
or the study of astrophysical influences on the evolution of life on the
planet Earth.
As described in previous chapters, there are compelling reasons to look
for evidence of the origin of life on Mars during a more clement epoch of
that planet's history. On the other hand, the prospects for detecting extant
life on Mars seem remote. To find such life, our vision must expand
beyond the realm of the known planets and moons.
Recent strategy reports from this committee (SSB, 1981, 1986) empha-
sized the dynamic and complex interactions between life and its terrestrial
environment. Those reports focused on the opportunities afforded by suit-
able airborne and orbital platforms for the study of the terrestrial biosphere
as a single, complex but closely coupled system. Now it is time to
extend this perspective to other planets and satellites in other planetary
systems. The same kind of evidence that reveals the presence of life on
Earth can, in time, be sought in other planetary systems. If the search is
successful, it will mark the beginning of a new science. It will then be
possible to examine life on Earth as just one of several examples of a
remarkable property of matter, rather than the only example. This chal-
lenge was recognized by the authors of the 1981 SSB report: "The emer-
gence of complex societies capable of extraterrestrial communication is an
evolutionary biological phenomenon. Thus, the problems of the search for,
and attempt to communicate with, extraterrestrial life lie at least in part
within the province of planetary biology and chemical evolution."
APPROACHES TO THE DETECTION OF
LIFE OUTSIDE THE SOLAR SYSTEM
If we are to attempt to sense the impact of life on distant planets from
this remote vantage point, those planets must first be located. At present,
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SEARCH FOR LIFE OUTSIDE THE SOLAR SYSTEM
107
there is no unambiguous evidence of the existence of an extrasolar plane-
tary system, although there are many tantalizing clues.
The detection of even a single example of an extrasolar planet that has
been modified by the evolution of life would have extraordinarily profound
consequences. It would then be possible to bring to bear the full power of
the scientific method in extracting laws of nature from several examples of
the same phenomenon. Despite its extraordinary diversity, life on Earth
offers just a single example from which it is impossible to generalize to
other systems. To escape from this dilemma, those other systems must be
found in order to achieve the fundamental goal of understanding the distri-
bution of life in the universe.
The basic techniques for detecting extrasolar planets are well understood
(a detailed discussion of this subject is given in reports of the SSB, 1988a,b).
Individual planets, once formed, will have a number of perturbing effects
on the motion and apparent brightness of their host star, but these effects
will be very small and will require extreme measurement accuracy. For
example, the accuracy required to detect Jupiter-mass planets around the
nearest stars has been achieved with heroic efforts during the past two
decades. Recent technological advances now allow previous accuracies to
be obtained more rapidly and make additional detection schemes feasible.
Planets with the mass of Neptune may become detectable within the decade,
but the detection of Earth-mass planets will require dedicated space-based
systems, not yet approved by any funding agency. It is appropriate for the
purposes of this report to consider what systems are available now, which
are planned for the immediate future, and, especially, what must be studied
to ensure that instruments of significantly improved capability become avail-
able to the next generation of planet seekers. These instruments must be
systematically and exhaustively employed for long periods of time if a
census of even the nearest stars is to provide a basic understanding of the
frequency of planetary systems. Furthermore, we must be able to extend
what is learned from studies of nearby planetary systems to the more distant
stars around which we cannot hope to detect planets by these simple tech-
n~ques.
Programs to search for other planets have been endorsed repeatedly. These
endorsements by astrophysicists and planetary scientists are concurred in by
scientists interested in studying the origin and evolution of life. In this
regard, it should be noted that the desire of exobiologists to examine spec-
troscopically the global chemistry of a distant planet imposes the most
extreme instrumental requirements.
In parallel with this search for passive evidence of the existence of life
beyond the solar system, a different kind of search can be carried out: that
is, the search for evidence of technologically advanced civilizations. The
unambiguous detection of an extrasolar technology would have profound
implications. Not only would we know that life in the universe is not
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THE SEARCH FOR LIFE'S ORIGINS
unique to the planet Earth, but we could contemplate the possibility of
communicating with, and learning from, the intelligent life that created the
detected technology. It is conceivable that the distribution of life in the
universe could come to be understood in the larger context of the evolution
of the cosmos. These possibilities have previously been endorsed by the
National Research Council ( 1972, 19821.
Since the 1940s, there has been a rapid growth in the techniques and
technology of communications and radio astronomy that can improve the
chances of detecting signals at great distances. These tools have not yet
been brought to bear SETI in a systematic way. It is now possible, by using
state-of-the-art instrumentation, to expand the search domain for extrater-
restrial signals far beyond anything that has been done to date. These
searches for deliberately generated signals should be complemented by
searches for other indirect manifestations of a distant technology.
Although the two approaches to searching for life beyond the solar sys-
tem can provide very useful inputs to each other, they may be pursued
independently and simultaneously at a pace set by the rate of maturation of
the requisite technologies. NASA is the leading agency in the development
of most, if not all, of the necessary instrumentation. Furthermore, NASA is
the agency responsible for providing the orbital platforms required by some
of the observational techniques (discussed below). The readiness of some
of the requisite technology and the concurrent development of orbital astro-
nomical facilities invite rapid implementation of search programs utilizing
these capabilities.
SCIENTIFIC OBJECTIVES
To achieve an understanding of the nature and distribution of life in the
universe, a number of discrete scientific objectives must be carried to com-
pletion. In many cases, these objectives overlap those already enunciated
by the Astrophysics and Solar System Exploration program offices within
NASA. However, it is important to remember that the desire of exobiolo-
gists to study both the nature and the distribution of life in other potential
habitats is likely to place more stringent technical requirements on the rele-
vant instrumentation than demanded by other objectives.
It is necessary to ask, in a cosmic context, how unique are this solar
system, the Earth, and terrestrial life? The precise mechanisms that led to
the formation of our solar system, with its small, dense inner planets and its
more distant gas giants, are the subject of much debate. Debatable as well
is the time scale over which these processes occurred. Consistent with
current theories of planetary formation, there does not seem to be anything
unique about the protosolar nebula; other nebulae around other stars should
also form planets by the same methods, whatever they are. Comparative
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SEARCH FOR LIFE OUTSIDE THE SOLAR SYSTEM
109
studies of protostellar nebulae currently in the process of collapse and,
perhaps, planet formation could provide a bound on the length of the pro-
cess, as well as an estimate of the mass ratio between the flattened nebula
and the centrally condensed protostar. The latter quantity is the key to
distinguishing among current models of planetary formation. There are a
number of Earth-orbital telescopes for infrared and submillimeter observa-
tions under design, or in the planning stage, but as currently conceived,
none will provide the resolution of 0.01 to 0.1 arc see necessary to observe
dimensions corresponding to our planetary system within distant protostel-
lar nebulae. The recent study Space Science in the Twenty-First Century
(SSB, 1988b) noted this same deficiency and recommended that advanced
technology programs be pursued to achieve this accuracy in future systems.
Perhaps the closest we can get to the process of planet formation is to
study the extended disks of dust and gas surrounding some young stars that
have just reached the main sequence. These stars were discovered in the
analysis of the IRAS four-color data. A significant infrared excess in the
60-,um band has been correlated with the existence of a dusty disk compo-
nent circling the distant star. These disks are typically much larger than our
solar system. Whether there is a selection effect favoring large disks, whether
the size of the disk is determined by the age of the star alone, and whether
planets have formed, are about to form, or have failed to form in these large
disks are questions whose answers are still unknown. These are certain to
be topics for thorough investigation in the coming years. Studies of these
disks and their frequency of occurrence have not yet produced any general
agreement among theorists as to how planets form.
During the coming decade it may not be possible to determine observa-
tionally the processes by which other planetary systems (and by extension
our own) formed. It should, however, be possible to verify observationally
that other planetary systems have indeed formed. The unambiguous detec-
tion of the first extrasolar planet will validate the hypothesis that planetary
formation is not uniquely related to the Sun.
OBJECTIVE 1: To determine the frequency and morphology of nearby
planetary systems.
For the purposes of exobiological understanding, a significant planetary
census will be required. Only this can provide a meaningful estimate of the
frequency of occurrence of terrestrial-mass planets at orbital distances from
the primary star that are suitable for the maintenance of surface tempera-
tures amenable to life.
A survey of nearby stars must be made initially to determine the fre-
quency of planetary-system formation in our local galactic neighborhood.
This survey must be augmented with a study of distant protoplanetary disks
and protostellar nebulae to allow for extrapolation of the frequency of
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110
THE SEARCH FOR LIFE'S ORIGINS
planetary-system formation throughout the galaxy. Once detected, the distri-
bution of extrasolar planet masses as a function of the distance from the
parent star (location in the protostellar accretion disk) must be determined
and correlated with stellar type to better understand the processes that con-
trol the formation of planets and the role of the central condensed protostar.
From observations of protostellar nebulae and early stellar disk systems, an
evolutionary sequence of events and time scales should be developed to
predict the rate at which planets are formed within the Milky Way. The
sensitivity of available instrumentation will undoubtedly dictate that these
surveys proceed in a stepwise fashion. The detection or (equally signifi-
cant) nondetection of Jupiter-mass planets in the vicinity of the nearest few
stars and the study of the largest dust disks around young stars will come
first. These should be followed by the search for lower-mass planets (down
to terrestrial size). Investigation of protoplanetary formation in progress
around distant protostars can commence whenever instruments of sufficient
angular resolution permit. To implement these investigations properly,
advanced technology studies are needed that will lead to a new generation
of instruments capable of detecting low-mass planets and investigating solar-
system-scale phenomena within protoplanetary disks in nearby regions of
star formation.
OBJECTIVE 2: To determine the frequency of occurrence of condi-
tions suitable to the origin of life.
The actual surface temperature of any particular planet will depend upon
the abundances and chemical nature of its atmospheric constituents as well
as its distance from the host star. Obtaining information on surface tem-
perature and the chemical composition of an atmosphere requires direct
imaging and spectroscopic analyses of each distant planet body. The tech-
nology required for such spectral studies is not yet available and may not be
available when a statistically interesting number of extrasolar planets are
first recognized. Models for their atmospheric composition will have to be
constructed on the basis of comparative planetology (including the major
planetary satellites) in our own solar system, together with what is known
about the origin of the Earth's current and precursor atmospheres. Predic-
tions made on the basis of outgassed and accreted atmospheric constituents
must be compared with spectral data as soon as instrumentation permits.
The goal is to understand just how often conditions suitable for life occur in
the Milky Way galaxy.
Once terrestrial-mass planets have been detected, they must be analyzed
spectroscopically to assess their atmospheric composition as well as their
surface temperatures and pressures. This will provide an estimate of the
frequency of occurrence of conditions similar to those that are presumed to
have given rise to life on this planet. To appreciate the intrinsic difficulty
of this task, it is necessary to remember that we have only been able to
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SEARCH FOR LIFE OUTSIDE THE SOLAR SYSTEM
111
accomplish a similar assay of Titan in the past few years and are still
waiting for a probe mission to resolve speculations about a liquid organic
ocean there. Titan presents a far bigger and brighter target than a distant
Earthlike planet and one that can be observed without confusion from the
solar luminosity. For these studies, therefore, it is important to continue the
development of technologies for supersmooth mirror production, low-light-
scattering telescopes, and large-orbital infrared, submillimeter, and optical
telescopes that may eventually permit the direct imaging and gross atmo-
spheric characterization of distant planets.
OBJECTIVE 3: To search for presumptive evidence of life in other
planetary systems.
The probability that life, once started, will evolve to intelligence depends
on many things, one of which may well be the fortunate location of a
planetary system within the parent galaxy now or at some past epoch. If
evolution toward intelligence everywhere requires billions of years, then
changes in the astrophysical environment may play a central role in the
process. Extinction events and evolution itself have been episodic on Earth
(see Chapter 51. There is strong circumstantial evidence for an astrophysi-
cal connection in at least some of these episodes. Changes in the character-
istics of the solar system have been most convincingly cited, but changes in
our galactic location have also been suggested as causative agents.
In the foreseeable future, detection and spectroscopic study of extrasolar
planets will be confined to the immediate galactic neighborhood. The pecu-
liar velocities of our stellar neighbors will ensure that some of them have
sampled far different galactic environments than Earth has in the past bil-
lion years. Given a sufficiently large sample of planets examined for signs
of life and sufficiently accurate models for galactic dynamics, it might be
possible to draw some conclusions regarding the probability of life arising
and evolving as a function of galactic locale.
Any potentially suitable terrestrial-type planets must be studied in detail
to search for signs of nonequilibrium chemical constituents, possibly signi-
fying the action of some form of active biological system. Although the
overabundance of molecular oxygen in the Earth's atmosphere and its coex-
istence with methane is an example drawn from Earth that correlates with
life as we know it, life as we do not know it is likely to be no less surpris-
ing than the soil chemistry of Mars. The key to a distant atmosphere may
lie in the coexistence of two other highly reactive components requiring a
source function for which no natural planetological explanation can be found.
OBJECTIVE 4: To search for evidence of extraterrestrial technology.
Because the instrumentation for detecting evidence of extraterrestrial
technology is far more mature than the instrumentation necessary for exam-
ining distant planets minutely, another technology (and, by inference, an-
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2
THE SEARCH FOR LIFE'S ORIGINS
other biology exhibiting intelligence) may be detected before any other
evidence is found for extraterrestrial life. The examination of distant plan-
ets first requires the identification of such planets, but searches for other
technologies can be made in the direction of plausible targets without a
priori knowledge of the existence of a suitable planetary abode. Further-
more, searches can also be indiscriminate with respect to direction if the
technology of another advanced civilization is sufficiently "loud." Thus,
several different search strategies, based on different concepts of what con-
stitute the most detectable features of a distant technology, may have to be
employed in conducting an exhaustive search for signs of extraterrestrial
technology. Such searches can attempt to detect purposeful communication
signals, whether intended for Earth or for some other receiver. Because
such signals are intentional, they can be expected to present a high signal-
to-noise ratio for whatever communication scheme is utilized. In this case,
it is possible to define what constitutes an exhaustive search. One must
also use current terrestrial technology as a paradigm and formulate the
sensitivity required to "see" the leakage radiation that is generated on Earth
from across the galaxy. In the case of intentional beacons, it is not neces-
sary to achieve this extreme sensitivity because intentional beacons could
be much brighter. However, it is necessary to define this limit and use it as
a standard against which to measure the significance of negative results.
Searches can also be conducted to detect the by-products of noncommu-
nicative technologies of other civilizations. Intentional amplification is not
expected to be imposed on these signals, and it is far more difficult to
define what constitutes a definitive search for such evidence. Nevertheless,
searches that can achieve some well-defined detection sensitivity should be
pursued.
MEASUREMENT REQUIREMENTS
For the recommendations and scientific objectives outlined in the previ-
ous section to be accomplished, certain measurement accuracies must be
achieved in a number of different observational technologies. It is appro-
priate for this report to compare the required accuracies to those likely to be
achieved by various facilities, including mature missions nearing launch
and those whose planning is well under way. In those areas where the
available accuracies match or exceed the required ones, the research objec-
tives of exobiologists can probably be accommodated by the appointment of
specialists in this field as interdisciplinary scientists and by widespread
distribution of announcements of opportunity. Facilities whose near-term
available accuracies fall short of requirements will entail basic technologi-
cal development so that subsequent generations of instrumentation will be
able to provide the necessary capabilities for exobiological purposes.
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SEARCH FOR LIFE OUTSIDE THE SOLAR SYSTEM
Detecting Extrasolar Planets
113
An extensive literature exists on the techniques for carrying out searches
for extrasolar planetary systems and their current or expected capabilities.
Recent discussions may be found in Space Science in the Twenty-First
Century (SSB, 1988b). With the exception of direct detection techniques,
little has changed since these comprehensive reviews, and this report only
briefly summarizes the measurement accuracies required to detect either
Jupiter or the Earth in orbit about the Sun if these systems were located at a
distance of 10 parsecs (30 light-years).
Extrasolar planets may be imaged directly or detected by one of three
indirect techniques: astrometry, spectroscopy, and photometry.
indirect Detection
1. Astrometric detection: Astrometry uses distant"fixed" stars as a
reference frame against which to measure, over long periods of time, the
relative position of a candidate star. A star and planet orbiting a common
center of mass introduce a barely discernible reflex motion, or "wobble,"
into the stellar trajectory. The maximum amplitude of the reflex is 0.5
milliarc see (500 microarc see) for Jupiter and the Sun observed at a dis-
tance of 10 parsecs, and 0.3 microarc see for the Earth and the Sun at the
same distance. Atmospheric turbulence limits the obtainable ground-based
measurement accuracy to something in excess of 100 to 300 microarc sec.
Although nearby Jupiters can be detected from good ground-based sites,
astrometric detection of even the closest Earthlike planet will require a
space-based platform. Any instrumental system must possess extreme sta-
bility over very long time scales commensurate with planetary orbits, be-
cause it is the periodic nature of this reflex that ultimately distinguishes
measurement from noise.
2. Spectroscopic detection: The same wobble in the star's motion in-
duced by a star and planet orbiting about their common center of mass can
be seen spectroscopically if the plane of the orbits is nearly parallel to the
line of sight. In this case the change in relative velocity introduced as the
stellar candidate moves toward or away from the observer can be seen as a
periodic Doppler shift in the absorption line spectrum of the stellar photo-
sphere. The effect of Jupiter on the Sun is to cause a maximum shift in
velocity of 12 m/s, whereas the maximum effect induced by a terrestrial
mass planet is 0.1 m/s. These small velocity shifts must be detected on top
of photospheric turbulence and the peculiar velocities of the stars moving
through space at tens of kilometers per second with respect to the observer.
Except for accuracy limits imposed by the brightness of the star, this tech-
nique is independent of the distance to the star. Again, the measurements
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THE SEARCH FOR LIFE'S ORIGINS
demand extreme instrumental stability over long periods of time and a peri-
odic signature associated with the velocity shift to distinguish signal from
noise. Because the atmosphere does not seriously impede these spectro-
scopic studies, they can be conducted from the ground, and several facilities
have already achieved instrumental accuracies sufficient to detect the ef-
fects of a Jupiter-mass planet. The recent announcement of seven stars that
exhibit spectroscopic shifts >10 m/s is extremely exciting. Measurements
must be continued over the next decade to identify the underlying periodic-
ity before definitive conclusions can be drawn. The limiting factor in this
approach is the lack of detailed knowledge about any periodic or quasi-
periodic processes in stellar photospheres that might induce Doppler shifts
of this order internal to the star. Continued observations of the Sun's whole
disk are required to determine the eventual sensitivity limit of this method.
3. Photometric detection: Astrometric and spectroscopic detection
schemes have a long history, and both have recently benefited from a new
generation of instrumentation; The next generation or two of astronomical
instrumentation may enable a third detection scheme.
If the orbit of a planetary system is very well aligned with the observer's
line of sight, then it is theoretically possible to detect the periodic decrease
and slight change in color of the stellar luminosity produced when all or
part of the stellar disk is occulted by a planet. This measurement requires
extreme accuracy, far beyond current capabilities. Absolute photometry to
a precision of 1 ppm in two different color bands is required to conclude
reliably that the diminution of stellar luminosity is due to an occultation
and not intrinsic fluctuations. The periodic nature of the effect is also a
necessary characteristic. Since this method of detection was reviewed (SSB,
1988), ground-based measurements have achieved a photometric accuracy
of 2 x 10-5. Observations from Skylab indicate that Jupiter occulting the
Sun would be relatively easy to detect from afar but that the Earth would be
detectable only in quiet phases of solar activity. Because it is impossible to
know in advance the inclination of a planetary system's orbital plane or
when an occultation might occur, this technique is intrinsically a statistical
one. Many stars must be monitored simultaneously and more or less con-
tinuously. Debatable, and rather optimistic, calculations suggest that moni-
toring 4000 stars with the requisite photometric accuracy will provide a
detection rate of about one planet per month. Because the atmosphere se-
verely degrades photometric precision, this detection scheme requires an in-
strument in Earth orbit. The double differential photometer required for the
job is at least one generation of technology away, but this particular method
may be worth pursuing because it is best suited to the detection of the
small-orbit, short-period planets within any system. The real utility of this
approach may eventually be to search for the inner planets of planetary
systems already detected by other methods. In general, all the other meth-
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SEARCH FOR LIFE OUTSIDE THE SOLAR SYSTEM
115
oafs work best for low-mass stars and high-mass planets. It is Jupiters that
will be found (if anything) in the near future, not other Earths.
Direct Detection
To image a planet directly it is necessary to resolve its combined internal
luminosity and reflected starlight from the overwhelming luminosity of the
nearby star. For the Jupiter-Sun system at 10 parsecs this requires being
able to achieve a brightness contrast ratio of 2 x 10-9 at visible wavelengths
(or 10-4 at infrared wavelengths) just 0.5 arc see away from the star. The
Earth-Sun system at 10 parsecs would require a contrast ratio of 2 x 10-'°
(or 10-6) at a distance five times closer to the star (0.1 arc see). No system
in existence on Earth or planned for orbit comes close to achieving these
accuracies. In the near future the only planets to be directly imaged will be
massive, in large orbits about low-mass stars, or much closer than 10 par-
secs.
Since the SSB reviewed the potential for direct imaging and recom-
mended consideration of low-light-scattering requirements for future tele-
scopes (A Strategy for the Detection and Study of Extrasolar Planetary
Materials: 1990-2000, SSB, in press), there have been new studies yield-
ing promising results. The combination of a coronagraph to reduce the
diffraction of starlight by the telescope and a supersmooth mirror to reduce
scattered light holds great promise for the future. One system currently
under study is called the Circumstellar Imaging Telescope (CIT). This
telescope should greatly reduce diffracted light in the wings of an image of
a point source (Airy pattern) over the entire field of view of the instrument.
Such an instrument would have to orbit the Earth to get above the detrimen-
tal effects of atmospheric viewing. A 2-m-diameter instrument in Earth
orbit would allow the direct detection of a Jupiter-sized planet around a
solar-type star out to a distance of approximately 10 parsecs. A larger in-
strument 10 m in diameter would be required to detect the Earth out to these
distances, and the technology for such a large mirror has not been demon-
strated.
In the CIT, diffraction is controlled by a Lyot coronagraph. Combining
a Lyot coronagraph with anodizing occulting masks in the first focal plane
can probably reduce the contribution from diffracted starlight by a factor of
1000. However, this factor of improvement in diffracted light can be util-
ized only if the scattering due to figure error in the primary mirror and to
surface dust or scratches is at least 1000 times less than the diffraction of a
conventional mirror.
Supersmooth mirrors have been manufactured for use in the fabrication
of microelectronics. These mirrors (0.5-m-diameter spherical mirrors) have
been characterized as approximately five times smoother than the Hubble
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THE SEARCH FOR LIFE'S ORIGINS
Space Telescope mirror. During the fabrication of supersmooth mirrors,
figure errors decrease monotonically until the desired specification is reached
and then polishing stops. The metrology used to measure the figure of the
mirrors can go beyond the current specifications, and there appears to be no
inherent reason why much smoother mirrors could not be fabricated. This
promising technology should be pursued in coming years.
Studying Nebulae and Disks
The study of protoplanetary nebulae and young stellar disk systems also
requires direct imaging techniques. Estimates of the required measurement
accuracies for studying solar-system-scale objects at 10 parsecs or at 140
parsecs (Taurus molecular cloud star-forming region) follow. To resolve a
linear dimension of 1 AU at 10 parsecs requires a resolution of 0.1 arc see,
whereas the diameter of our solar system (100 AU) can be resolved with a
resolution of 10 arc sec. In the Taurus cloud, the corresponding resolu-
tions become 0.007 arc see and 0.7 arc see, and resolving something even
as large as a dusty protoplanetary nebula for a 1-solar-mass star in the
Taurus cloud still requires a spatial resolution of 80 arc see (1.33 arc min).
To study dusty disks around young stars or protoplanetary nebulae in the
process of collapse requires more than just spatial resolution; spectral reso-
lution and good sensitivity are necessary. Resolving powers (~//~) as high
as 105 may be needed to use far-infrared, submillimeter, and millimeter
molecular lines as tracers of the kinematics and chemistry of the collapsing
nebulae. To locate the inner cutoff (if any) of a dust disk around a young
star and to characterize the particle size distribution and density as a func-
tion of radius require the ability to suppress the luminosity of the central
star, as in the case of individual direct planet detection. Once again, corona-
graphs can be used, but their physical size will ultimately determine how
close to the star the measurements can be made. Ground-based interfer-
ometers at millimeter, submillimeter, and infrared wavelengths may prove
to be the instruments of choice to study the radial dependence of the contin-
uum emission from the dust. Long baselines, large collecting area ele-
ments, or many elements are required for sensitivity.
Spectroscopic Analyses of Planetary Atmospheres
To achieve the most ambitious objectives of remotely detecting, charac-
terizing, and searching for evidence of life on an extrasolar planet, direct
light from the planet must be examined without contamination from the
light of the parent star. Only direct methods of planet detection are suitable
for obtaining these observations. In the future, large systems employing
coronagraphs, anodization, and supersmooth mirrors with diameters 210 m
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117
may allow full characterization of the temperature, pressure, atmospheric
composition, etc., of extrasolar planets. These large-aperture instruments
may be monolithic but will most likely be composed of multiple phased
elements. Therefore, additional technologies involved in stabilization of the
point spread function of a multiple-element telescope will be needed.
An alternative to a very large-aperture telescope has recently been sug-
gested for imaging a distant Earthlike planet and spectroscopically analyzing
its atmosphere in search of oxygen and ozone: an optical aperture-synthesis
interferometer. In particular, a 25-element array of 2.8-m anodized mirrors
with surface accuracies comparable to the Hubble Space Telescope might
be able to achieve sub-arc-second resolution sufficient to isolate planet and
star. The stellar light can be satisfactorily suppressed by the combined ef-
fects of anodization and precise positioning of the star in a null of the inter-
ferometer. Very preliminary calculations suggest that 60 hours of integration
time would be required for a 10-sigma detection of the 7600-5 oxygen A-
band absorption feature for an Earthlike planet at the distance of Tau Ceti
(~1 parsec). If the distance is increased to 10 parsecs, the required integra-
tion time increases to 300 hours. Whether such a scheme is feasible will
depend ultimately on the accuracy with which the elements of the array can
be maintained in relation to one another so that the star can be kept pre-
cisely at the interferometer null and the signal phase can be maintained.
The positional requirements are extraordinary, far in excess of today's
capabilities.
Unless they are fortuitously close, the direct imaging and spectroscopic
analysis of Earthlike planets are not likely to occur within the time scale
envisaged in this report. Continued development and research are required
to ascertain whether very large smooth mirrors or extreme positional accu-
racy in space can actually be demonstrated by the next generation of tech-
nology. The search for primitive life may not yet be timely, but the devel-
opment of technology to meet required measurement accuracies is. The
problem of finding life beyond the solar system may become more tractable
if there exist extraterrestrial technologies engaged in activities that can be
detected remotely.
Searching for Extraterrestrial Technologies
Technologies That Generate Electromagnetic Signals
One very practical test of the idea that intelligent life exists beyond our
solar system is based on the postulate that other technologies have transmit-
ted (either deliberately or unintentionally) electromagnetic signals that can
be received and recognized with extant technology here on Earth. In 1959,
it was proposed that transmissions in the neighborhood of the spectral line
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of neutral hydrogen (1420 MHz) might be a means by which extraterrestrial
technologies communicate with each other over interstellar distances. More
than two decades of scientific debate and review have expanded this idea
into a plan to systematically search through the terrestrial microwave win-
dow for signals originating from an extraterrestrial source of intelligence.
The plan calls for the use of existing radio telescopes, mature microwave
technology, and very large special-purpose multichannel spectrum analyz-
ers and signal-processing systems to carry out a promising set of explora-
tory search strategies. The entire sky is to be scanned with moderate sensi-
tivity over the frequency range of 1 to 10 GHz. A set of about 800 nearby
solar-type stars will be targeted for much more sensitive searches over the
1- to 3-GHz frequency range. The types of signals sought are those be-
lieved never produced by any natural process; they are compressed in fre-
quency and perhaps in time as well. If implemented soon, this plan (which
was recommended by the Astronomy Survey Committee in Astronomy and
Astrophysics for the 1980s, Volume 1, National Research Council, 1982)
should occupy most of the decade to be covered by this document.
The measurement accuracy appropriate for such a search strategy may be
determined by establishing the sensitivity required to detect the artificial
microwave signals generated by current terrestrial technology, if the planet
Earth was assumed to be located across the Milky Way galaxy from us.
Since we do not deliberately transmit signals intended for communication
with another technology, the Earth model must consist of signals generated
for our own purposes that leak into interstellar space. The weakest, but by
far the most numerous, signals are the narrowband carriers for radio and
television broadcasts; these are rated at 106 to 107 W of effective isotropic
radiated power (EIRP). The single strongest, though very infrequent, signal
transmitted is the Arecibo planetary radar at 10~3 W EIRP. If these trans-
mitters were located across the galaxy at a distance of 20 kiloparsecs (60,000
light-years), detection would require a sensitivity ranging from 10-37 to 10-3°
W/m2 for signals with EIRP ranging from 106 to 10~3 W. For comparison, a
typical molecular line survey made by radio astronomers achieves a sensi-
tivity of 10-2° W/m2. The planned NASA Sky Survey will achieve 10-23 W/
m2 and, at its most sensitive, the planned NASA Targeted Search will achieve
10-27 W/m2. Successful detection by this planned search will require the
existence of extraterrestrial transmitters that are more powerful (perhaps
intentional) or closer than the other side of the Milky Way.
Although the planned microwave search will not conclude until nearly
the end of the next decade, advanced planning for follow-on searches should
be conducted concurrently. The planned microwave search may fail to
detect any signals, either because the strategy is flawed or because the
sensitivity and coverage of parameter space were inadequate. The micro-
wave region of the spectrum is "preferred" for such signal detection be-
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119
cause the natural astrophysical background radiation is least at those fre-
quencies. The naturally quiet microwave window extends to at least 100
GHz, and signals from orbital transmitters may occupy the upper end of the
window. The planned microwave search is restricted to the lower portion
of the window by the increased atmospheric noise inherent to any terrestrial
ground-based search. Future searches will require access to space to extend
the search to higher frequencies and to escape the increasing interference
generated by terrestrial communication technology. Conclusions concern-
ing the best possible signal-to-natural-noise ratio as a function of frequency
(if space-based transmitters and receivers are assumed) depend upon scaling
laws for the construction of orbital structures. Under certain scaling laws,
the infrared appears to provide the best signal-to-noise ratio; under others,
the microwave region is still preferred. Experience with on-orbit construc-
tion in the coming decades will allow an empirical determination of how
the size of an antenna scales with wavelength. Therefore, it is essential to
consider and review other plausible strategies for the detection of electro-
magnetic signals from technologies that generate them and to support devel-
opment of those that are worthwhile, should they be required.
Technologies That Do Not Generate Electromagnetic Signals
Even a highly technological civilization might not deliberately generate
signals that are detectable over interstellar distances. The question is then
whether the technology of such a civilization could be detected in some
other way. It is impossible to extrapolate the likely progress of technology,
here or elsewhere, with any degree of certainty. Although speculative,
however, it is appropriate to consider those technological activities in which
we now engage and ask how they might appear from afar if they were
increased in scale and intensity as some scientists and engineers have pro-
posed.
Energy production or transformation will probably be an important con-
cern for any advanced technology. Collection of stellar radiation on a
planetary-system scale, large-scale stellar disposal of fissionable waste, and
tritium leakage from orbital fusion plants might produce anomalous radia-
tion in the infrared, in the optical lines associated with rare earth elements,
and at microwave frequencies, respectively. Thus, there could be observ-
able signatures of energy production by advanced civilizations, but it is not
possible to make any quantitative predictions. However, in the event of
unexpected or unusual observational results during the pursuit of more tra-
ditional research, such explanations should be kept in mind if known astro-
physical causes cannot be found easily. No separate or discrete efforts
seem to be indicated in this line of research. It is important to lend support
to the development and deployment of new and more capable infrared in-
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THE SEARCH FOR LIFE'S ORIGINS
strumentation for the study of protoplanetary nebulae and dusty disks around
young stars. Continuation of the classical study of stellar spectra and im-
proved instrumentation to make the process more efficient are important in
their own right. Detection of another technology might be a serendipitous
bonus.
Interstellar travel and colonization, if undertaken by a distant technol-
ogy, might well have extraordinary observational consequences. However,
because this takes us beyond the realm of extrapolation of our current tech-
nology, it is not appropriate for this report.
Available Measurement Accuracies
Now that the measurement accuracies required have been derived to de-
tect extrasolar planets, spectroscopically scan their atmospheres, and detect
evidence of extraterrestrial electromagnetic technologies, it is necessary to
compare them with what is likely to be available in the near future.
Tables 6.1 through 6.4 provide a summary of existing instrumental capa-
bilities and possible future prospects for the four methods of extrasolar
planetary detection discussed above. Table 6.5 is a summary of the obser-
vational capabilities for NASA's proposed Microwave Observing Project.
Innovations, breakthroughs, and perhaps completely new technologies
are required to directly image terrestrial planets, perform chemical assays
TABLE 6.1 Instruments for the Detection of Extrasolar Planets:
Astrometric Methods
Resolution
Telescope/Spacecraft Launch Instrument (arc see)
Allegheny Observatory Now MAP 10-3
Lick Observatory Now Image detector 10-3
Hubble Space Telescope 1990 WFC 10-3
FGS 1 0-3
Dedicated new ? MAP 10-4
astrometric telescope
on Mauna Kea
Imaging astrometric ? MAP 10-4
free flyer
ATE on Space Station 1995 MAP 10-5
Free-flying astrometric ? Moving grating 10-6
interferometer
NOTE: ATE = astrometric telescope facility; FGS = fine guidance sensor;
MAP = multichannel astrometric photometer; WFC = wide-field camera.
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TABLE 6.2 Spectroscopic Detection
121
Telescope/Spacecraft
Launch Instrument AV (m/s)
Ground-based facilities (6)
spectrometer
Now Radial velocity 10
TABLE 6.3 Photometric Detection
Telescope/Spacecraft Launch Instrument /~\lk
Space Station 1999 Double 10-5
Block II differential
spectrometer
TABLE 6.4 Direct Detection
Resolution
Telescope/Spacecraft Launch Instrument (arc see) Contrast
Ground-based Now IR CCDs 1-2 5 x 10-2
speckle and (Keck
coronagraphs in 1991) 0.5 10-3
Hubble Space Telescope 1989 FOS and 1 2 x 10-7
coronagraph
Hubble Space Telescope, 1994 NICMOS or 0.2 10-5
2nd generation HIMS 0.1 10-5
ISO 1992 ISOCAM 6 10-3
SIRTF 1995 MIPS 6 10-3
CIT 1995 Coronagraph 1 10-9
FIRST ? 1 10-3
LDR ? 1 10-3
Dedicated IR ? 0.5 10-4
interferometer
Optical ? 0.3 10~~°
interferometer
NOTE: CCD = charge coupled device; CIT = Circumstellar Imaging Telescope;
FIRST = Far-Infrared Space Telescope; FOS = faint-object spectrograph; HIMS =
Hubble imaging Michelson spectrometer; IR = infrared; ISO = Infrared Space Ob-
servatory; ISOCAM = Infrared Space Observatory camera; LDR = large deployable
reflector; MIPS = multiband imaging photometer; NICMOS = near-infrared camera
and multiobject spectrometer; SIRTF = Space Infrared Telescope Facility.
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TABLE 6.5 Search for Technological Civilizations That Generate
Electromagnetic Signals
Dual Mode Search Strategy for NASA's Microwave Observing Project
Targeted Search
800 nearby solar-type stars
Up to 1000 s of observation per star at each frequency band
1- to 3-GHz frequency coverage
Dual circular polarization
High resolution and sensitivity
Sensitivity to a wide variety of signals pulsed and drifting continuous wave
(COO)
On-line RFI subsystem
120 million channels (1-, 2-, 4-, 8-, and 16-Hz and 74-kHz resolution band-
widths)
Sky Survey
All-sky coverage
0.3 to 3 s of observation per beam at each frequency band
1- to 110-GHz frequency coverage
Dual circular polarization
Moderate resolution and sensitivity
Primary sensitivity to CW signals
RFI data base
15 million channels (30-Hz and 74-kHz resolution bandwidths)
thereon, search for leakage radiation throughout the galaxy, and move the
search for extraterrestrial technologies to higher frequencies or to other
search concepts.
Advances will be needed in technologies required for interference pro-
tection of a large dedicated SETI facility in high Earth orbit or on the lunar
farside; for large-scale optical, infrared, and submillimeter arrays in Earth
orbit or on the lunar farside for direct imaging and spectroscopic examina-
tion of extrasolar planets and protoplanetary nebulae; and for advanced
data-processing techniques.
Representative terms from entire chapter:
extrasolar planets
