The Search for Extra-Solar Terrestrial Planets: Techniques and Technology: Proceedings of a Conference held in Boulder, Colorado, May 14–17, 1995

158 Pages·1997·7.76 MB·English

by  James F. Kasting (auth.), J. M. Shull, H. A. Thronson Jr., S. A. Stern (eds.)

THE SEARCH FOR
EXTRA-SOLAR TERRESTRIAL PLANETS:
TECHNIQUES AND TECHNOLOGY
Proceedings of a Conference held in Boulder, Colorado,
May 14-17, 1995
Editedby
J.M.SHULL
University ofColorado, Boulder, Colorado, U.S.A.
H. A. THRONSON, Jr.
University ofWyoming, Laramie, Wyoming, U.S.A.
andNASA Headquarters, Office ofSpace Sciences,
Washington, DC, U.S.A.
and
S.A.STERN
Southwest Research Institute, Boulder,
Colorado, U.S.A.
Reprinted from Astrophysics and Space Science
Volume 241, No. 1, 1996
SPRINGER SCIENCE+BUSINESS MEDIA, LLC
A C.I.P. Catalogue record for this book is available from the Library of Congress.
ISBN 978-94-010-6453-8 ISBN 978-94-011-5808-4 (eBook)
DOI 10.1007/978-94-011-5808-4
Cover photo: Jmage of the Earth--Moon system taken from the Galileo spacecraft during its flight
to Jupiter. The detection of Earth-like planets around other stars is an exciting long-range goal
for 21 st -century astronomy. Photo credit: NASA.
Printed on acid-free paper
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@1997 Springer Science+Business Media New York
Originally published by Kluwer Academic Publishers, New York in 1997
Softcover reprint ofthe hardcover lst edition 1997
No part of the material protected by this copyright notice may be reproduced or
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TABLE OF CONTENTS
J.M. SHULL, H.A. THRONSON, Jr. and S.A. STERN / Preface 1
J.P. KASTING / Planetary Atmosphere Evolution: Do Other Habitable Planets
Exist and Can we Detect Them? 3
G.W. WETHERILL / Ways that Our Solar System Helps Us Understand the
Formation of Other Planetary Systems and Ways that it Doesn't 25
1. SCHNEIDER / Photometric Search for Extrasolar Planets 35
w.n. COCHRAN and A.P. HATZES / Radial Velocity Searches for Other
Planetary Systems: Current Status and Future Prospects 43
P. CaNNES, M. MARTIC and J. SCHMITT / Demonstration of Photon-Noise
Limit in Stellar Radial Velocities 61
H.A. McALISTER / Aspects of Astrometric Searches for Other Planetary
Systems 77
M. SHAO / Astrometric Detection of Earth-Like Planets with OSI 85
S. CASERTANO, M.G. LATTANZI, M.A.C. PERRYMAN and A. SPAGNA /
Astrometry from Space: GAIA and Planet Detection 89
M. SHAO / Ground-Based Interferometry 105
w.J. BORUCKI, E.W. DUNHAM, D.G. KOCH, W.D. COCHRAN, J.D. ROSE,
D.K. CULLERS, A. GRANADOS and J.M. JENKINS / FRESIP: A Mission
to Determine the Character and Frequency of Extra-Solar Planets Around
Solar-Like Stars 111
A. LEGER, J.M. MARIOTTI, B. MENNESSON, M. OLLIVIER, J.L. PUGET,
D. ROUAN and J. SCHNEIDER / The Darwin Project 135
P. DASCH / Public Involvement in Extra-Solar Planet Detection 147
Index 155
PREFACE
J. MICHAEL SHl:LLJ , HARLEY A. THRO:\SOX, JR. 2, A:'>D S. ALAN STER:\3
I University of Colorado, Dept. of Astrophysical. Planetary, &. Atmospheric Sciences
2 University of Wyoming and KASA Headquarters, Code SR
3 Southwest Research Institute, Boulder Office
On May 15-17. 1995, three Rocky Motultain research institutions hosted a confererJce to dis-
cuss the scientific basis, teclmological options, and programmatic implications of a large-scale
effort to find and study Earth-like planets outside the Solar System. Our workshop attracted
scientists, erJgineers, space agency administrators, and the public media to discuss and debate
the most promising teclmological options and opportunities. Major programs and proposals to
search for and study exo-planets were preserJted and discussed. In addition, our meeting c0-
incided ·with NASA's "roadmap" study for the Exploration of Neighboring Planetary Systems
(~"'\PS). Our meeting was the first international confererJce on this subject, affording an op-
portunity for several members of this study to participate in the debates over new technologies.
Our meeting proyed to be timely. Shortly thereafter, in late 199·5 and early 1996, two groups
of astronomers annotulced the first discoveries of planetary companions to nearby stars. using
high-precision radial velocity measuremerJts to detect the gravitational reflex motion of the star.
The first three detections include a Jupiter-mass companion to the solar-like star. 51 Pegasi,
and two remarkable objects of mass at least 2.3 and 6.5 Jupiter masses arotuld the stars 47
Ursae :Majoris and 70 V"rrginis, respectively. A.s is common "ith new discoveries, many more
have followed; extra-solar planets now number at least terJ.
The papers preserJted in this book prmide the currerJt early status of the search for e. .c tra-solar
planets. What sort of objects are we looking for in planetary atmospheres and planetary-system
architectures? \\b at techniques are currerJtly feasible, both from the grotuld and in space? How
can interferometers be optimized to di.scern faint planets in the glare of their parerJt stars? We
also made an e:x.-plicit effort to include the media at our confererJce, including a panel di.scussion
on Progmmmaiic Directions for the Future, recognizing that the public is an essential partner in
this effort. The book concludes "ith a cogerJt article on the role of public and media involvement
in this quest, an activity likely to be both lerJgthy and expensive.
The papers in this volume just. scratch the surface of the planetary-search technologies that
"ill be applied over the coming decade. Most searches "ill invoh'e studies of radial-velocities,
photometric variations, and direct imaging. However, the tulbiased distribution of planet masses
may first be characterized by grotuld-based monitoring of thousands of stars for gra,itational
micro-lensing by planetary systems. lltirnately. imaging and spectroscopy may be done in the
infrared, using large interferometers in space. possibly the most promising candidate technique
at this time. However, since the most successful techniques cannot easily be predicted for the
Astrophysics and Space Science 241: 1-2, 1996.
© 1996 KJuwer Academic Publishers.
2
long-term, the scientific funding agencies should rely on a variety of techniques. And, because
the ent.erprise may require a long "attention span", a thirty-year effort by some estimation;;, the
public and their go\'ernments should be prepared for a step-by-step approadl with appropriate
milestones.
The results of such an enterprise are likely to be both spectacular and profound. The detec-
tion and study of Earth-like planets outside our Solar System will be one of the great scientific,
technological, and philosophical events of our time. :\0 scientific activity is more likely to cap-
ture as strongly the public imagination and support. The outcome, either positive or negative,
will have a profound effect upon our understanding of the Universe and our place v.ithin it.
Furthermore, the facilities proposed to undertake this program may be eJ..-pected to be powerful
general purpose astronomical obsenatories. At the same time, it is possible that no scientific
enterprise will be more technically challenging, as it appears that such a program ",ill require
sensitive operation of large optical S}'stems in space. \\i th these motivations. we convened this
conference and collaborated to produce this book.
To support this conference, we benefitted from the financial support of our institutions. \\e
take this opportunity to thank: the Center for Astrophysics & Space A.stronomy (University of
Colorado); the Department of Physics & Astronomy CCniyersity of \Vyoming): and the South-
west Research Institute. For help at the meeting, we thank our local organizing committee,
particularly Kachun Yu. Janet Shaw provided valuable computer-systems help at \anous stages
of the enterprise. We also thank our colleagues at both :\ASA and ESA for their encouragement
and tlleir attendance at the meeting. The help of \Yes Huntress (:\A5A) and Serge Volonte
(ESA) was especially welcome in setting the visionary tone of our meeting.
Alan and Carole Siern with children
PLANETARY ATMOSPHERE EVOLUTION:
Do Other Habitable Planets Exist and Can We Detect Them?
JAMES F, KASTING
Department of Geosciences
Penn State University, University Park, FA 16802
Abstract. The goal of this conference is to consider whether it is possible within the next
few decades to detect Earth-like planets around other stars using telescopes or interfer-
ometers on the ground or in space, Implicit in the term "Earth-like" is the idea that such
planets might be habitable by Earth-like organisms, or that they might actually be inhab-
ited. Here, I shall address two questions from the standpoint of planetary atmosphere
evolution. First, what are the chances that habitable planets exist around other stars?
And, second, if inhabited planets exist, what would be the best way to detect them?
1. Climate stability on the Earth
A planet must satisfy a number of conditions in order to support life as
we know it. It must have water, carbon dioxide (for photosynthesis), and
other volatile compounds (e.g., ones containing N, P, and S) available at
its surface. It must have sufficient mass to hold onto an atmosphere, and it
must be in an orbit that is stable over long periods of time. It also needs
to have a stable climate that is, at the very minimum, conducive to the
continued presence of liquid water. Liquid water is required by all known
organisms during at least part of their life cycle and should be considered
as a fundamental requirement for life elsewhere. It may be, of course, that
some extraterrestrial life form does not require water but, if so, we would
have little idea of what to look for or where it might exist. The practical
search for habitable planets and for extraterrestrial life should be based on
life forms that we know are possible, that is, on organisms that are basically
similar to those on Earth.
Climate stability is often taken for granted here on Earth. The geologic
record shows that liquid water has existed for at least 3.8 billion years (b.y.)
of the Earth's 4.6 b.y. history and that life has been around for at least the
last 3.5 b.y (Schopf 1983). But it is not really obvious why this should have
been the case. During the last 4.6 b.y., the Sun's luminosity has increased
by some 40%, according to stellar evolution models (Fig. 1). This conclusion
is considered to be robust because the luminosity increase occurs as a direct
consequence ofthe fusion of hydrogen into helium and the attendant increase
in the density of the Sun's core. Sagan and Mullen (1972) showed that, all
other factors being equal, this change in solar luminosity would imply that
Astrophysics and Space Science 241: 3-24, 1996.
© 1996 Kluwer Academic Publishers.
4 JAMES F. KASTING
the mean surface temperature of the Earth was below the freezing point of
water prior to about 2 b.y. ago. Their finding is corroborated in Figure 1,
which shows calculations of the Earth's effective radiating temperature, Te ,
and mean surface temperature, Ts , made with a one-dimensional, radiative-
convective climate model (Kasting et al. 1988; Kasting and Toon 1989). Te
is calculated from the energy balance relation,
4 S
O"Te = "4 (1 - A) , (1)
where S is the solar flux at Earth's orbit, A is the planetary albedo, and
0" is the Stefan-Boltzmann constant. Ts is computed under the assumption
that the atmospheric C02 concentration remained constant at 350 parts
per million and that the tropospheric relative humidity remained constant
as well. The difference between Te and Ts represents the greenhouse effect,
the magnitude of which increases with time because of an increase in the
absolute abundance of water vapor.
Of course, no one really believes that Figure 1 represents the actual cli-
mate evolution of the Earth. The fact that liquid water was present from
very early on indicates that either the Earth's albedo was lower in the past or
its atmospheric greenhouse effect was larger. The modern planetary albedo
is about 0.3. Lower values are possible in the past if cloud cover was signifi-
cantly reduced (Henderson-Sellers 1979; Rossow et al. 1982). However, this
effect would likely have been compensated by an increase in surface albedo
caused by ice and snow, unless other warming mechanisms existed (Kasting
et al. 1984; Kasting 1989). A larger greenhouse effect is a more plausible
solution. Sagan and Mullen (1972) suggested that higher concentrations of
ammonia might have provided the necessary warming. This is now consid-
ered unlikely because ammonia should have been rapidly photolyzed to N 2
and H2 (Kuhn and Atreya 1979; Kasting 1982). Methane is another possible
early greenhouse gas (Kiehl and Dickinson 1987). However, its sources today
are almost entirely biological; hence, it is unlikely to have been present at
high concentrations on the prebiotic Earth, although it may have contribut-
ed to the greenhouse effect during the late Archean, around 2.5 b.y. ago
(Rye et al. 1995). The best candidate, though (for reasons described below),
is C02, which could have kept global surface temperatures above freezing
if it was present at levels of a few tenths of a bar, or about 1000 times its
present concentration (Owen et al. 1979; Kasting et al. 1984).
Could atmospheric C02 concentrations have actually been 1000 times
higher in the past? Although this may sound like a large amount, it is small
compared to the "-' 60 bars of C02 tied up in carbonate rocks in the Earth's
crust. This carbon interconverts with atmospheric C02 on time scales of mil-
lions of years by way of the carbonate-silicate cycle (Fig. 2). Atmospheric
C02 dissolves in rainwater, and the resulting weak acid dissolves silicate
PLANETARY ATMOSPHERE HABITABILITY 5
300r---,--------.---------r--------.-------~
....
c:
Q)
VI
Q)
:::.:::
Freezing Point of Water cL..
LQ.) 275 .9 .o...
.:.:.l (!)
I<
Qt.)I .<.~.t.I
Co Q)
E a:
~ ~
250 .8 VI
o
J:
E
::s
...J
L.
<tI
'0
L...--....J4~---~3-----2l::----......L..----""""'-O·7 (J)
Billions of Years Before Present
Figure 1. Diagram (Kasting and Toon 1989) illustrating the faint young Sun problem for
Earth. Solid curve is solar luminosity relative to today's value, as calculated by Gough
(1981). Dashed curves represent effective radiating temperature, Te , and mean global
surface temperature, T" as calculated by a one-dimensional climate model, assuming
constant atmospheric CO2 fixed relative humidity, and no cloud feedback.
rocks on land. The bypro ducts of silicate weathering, which include calci-
um ions (Ca++), bicarbonate ions (HC03), and dissolved silica (Si02), are
taken up by streams and rivers and carried to the ocean. There, organisms
use them to make shells of calcium carbonate (CaC03). Other organisms
make shells out of silica. When the organisms die, they settle into the deep
ocean, and some of their shells are preserved as carbonate and opal (sili-
ca) sediments on the seafloor. The seafloor, however, is not static; it slowly
spreads out from the mid-ocean ridge spreading centers as part of the global
plate tectonic cycle. At certain plate boundaries, the seafloor is subducted,
and its carbonate-rich sediment load is carried downwards with it. The high
temperatures and pressures encountered at depth cause calcium carbonate
and silica to recombine into silicate minerals, releasing C02 in the process.
This C02 eventually makes its way back to the surface and is released into
the atmosphere by volcanos, completing the cycle.
6 JAMES F. KASTING
THE CARBONATE-SILICATE CYCLE
land
Ca5103 + 2 CO2 Ocean
+H20 weathering
Ca++ + 2 HCOi + si02
Ca+++2 HCOi
- + CaC03 t CO2 + H20
CaC03 + 5102
melamorphlsm
Ca5103 + CO2 ..
Figure 2. Schematic diagram of the carbonate-silicate cycle, which controls atmospheric
C02 concentrations over time scales in excess of one million years.
The carbonate-silicate cycle just described provides a natural explanation
for why atmospheric C02 levels should have been higher in the past (Walker
et al. 1981). If the early Earth actually had been frozen as a result oflow solar
luminosity, silicate weathering would have ceased and volcanic C02 would
accumulated in the atmosphere. Eventually, the greenhouse effect would
have become large enough to melt the ice, restoring the normal hydrologic
cycle of evaporation and precipitation, and allowing silicate weathering to
proceed. Conversely, if the Earth were to become significantly warmer, the
rate of silicate weathering would increase, atmospheric C02 levels would
fall, and the climate would begin to cool. So, a planet like Earth that has a
large carbonate rock reservoir and an active plate tectonic cycle also has a
built-in negative feedback system that tends to stabilize its climate within
the liquid water regime.
If one accepts the argument given above, one may then invert the logic of
the faint young Sun paradox and use the predicted variation in solar lumi-
nosity, along with known climatic constraints, to estimate the atmospheric
C02 concentration at different times in the Earth's history (Fig. 3). The
PLANETARY ATMOSPHERE HABITABILITY 7
10
:-0. ,=> earth
::::::::~: .:
: : : : : :
: ~:,.
::::~ H
(5u-r2o0n°iCan) glaciation 103 :J
I : ..ate Precambrian ~-c
0
1~-20 DC)
10 2 .~
30% Solar flux 'E
.reduction (ODC) ~~" cfl
(0)
10'
(0). .
10.J ;§
"i
Terrestrial -: . ::::v
C3 photosynthesis
10-4 ~~'
4.5 3.5 2.5 1.5 0.5
lime before present (Ga)
Figure 3. Atmospheric CO2 concentrations required to offset decreased solar luminosity
in the past (Kasting 1992). The vertical bars represent periods of glaciation. The arrow
near 0.4 Ga marks the lower limit of CO2 for C3 photosynthesis (about 150 ppm).
shaded area on the graph represents the range of atmospheric C02 concen-
trations that are consistent with the geologic record (Kasting 1987; Kasting
1993). The vertical bars at 2.5 b.y. ago and 0.65 b.y. ago represent periods
of glaciation. During these times the C02 concentration could neither have
been too low (because the oceans would have frozen) nor too high (because
the glaciers wouldn't have existed). The suggested upper limit on atmo-
spheric C02 at 4.5 b.y. ago is 10 bars. This value comes from considering
the nature of the carbon cycle on a hypothetical ocean-covered early Earth
(Walker 1985). Much of Earth's carbon could have been in the atmosphere
in this case because silicate weathering would have been inhibited by the
absence of exposed land surfaces and because the planet would have lacked
stable continental platforms on which to store carbonate rocks. The mean
surface temperature of a primitive Earth with a 10-bar C02 atmosphere has
been calculated to be about 85°C (Kasting and Ackerman 1986). On the
other hand, if weathering of impact debris removed significant amounts of
C02 (Koster van Groos 1988), then the early atmosphere could have been