Monday, July 9 2001
Planet-Hunting: The Search for Planets Outside the
Solar System
- Vandana SinghVandana Singh
is a former physicist, now a writer of science fact,
science fiction and fantasy. She is also an editor of
scientific and other works, and a member of Saheli,
Austin. She is based in the greater Boston area.
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A simple glance at the night sky stimulates the human
imagination to soar beyond the terrestrial. As much as pre-flight
humans once marveled at the freedom of birds, we in the twenty-first
century dream about escaping the gravitational shackles of our
planet. What will we find there, in the void of space, among
distant suns? Will there be planets like those in our solar
system? And if so, will there be life?
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For most of humanity's sojourn on earth we have been unable to
answer this question. Science fiction writers have spent decades
joyfully speculating on the subject, but it is only now, at the
close of the twentieth century, that we are beginning to read the
answer in the night sky. There is already some evidence (see last
time's column) that some planets and moons of the solar system
may have conditions that support life. But what about worlds
outside our cosmic backyard? Do any exist, and if so, what are
the possibilities for life?
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Looking for a Needle in the Cosmic Haystack
The task of finding extra-solar planets is a daunting one.
Planets have no innate luminosity; they shine dimly in reflected
light from the stars they orbit. Their presence must, for the
most part, be deduced rather than detected directly.
Some of the ways that astronomers have developed to look for
planets are nothing short of ingenious. Most stars are so distant
from earth that they appear fixed in our night sky (their
apparent movement across the sky is only due to the earth's
motion). It takes hundreds or thousands of years for the stars'
relative positions to change. If a star appears to move slightly
against this relatively unchanging canvas, there is a chance that
this motion might be caused by an unseen planet.
Planetary system around the sun like star HD168443
The sun like star HD168443 is located 123 light years away in the constellation Serpens. It is orbited by both a massive planet and another bulkier object of 17 - 40 Jupiter masses. Whether this bulky companion is a planet, a dim failed star called a brown dwarf, or some as-yet unidentified astronomical object is unclear. The star and inner planet are visible in the upper left, with the outer companion shown as a ringed world with a flare. Two rocky satellites orbit this large body.
Copyright © Lynette R. Cook, All Rights Reserved, Used with Permission.
This slight movement, or wobble, is the effect of the small
gravitational pull of the planet. The to-and-fro movement of the
star appears as a Doppler shift in the star's spectrum that
alternates between the blue and red ends. (The Doppler effect is
the change in frequency of a wave due to the motion of its source
relative to us --- in the case of sound waves, it is familiar to
any one of us who has heard the whistle of an approaching train
drop in pitch as it moves away from us). The wobble can also be
detected via astrometry --- careful measurements of the star's
position over time. Until very recently astronomers were
handicapped by the fact that the earth's atmosphere prevents such
precise measurements of tiny motions --- it is like trying to see
through a shifting veil. Two recent developments --- the Hubble
Space telescope in orbit around the Earth, and the advent of
adaptive optics, an ingenious way of compensating for the effects
of the atmosphere on ground-based telescopes --- now enables us
to see the night sky with startling clarity.
But because this wobble is so small, astronomers expect this
method to work best for very massive Jupiter-like planets that
are relatively close to the parent star. Similarly, occultation
of the star by a planet, resulting in a change in brightness of
the star as the planet passes briefly between it and us, can
detect large planets. Earth-sized planets, however, are still
beyond our detection capabilities, except for one very bizarre
class of star called a pulsar. A pulsar is the remnant of a
supernova explosion. It is a star only about 20 km across and is
typically about one and a half times the mass of our sun. Matter
is so compressed in such a star that a pin-head-sized peck of
material would weigh as much as a battleship. The magnetic field
of such a star is about a million million times that of earth.
Pulsars also rotate rapidly. As a result, they can emit radio
waves or X-rays or gamma-rays, which sweep out towards earth
rather like a lighthouse beam, at extremely precise intervals. A
disturbance in the timing of the radiation from the pulsar can
indicate a planetary companion.
But the extreme conditions on a pulsar's planets make it
unlikely that life as we know it can exist there. What prospects
are there for detecting earth-sized planets of less violent
stars? One method that has great promise for detecting earth-sized
planets is called gravitational microlensing. According to
Einstein's theory of General Relativity, gravitation bends light
rather as a lens does. If we have two stars roughly in line with
each other as seen from earth, the closer star can act as a
gravitational lens, producing more than one image of the distant
star. If the closer star has a planetary companion, the resulting
variation in the brightness of the image can reveal the planet's
existence.
One of the most dramatic methods of looking for extra-solar
planets is to look at star nurseries. Stars typically form when a
cosmic dust cloud begins to condense due to gravitational forces.
If conditions are right, the star will ignite at the center,
while planetary material will begin to condense into planets
further from the star. Once the star has shed its embryonic veil
of gas and dust, the still-glowing proto-planets may be clearly
visible.
A Brief Chronology of Planet-hunting
The dream of other worlds is a compelling one indeed. Way back
in 1916, astronomer E.E. Barnard discovered a star about 6 light-years
away that had a very large proper motion (the motion against the
backdrop of fixed stars). This led to speculation as to whether
Barnard's star had planetary companions. Astronomer Peter van de
Kamp spent most of his life studying this star, looking at over
2000 photographic plates to see if there was a wobble in the
star's motion. After 40 years of devoted study he concluded that
Barnard's star did indeed harbor a Jupiter-like planet. However
subsequent studies, including observations through the Hubble
Space telescope, failed to confirm van de Kamp's findings. If
Barnard's star does indeed have planets, they are likely to be
small ones.
The first confirmed discovery of extra-solar planets occurred
in 1992. Astronomers Wolszczan and Frail, studying timing signals
from a pulsar named PSR 1257+12 in the constellation Virgo,
deduced the existence of two planets. Subsequent observations
raised the number of planets to four, at least two of which are
about 3 times the mass of the earth, and one is far smaller.
These planets are bombarded by intense radiation and as such are
thought to be unlikely candidates for life-bearing worlds.
The pulsar PSR 1257+12 has three confirmed
planets
Discovered by Alex Wolszczan before planets were ever discovered
around sunlike stars. The pulsar¹s radiation regularly sweeps
over the planet, energizing its atmosphere, causing auroras, and
bathing it in an eerie green light.
Copyright
© Lynette R. Cook, All Rights Reserved, Used with Permission.
Then, in 1995, the year Peter van de Kamp died, Mayor and
Queloz of Geneva Observatory detected a wobble around the star 51
Pegasi. It is now believed that a Jupiter-like planet circles
this star at a distance about one-eighth that of Mercury from our
sun. The surface temperature of the planet is estimated to be a
blistering 1300 degrees celsius!
After 1995 there was a flurry of new discoveries, some of
which had to be retracted after mistakes in the data and their
interpretation were found. To date, there are some sixty
confirmed planets orbiting suns other than our own. Most of them
have been found via the wobble method --- these are massive
giants about 1 to 13 Jupiters in mass. A multi-university team
led by astronomers Geoffrey Marcy and R. Paul Butler have
discovered the largest numbers of planets.
The planetary status of some of these giants is still in
question. It is possible that some of these alleged planets ---
particularly the heavier ones --- are actually brown dwarfs,
which are essentially failed stars. Brown dwarfs are typically
about 10 - 80 times the mass of Jupiter but shine only briefly by
burning deuterium.
The next significant discovery in the planetary pageant was
that of a multi-planet system, the first one found orbiting a
normal star (as opposed to a pulsar). In 1999, Marcy and Butler's
team found three planets circulating a sun-like star called
Upsilon Andromedae about 44 light-years away from us. The planet
masses are 0.71, 2.11 and 4.61 times that of Jupiter, and the
farthest is about 2.5 AU (I AU is the distance between the Earth
and our sun). This is a significant discovery as it allows us to
compare, for the first time, our own solar system with another
one.
The Upsilon Andromedae System
Copyright
© Lynette R. Cook, All Rights Reserved, Used with Permission.
One of the most bizarre recent discoveries is that of "free-floaters".
In 2000, astronomers led by Philip Lucas and Patrick Roche found
twenty drifting bodies in the Orion nebula. By looking at the
spectra of these objects they found hints of the presence of
water vapor, which indicates that these bodies are rather light.
Whether they are planets is a subject of controversy as also a
matter of definition. Detractors believe that they are simply
lightweight objects formed by the fragmentation of gas clouds.
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NEWS FLASH!! Possible Sighting of Free-Floating
Planets! As we go to press, a team led by Kailash Sahu
has observed, through the Hubble Space telescope, six
freely floating objects in the star cluster M22. These
were observed through gravitational microlensing --- a
phenomenon that causes a momentary brightening of a
distant star as the foreground object passes in front of
it. These objects are estimated to be only about 80 times
the mass of our Earth, and if confirmed, may be the
smallest objects found outside the solar system!
For the full story as well as pictures and animation
showing how gravitational microlensing works, click here
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But the latest evidence we have of an extra-solar planet
actually consists of an image captured on a photographic plate
--- a picture of a newly formed star and its possible planetary
companion. The still-aglow companion may in fact be a distant
celestial object, but Ray Jayawardhana, the young Sri Lankan
astronomer who discovered it in 1999, estimates that we shall
know for sure by the end of this year. If the object appears to
tag the star, it is very likely a planet about twice the mass of
Jupiter, orbiting the star at a distance about thrice that
between Pluto and our sun. The star is part of a small nursery of
a few dozen stars called TW Hydrae. There are 9 other star
nurseries near us that are being studied.
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Mysteries Galore
Each discovery of an extra-solar planet has expanded our
knowledge of our universe, and at the same time, has left us with
deeper mysteries. For instance, standard theories of planetary
formation hold that gas giants like Jupiter typically form far
from the star, as is the case in our solar system. However many
extra-solar planets appear to be uncomfortably close to their
sun, less than 1 A.U.! How can this be possible? Could they have
been knocked closer to the sun because of gravitational
interaction with another planet? Theorists are busy
reconstructing their ideas.
Our solar system has nearly circular orbits. Before the
discovery of extra-solar planets, we had no way of knowing if
this was the norm or whether we were some kind of fortunate
exception. So far, many of the planets found have highly
eccentric orbits (that is, orbits far from being circular ---
oval or elliptic). This also makes it less likely that there is
life (as we know it) on these worlds. A circular orbit means that
the planet has more or less stable temperature, a condition
conducive to life. In multi-planet systems, highly eccentric
orbits might mean that some planets get too close to each other,
which can cause disturbances ranging from massive tidal forces
and disruption of the planet's orbit to destruction of the planet
itself.
Then there is the question of planets around pulsars. How
could planets end up circling the remnant of a supernova
explosion? Are they later captives of the pulsar, or were they
old companions orbiting the original star at some safe distance,
only to be reined in closer after the explosion?
It is clear that our old view of the universe, obtained
through the shifting veil of our planet's atmosphere, must now
give way to new insights. Our universe is far more complicated,
beautiful and surprising than we had imagined. As for the old
question "Are we alone?" we have taken only a very
small step in the direction of the answer. Perhaps this is a good
thing. Perhaps our enforced isolation from other worlds will give
us time --- time to appreciate the uniqueness of our own blue-green
jewel of a planet, swimming bravely through the void of space,
brimming with life. And time also to develop the wisdom we shall
surely need if we do indeed encounter intelligent life on other
worlds.
Addendum
>NASA Science News for November 27, 2001
Astronomers using the Hubble Space Telescope have detected the atmosphere
of a planet circling a Sun-like star 150 light years away. Their
ground-breaking discovery shows it is possible to measure the chemical
makeup of distant planets -- and to search for chemical markers of life
far beyond Earth.
FULL STORY at: http://science.nasa.gov/headlines/y2001/ast27nov_1.htm?list552238
References
This article is based to a large extent on the following excellent reports and websites
and the references therein. (I claim full responsibility, however, for any errors or omissions).
- The graphics on this page are paintings and not real pictures of extrasolar planets. For a breathtaking array of such paintings by master artist Lynette
Cook, who has produced imaginative perspectives of the planets discovered
by several groups, including the Marcy and Butler team, visit:
http://extrasolar.spaceart.org/
- Science News, October 21, 1995, May 19, 2001, May 26,
2001
- http://www.sciencenews.org
- http://exoplanets.org,
the site of the Marcy and Butler team
- http://www.public.asu/~sciref/exoplnt.htm,
a very thorough catalog and historical report from
Arizona State University.
- http://www.obspm.fr/planets,
the site of the Observatoire de Paris, containing the
most up-to-date information and a catalog of extra-solar
planets.
- http://www.rog.nmm.ac.uk/leaflets/pulsars/pulsars.html,
pulsar pages of the site of the Royal Observatory in
Greenwich.
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