The search for other "Earths"
Professor Eric Wilcots
Fifteen years ago we knew of exactly one solar system - a collection of nine
planets, untold numbers of small rocky bodies called asteroids and lumpy
mixtures of ice and rock called comets orbiting around a rather average star.
The third of those nine planets is a small, blue-green world, two-thirds of
which is covered by water. It is Earth, and the solar system is our own. Since
then astronomers have discovered dozens of other solar systems - other stars,
very much like our own, with at least one planet in orbit around them.
The discovery of planets around other stars has revolutionized the field of
planetary sciences, causing us to throw out old ideas and look for new
explanations of how planets and their stars form. It has ignited a drive to find
more planets and smaller planets around more stars. And it made very real the
possibility of discovering life outside of our own solar system.
How Do We Find Other Solar Systems?
What Isaac Newton realized around 1700 was that the same phenomenon that
keeps you and I stuck to the Earth is the same phenomenon that keeps the moon
going around the Earth and the Earth going around the Sun. The simple thing to
remember is that gravity makes things move and how much they move depends on how
massive they are and how far apart they are. We all know that the Earth exerts a
gravitational pull on the Moon, but the Moon also exerts a gravitational pull on
the Earth. The result of all this is that both the Earth and the Moon are
orbiting around something called the center of mass and because the Earth is so
much more massive than the Moon the center of mass is very close to the center
of the Earth. So, to you and I it looks as if the Moon is going around the
Earth.
What does this have to do with finding planets? The largest planet in our
solar system is Jupiter. Surely, the center of mass of the Sun-Jupiter system is
well inside the Sun, but it is not at the exact center. This means that as
Jupiter moves along its orbit, the Sun is (very slowly) orbiting around its
center of mass. So, to an observer very far away it would appear that sometimes
the Sun is moving ever so slightly towards her, and sometimes the Sun is moving
ever so slightly away from her. It is only the motion of the star as it responds
to the orbital motion of the planet that allows us to detect the presence of the
planet.
 What
we see illustrated in the figure is that as the star moves towards us, its light
is slightly blueshifted, and when it moves away from us, its light is slightly
redshifted. Astronomers can detect this slight shift, very much in the way that
you and I can pick up a shift in the pitch of a train whistle as it first
approaches us and then recedes. When it comes to finding planets, the thing to
remember is that the amount of the shift and the amount of time it takes from
being blueshifted to being redshifted tells us something about how massive the
planet is, how far away it is from the star, and how long it takes to go around
the star.
This "Doppler" technique has been used to detect all of the other
solar systems we now know about. Other approaches have been used to confirm the
detection of an extrasolar planet and learn much more about the nature of the
planet in question. One of the most fruitful techniques is to measure the
dimming of the star caused by having the planet pass in front of it.
What's Out There?
To date we have detected about 105 planets around 91 different stars, and 39
of those stars are visible to the naked eye. (Link)
All told, 75 of the stars we have looked at carefully enough have at least one
planet orbiting around them. All of the planets we have detected around other
stars are quite massive, ranging in size from that of Saturn to planets 10 times
larger than Jupiter. This isn't to say that smaller planets aren't out there;
it's just that we don't have the technology to detect them.
The amazing thing about these other solar systems is that they don't look at
all like ours. My daughter's first grade classmates can all tell me that only
small rocky planets are found in the inner solar system, while very large
"gas giants" are found in the outer part.
All of the models we had to explain how the solar system formed in the first
place were based on the notion that only small rocky planets formed in close
proximity to the central star. But all of the extrasolar planets we have
discovered so far are very massive, and they reside extremely close to their
central star; some would even reside inside the orbit of Mercury!
Finding Earth
Despite our success in finding dozens of other solar systems, we have yet to
identify an Earth-like planet around another star. The reason is very simple.
The wobble of a star that results from having a small planet orbit is very, very
small - too small to be detected with the current suite of instruments. We will
probably need another approach to find Earth-like planets around other stars.
One way is to actually detect the motion of the star caused by the orbiting
planet, not just the shift of its spectrum. This is a technique known as
astrometry and it is the art and science of measuring the positions of
individual stars to an amazingly precise accuracy and of measuring how the star
moves over the course of years. Over the next decade or so the U.S. and European
space agencies will launch telescopes into orbit that are capable of providing
the type of accuracy that will allow us to detect the presence of planets a
little bigger than Earth around nearby stars.
Another approach is to directly observe the planet. While this might seem
obvious, it is extremely difficult to do. The light from the star is so bright
that it will simply wash out whatever light is coming from the orbiting planet.
The key is to be able to observe the star with tremendous resolution and be able
to artificially block out the light from the stars. This technique should
provide our very first images of planets around other stars, and it will be able
to identify Earth-like planets.
Lastly, and perhaps most interesting, is to look for the spectroscopic
signature of Earth. This is more than just a quest to find an Earth-like planet,
it is a quest to find signs of life elsewhere in the Galaxy. The terrestrial
planets in our solar system all have very specific spectroscopic fingerprints
that tell us quite a bit about their atmospheres. Venus and Mars reveal a clear
signature of the CO2, the most common molecule in their atmosphere. Earth's
spectroscopic signature, however, has more than just CO2. It has clear
signatures of water and ozone, the strongest indicator of the presence of life
on Earth.
In the coming decades we will peer out into our Galactic neighborhood with a
new suite of instruments and a new generation of planetary scientists. There is
limitless room for discovery, but there is someone alive today who will be the
very first person to discover an Earth-like planet orbiting around some other
"average" star. And she or he may be the very first person to discover
the presence of life elsewhere in the Universe.
Professor Eric Wilcots lectures in the Department of Astronomy at the
University of Wiscosin. Find out more about this topic at his lecture at the
Sasol Scifest in Grahamstown, South Africa from 26 March to 1 April 2003. His
other talk at the Sasol Scifest takes an in depth look into the Southern African
Large Telescope (SALT) under construction near Sutherland, South Africa. SALT
will be the largest telescope in the Southern Hemisphere and will be at the
forefront of astronomical research for years to come. Visit www.scifest.org.za
for more information.
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