Jupiters not Earths
Figure 1 shows what most people think we
should be looking for: a blue-white
rocky planet with a surface and atmosphere, and a stable orbit in a zone where
water is liquid and life can be comfortable.
Captain Kirk, find us some more of these!
Figure 1. Earth
But, Figure 2 shows the kind of planet we
think astronomers have been finding so far:
Jupiter, 300 times more massive than the earth, with no surface and
shrouded in a dense atmosphere composed of noxious gasses such as methane and
Figure 2. Jupiter
Moreover, the extrasolar planets found so far are all in orbits much
closer to their parent stars than Jupiter is to the sun. This was completely unexpected ten years
ago, when the first extrasolar planet candidate, the unseen companion of HD
114762, was discovered in an orbit similar to Mercury's. The thinking at that time was dominated by
the one example of a solar system that we knew about, our own, where the
largest planets, Jupiter and Saturn, orbit majestically in wide, long-period
orbits out in the icy nether regions where it was cold enough for them to form
by the accumulation of gases and volatile compounds onto rocky cores 10 to 20
times the mass of the earth. Although
Jupiter is the largest planet in the solar system, it is puny compared to the
sun, and pulls with only one thousandth of the sun's mass.
The inner terrestrial planets in our solar
system are in turn puny compared to Jupiter.
The earth is ten times smaller in diameter and is composed mostly of
dense refractory materials, such as minerals and metals that were able to stand
the heat close to the sun when the earth formed. Nobody expected to find giant planets so close to their parent
stars, because nobody thought that giant planets could form in a region where
all the ices would have melted into volatile gases.
Contributed by: Dr. David Latham