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 ammonia.

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