Are the properties of extrasolar planets known?
As of today, 101 extrasolar planets have been discovered, in 87 different planetery systems (4 of the planets are around pulsars). You can get an up to date list of the planets discovered at this website .
We classify the stars by their spectral type, which corresponds to a temperature and brightness scale. The sequence is O, B, A, F, G, K, M , in order of decreasing temperature and luminosity. All the planets discovered were around F, G, and K stars. (but we don't know if that's a real effect, or if it is just because it is harder to detect planets around O, B, and A stars because they are very bright) There have been no planets discovered around stars in globular clusters. The abundance of metals and heavy elements is smaller in globular clusters than around other stars, which tells us that there must be some of these heavy elements to form planets. (by 'heavy elements', I mean stuff more massive than hydrogen and helium).
We don't know the properties of extrasolar planets, we only know some properties of the extrasolar planets we are able to detect, which is not the same thing. Most of the planets discovered are very large (larger than Jupiter), and orbit very close to their stars. But this is because these are the planets that are easiest to detect. So if until now no Earth-like planets have been discovered, it doesn't mean that such planets do not exist, but rather that the observation techniques are not sufficient.
Now astronomers are trying to understand what the atmospheres of these planets are like.
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