Expert: Courtney Seligman - 6/16/2009

Question
QUESTION: on bbc's the sky at night, they were looking at possible planet formations around other stars,they seem to conclude that at certain size eg jupiter etc these distant objects are gas giants ,why cant earth type or for that matter mars like planets be large and if not what is the limit for such bodies, twice earth size, three times ?

ANSWER: I don't know what the limiting size would be, but there are two factors which argue against very large rocky bodies. First, there isn't that much rocky material in the clouds of gas and dust out of which stars and planets form, so rocky objects are always going to be smaller than icy or gaseous ones formed from the same clouds. Second, if very massive rocky objects could form, there's a good chance they'd pull light gases toward them, and end up as the rocky cores of apparently gaseous planets. Both possibilities argue against very massive (tens or hundreds of Earth masses) planets being like the Earth or Mars.

However, I don't see any reason why a massive stellar nebula with relatively large amounts of dusty material couldn't produce rocky bodies with a few Earth masses. Whether the limit of mass would be five to ten Earth masses, which would produce twice-Earth-size objects, or twenty to thirty Earth masses, which would produce three-Earth-size objects, would be merely a guess, as I've never read a discussion of such a possibility.

In my mind the main question would be, if such objects could form, would they be able to avoid pulling large amounts of gases toward themselves, or would they always end up as the rocky cores of apparently liquid or gaseous planets. To avoid that would require their star to form relatively quickly, so that any gases were blown away before the bodies were large enough to hold onto gases. Stars substantially heavier than the Sun blow away the gases surrounding them while they are still forming, which might leave them with primarily rocky planets. However, they aren't studied much, (a) because their large mass makes it harder to detect planets orbiting them and (b) because their lifetimes are much shorter than the age of the Solar System, making the development of interesting forms of life (during the star's lifetime) relatively unlikely. For both those reasons, efforts to look for extrasolar planets have concentrated on Solar-size and smaller stars.

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QUESTION: excellent,again when looking for planets around other stars although above answer partly explains,how do asronomers choose which stars to study,do they pick these at random,surely they would start at those closest,alpha centuara,beta etc working outwards where possible or even well known stars that the guy on the street is familiar with polestar, the pleiades cluster etc?

Answer
Although looking for exoplanets is interesting in its own right, and might greatly increase our understanding of the origin of the Solar System, the main impetus is the search for extraterrestrial life forms, particularly those sufficiently advanced technologically that we could communicate with them (presuming that is a wise thing to do).

Given that, the emphasis is on searching for planets around stars similar to the Sun, or smaller and less massive. It is easier to find such planets, because they can have more noticeable effects on their stars, and they could have a chance of developing complex life forms, since their stars have very long lifetimes. Substantially brighter stars (which happen to include Polaris and the Pleiades) may be studied for other purposes, but finding exoplanets around them would be difficult, and finding exoplanets around them with advanced forms of life unlikely.

Within the class of suitable stars, we do prefer nearby ones. Being closer, they are brighter and easier to study. Also, if through some very lucky (or unlucky) chance we discovered such a world with technologically advanced lifeforms capable of communicating with us, the closer they are, the better the chances of meaningful communication. If we were to detect such lifeforms in a cluster or stars 30,000 light years from here, there couldn't be any meaningful interchange, because of the 60,000 years between sending a message and receiving a reply. But if they were only ten light years away, although communication would be slow, it would at least be feasible.