Extra-solar planets

Telescopes are nowhere near good enough to see features on planets orbiting stars other than the sun. But if a planet passes in front of its parent star they can study the spectrum of starlight passing through its atmosphere. This shows what the atmosphere is made of and how cloudy it is. A planet’s size can be estimated from how much light it blocks during such a passage. And telescopes can, by measuring a planet’s distance from its parent star, reveal its temperature. Such data are sufficient to draw plausible artists’ impressions. These ones, by an artist at the European Space Agency, are of “hot Jupiters”—gassy giants that orbit their parent stars more closely than Mercury does the sun. The planets’ spectra were collected using the Hubble space telescope by David Sing, of Exeter University, in Britain, and his colleagues, and have just been published in Nature. The planets’ names are the star-catalogue names of their parents, followed by the letter “b” to indicate they are the second-known body in that system. The smallest, HAT-P-12b, is about the size of Jupiter. The others are to scale.