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Asked by to Sarah, Sam, Roberto, Nate, Nat on 16 Jun 2014. This question was also asked by .
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Sarah Casewell answered on 16 Jun 2014:
That is a really good question! At the moment we look at the light that comes from the planet as it goes behind it’s star. We can then see the plant’s atmosphere. This is called transmission spectroscopy. When we see what the atmosphere is made, of, we can determine if it’s earth like. Not many of the planets discovered so far look like earth, and they’re mostly gas giants, but we’ve been able to tell if they have water in their atmospheres – and quite a lot do!
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Nate Bastian answered on 17 Jun 2014:
In addition to Sarah’s answer, we can work out the size of the planet (i.e. if it is the of earth, or of Jupiter) by studying all the planet moves around the star (and how the star is affected by the planet). From this we can work out if a planet is mostly made of rock (like earth, or venus or mars) or mostly made of gas, like Jupiter, Saturn and Neptune).
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Sam Connolly answered on 17 Jun 2014:
If you want to specifically know about the atmosphere of a distant planet, you have to look at the light you see when the planet is in front of its star, as Sarah says. All gases absorb certain colours of light, similar to a filter. If you have piece of transparent red plastic, for example, its acting as a filter by absorbing green and blue light, which is why only red light can get through, so everything looks red if you look through it. So as long as you know how much of each colour of light (called the spectrum) is coming from the star before the planet goes in front of it, you can work out which colours you can’t see any more when the planet does go in front. Then if you know that the colours you can’t see are the ones absorbed by say Oxygen, you know there must be Oxygen in the atmosphere. So if we look at the spectrum of light when a planet is in front of its star and find its mostly Nitrogen, with some Oxygen and a bit of CO2, we know it’s Earth-like!
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Natasha Stephen answered on 18 Jun 2014:
I think the others have covered this one pretty well; it’s all about the spectroscopy!
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Roberto Trotta answered on 19 Jun 2014:
We would need to look at the star around which the planet orbits very very carefully. Then the light of the stars goes through the planet’s atmosphere, the elements in the atmosphere will absorb certain colours in the light from the star. So by looking at the light you can tell which elements are in the planet’s atmosphere from the missing colours (we call those colours “spectral lines”).
This way we can tell what the atmosphere is made of, and whether it’s similar to ours.
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