Why are we looking for Earth-sized planets?

We put Sasha's question to our captain planet: David Rothery.

David - There is something special about the size of the Earth, Sasha, but we’re not searching specifically for Earth sized planets, we’re searching for any planets around other stars that we can find, and most of them are giant planets like Jupiter because they’re the easiest ones to find. If one of those goes in front of a star it cuts out more of the star’s light, for example.

But we are now beginning to find Earth sized planets transiting (going in front of their stars), and causing a tiny dip in the brightness and that is what get’s people most excited, and that’s what hits the news. Just about ten days ago there was a story broke with seven Earth-sized planets orbiting a star known as Trappist-1. We’ve known about three of them for a couple of years actually, and four more have taken the total up to seven. That was very exciting. It was a red dwarf star; seven Earth sized planet. At least three of them are going to have the right temperature for there to be liquid water on the surface. So it’s a place where life that we understand could form, and where life as we understand would be detectable by means that we have.

Nobody’s saying you’re not going to get life in the clouds of a Jupiter like planet but that is entirely speculative. So, if we want to find life elsewhere, the most sensible place to focus on to begin with, at least, is planets that look similar to the Earth. Then at least we have a chance of looking for something that we could recognise.

Chris - One point that was made to me this week in the wake of this discovery was that because those small planets are so close in to that host star which is, as you say, not much bigger than Jupiter they will, therefore, become what we call tidally locked. So they’ll always show the same face to that star in the same way that we always see the same face of the Moon, and that will mean that one side of those planets is going to be roasting hot or nice and warm, the other side very, very cold. So it’s not going to be ideal and not like the Earth at all, potentially.

David - Unlike the Earth in that respect, this tidal locking is because it’s a small dim, red star, and to be at the right distance from it to be warm enough you’ve got to be very close and you get tidally locked. But that doesn’t mean one side is roasting hot and the other side is absolutely freezing.

If it has an atmosphere and some circulation, the heat would be distributed around the planet so you could have life all over the place. No sunlight, no starlight on the far side but there are ways to feed off chemical energy. So they are in the habitable zone of temperature we believe is conducive to liquid water, and if you've got the right chemistry you can have life. So being tidally locked - most of our life-bearing planets in our galaxy could be these Earth like planets around red dwarf stars, tidally locked. We could be the unusual case; not the norm. We don’t know yet. It’s exciting that we’re beginning to find out.