Seven new planets discovered

28 February 2017

Interview with 

Amaury Triaud, Institute of Astronomy, Cambridge University

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Sunset on an alien planet - TRAPPIST-1f.

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NASA announced the exciting discovery of seven brand new planets this week, in a solar system 39 light-years away. The planets orbit an ultracool dwarf star about the size of Jupiter, called TRAPPIST-1, and some of them are likely to contain liquid water, so could this mean they also host life? Georgia Mills spoke to one of the scientists who was part of the international team of astronomers to make the discovery: Amaury Triaud from Cambridge University’s Institute of Astronomy, and he explained how they found these new planets.

Amaury - We just wait for the planet to pass in front of the star. As it does so it casts a shadow and from how deep the shadow is, then we know that there is a planet and we also measure the size, and how frequently the shadow comes back we measure the orbital period of the planet.

Georgia - So you had your sights on this star and you just waited to see if planets passed in front of it?

Amaury - Pretty much, yeah. We had a survey, so we looked at different stars and on that one planets past.

Georgia - What do we know about them?

Amaury - What we know is their size and the masses, so we know they have sizes very similar to the Earth and masses similar to the Earth. Once you have radius and mass you know the composition and density and that is consistent, at the moment, with rocks which allows us to tell that they are Earth-like essentially.

Georgia - Oh wow! And what about things like their orbit time and say you and I were sitting on one of these planets, what would it look like, what would it be like?

Amaury - Well, the sight would be remarkable. The planets are really close to one another and really close to the star. The star being small to get good temperature all the planets are really compact. Say you were on TRAPPIST-1F, for instance, the star would be three and a quarter times larger in size, but in area, it would look ten times bigger than the Sun. Humans are very good at noticing area rather than size itself. The nearest other planet (TRAPPIST-1G) would be in the order of twice the size of the Moon but it would not always be there. Only when the planets are in conjunction, so once per orbit essentially, every nine or ten days or so, you would see this in the sky, which is another planet.

Georgia - Considering how close they are to their star, do we think they might be able to host life?

Amaury -  The star is small; it’s quite cool compared to the Sun and so the energy the planets receive is similar to what the Earth receives. We’re actually quite excited. There is a lot of potential that if the atmospheric and geological conditions at the the surface of the planet are good and if there is water then this could exist at the liquid form which is essential for life.

Georgia - Why is this thought that there might be liquid water there?

Amaury - We don’t know yet if there is liquid water. We actually say that all seven planets are temperate, meaning that under specific conditions they could host liquid water. For three of them I think the chances are much higher. But you could imagine that for the furthest planet out, which is quite cold, if you have a lot of greenhouse gas it would trap the heat and allow liquid water to persist. So what’s great is we now have seven targets that we can check.

Georgia - How do we check them?

Amaury - Using the same method as we did to detect them. The planet passes in front of the star, some of the starlight shines through the planet’s atmosphere and we notice a signal. That minute signal is what we will investigate now.

Georgia - Say, for example, there was alien life it would be very exciting, but what if it was like bacteria or these very small things. How would we know just from their imprint over the sun?

Amaury - Here we rely on what we know on life on Earth. Life on Earth has changed our atmosphere dramatically meaning that if similar amounts of life is over there, and changes in a similar amount of the atmosphere then we would notice that but it requires several ingredients. Life needs to be present and change things significantly.

Georgia - These planets, I think they’re 39 light years away. That’s quite far so we’ll probably never get there, so what’s the point of actually studying them?

Amaury - We only have one example for biology at the moment in the solar system so we want to go beyond a sample of just one. The key question here for an astronomer is not
is there life elsewhere? We think there must be somewhere but how frequently does it happen? If you have the right conditions is it every planet, every planet in ten, every one in ten thousand? That, I think, is a key question in understanding our position within the many outcomes of nature.

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