Seven Earth-Size Planets Identified: What Does it Mean?

7 Planets

Orbiting a star called TRAPPIST-1 not too far away, lie at least seven Earth-size planets that may have temperatures similar to Earth's. This discovery was announced today, February 22, 2017, at a news conference at NASA Headquarters. The study is published in the journal Nature

Here's a quick breakdown from some of our astronomers of why this discovery is important and just how Earth-like these planets really are. 

An artist’s rendering of the seven planets that orbit the star named TRAPPIST-1, in order of their distance from the star. Credit JPL-Caltech/NASA.

Question: What's all this about?
Answer: There's a star called TRAPPIST-1 about 40 light-years from here. We already knew it had three Earth-sized planets. Scientists just found four more Earth-sized planets in the system.

Q: So?
A: Astronomers are really interested in finding planets that look like the Earth, for two reasons. First, we know there's life on Earth, so we're hoping that if we find other planets like Earth, we'll find life there. Second, maybe future humans can settle Earth-like planets and free up some space on Earth.

Stars like TRAPPIST-1 are about as small as a star can get, and they are also the most common type of stars in the Galaxy. However, TRAPPIST-1 is too faint for radial velocity surveys such as Paul Butler’s that observe at visible wavelengths. TRAPPIST-1's serendipitous alignment made this detection possible through the transit technique, where a planet crosses in front of its host star.

Q: So how Earth-like are these planets?
A: Well, they're roughly the same size as Earth. Based on what we know about exoplanets so far, the discoverers think that all seven are rocky, meaning they have a solid ground you can walk around on, like Earth, instead of being giant balls of gas, like Jupiter. Three of the planets are in the "Goldilocks zone," the region of space around the star that's not too hot, not too cold, and might be the right temperature to allow water to be a liquid on the surface of the planets.

Q: So they're just like Earth?
A: There are a few important differences. First, they're probably tidally locked with their star, meaning that their spin periods are related to their orbital periods. One possibility is a synchronous rotation, with a spin period equal to the orbital period, so that one side of such a planet faces the star at all times (like how one side of the Moon always faces the Earth). That means on one side of such a planet, it's always day, and on the other, it's always night. However, these planets could also be tidally locked into resonances where, e.g., the spin period is 1.5 times that of the orbital period, as is the case with Mercury. As a result, Mercury's surface is not perpetually divided into dark and bright hemispheres. Even higher order spin-orbit resonance is possible as well.

Q: What do the planets look like?
A: We don't know. All the pretty pictures you'll see of the planets are just artist's conceptions. The planets are so small and far away from us and close to their star that we can't take pictures of them, even as tiny dots of light. The only way we can tell they're there is that they pass in between us and their star, and we can see the starlight dim a tiny bit every time that happens. But even that can give us a lot of information (like the size of the planets and how far they are from their star).

Q: What's with the weird name?
A: Modern astronomers aren't great at naming things. We name stars after the telescope or survey that found them. TRAPPIST-1 was the first star observed by the TRAnsiting Planets and PlanetesImals Small Telescope (TRAPPIST). We name planets after their stars, starting with the letter "b", so the planets are named TRAPPIST-1b, TRAPPIST-1c, etc. Don't give me that look, I didn't come up with this system.

Q: Now that we have this system, should we stop looking for planets?
A: Absolutely not! TRAPPIST-1 is a very neat system, but also not the easiest to learn more about. Eventually, we want to be able to observe planets directly, i.e. separate the light of the star from the light reflected or emitted by the planet, to get spectra of the planet. TRAPPIST-1d and 1e, which are the most Earth-like in terms of temperature and size, will be impossible to image directly without a space telescope about 100 m in size. The biggest space telescope ever launched will the James Webb, scheduled to go up next year, and it is 6.5 m.

Written by Erika Nesvold, Sergio Dieterich & Alycia Weinberger // February 22, 2017

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