Exoplanet Excitement, but still no Planet B.
Here is what we know: NASA announced on Wednesday the discovery of not one, not two, but SEVEN earth-sized exoplanets orbiting a small star within our galaxy. (An exoplanet is any planet outside of our solar system.) We know how far from their star they are; we know how big they are; and we know their masses. We do not know anything else, but what we know is plenty to be excited by.
The star, Trappist-1, is what we call a Red Dwarf. It’s thousands of times dimmer than our sun: so dim that even though it is only 40 light years away, it is not visible either to the naked eye or to amateur telescopes. (Betelgeuse, the red giant on the shoulder of Orion, is 642.5 light years distant, and you can see it any clear night until summer.) It is, however, much brighter in the infrared part of the spectrum (the longer-wavelength light we associate with a feeling of heat), and so the observations were made with the Spitzer Space Telescope, which sees in Infrared.
Even a space telescope cannot see Earth-sized exoplanets at 40 light years out, but we can still detect them. Spitzer watched Trappist-1 over a period of months, carefully measuring its light output. Whenever a exoplanet passed in front of the star, it cast a tiny shadow onto the telescope, blocking some of the star’s light. That is called a transit. Detecting exoplanets this way is called the Transit Method, and it has been used to great effect by the Kepler Space Telescope. The amount of light blocked during each transit let scientists figure out how big the exoplanet was; the timing of the transits told us how far each exoplanet was from its sun, because gravity makes exoplanets orbit faster the closer they are to their star. (You can explore this and the transit method with exhibits in the Space Place at Science North!)
From their shadows, we know these seven are all rocky exoplanets, close in size to Earth. The biggest is only 13% larger in diameter; the smallest just over three quarters our size. Their masses are Earth-like as well, so we can guess they’re made of the same stuff: rock, and, just possibly, water. Yes, water.
As it turns out, these exoplanets are very close indeed: the entire solar system could fit inside the orbit of Mercury. Creatively named in order of their distance from Trappist-1 b, c, d, e, f, g, and h, the furthest out, Trappist-1h (h to its friends), is 5 times as close to Trappist-1 as Mercury is to our sun. Remember, though, that Trappist-1 is 2000 times dimmer than the sun. Snuggled up close to that dim star, b and c are too hot for liquid water to exist on their surfaces. Even 5x closer than Mercury is to the Sun, h is too cold. Trappist-1d, e and f, though? They’re in the goldilocks zone. Not too hot, not too cold: juuuuuust right for liquid water… or life.
These exoplanets are so close together that each one would appear as large (or larger) in each other’s skies as the moon does to us. Imagine, standing on Trappist-1e, looking up and seeing whole other worlds- with mountains, continents, and maybe even the lights of an alien city on its surface. Just imagine. It would be quite a sight, wouldn’t it?
It would be, and it’s cool to imagine, but the odds are against it. All 7 exoplanets are very likely tidally locked to their star, which means one side will always face Trappist-1 and be baked under an endless noon, while the other side freezes in eternal night. For life to exist, it could only hold on in the tiny, twilight band in between, with howling, hurricane force winds constantly blowing from the day side to the night. It still sounds fascinating, but not terribly hospitable.
Even worse, as close as they are to the star, powerful solar flares could have long ago stripped any atmosphere (and any water) these exoplanets may have once had. Red dwarf stars are very unstable, and while Trappist-1 has calmed down in its middle age, it was probably once an active ‘flare star’, and could have easily fried its exoplanets in the early days. Without incredibly strong magnetic fields, even the regular solar (or rather, stellar) wind could strip a exoplanet’s air and water at such close distances. Of course, maybe not. One of the three potentially-habitable exoplanets could have gotten lottery-win lucky, possessing a strong magnetic field and missing the worst of Trappist-1’s early flares.
We will find out, and fairly soon. While current telescopes (even the Hubble) aren’t quite up to the task of analyzing the atmosphere of an earth-sized exoplanet, they will shortly begin to give us hints of what the exoplanets might be like. We can be certain that when the next-generation of telescopes — including NASA’s James Webb Space Telescope, the successor to the Hubble- become available, Trappist-1 and its fascinating, tiny solar system will top the list of objects to explore. We know enough, however, to say with confidence that none of these seven is going to be a second earth.
As interesting as the Trappist system is, there’s still no place like home.