This is the first time astronomers have discovered so many terrestrial planets orbiting a single star, and this new system could be the best target in the galaxy to search for extraterrestrial life.
Even they aren't home to aliens, TRAPPIST-1 will provide planetary scientists with an unprecedented new window on the formation of solar systems and the behavior of rocky worlds.
So, sure, this is a really cool discovery. But if you're daydreaming about visiting TRAPPIST-1, where transits are a daily spectacle and the star's dim glow gives the appearance of a perpetual sunset, don't pack your bags just yet.
Here's why:
• TRAPPIST-1 is 39 light years away.
Yes, that is crazy close in the scheme of the universe. The Milky Way galaxy alone is 100,000 light years across.
But even if humanity had a spacecraft capable of moving at the speed of light, it would take almost four decades to get to TRAPPIST-1. I don't know about you, but I don't have that many vacation days.
• It's kind of a wimpy star.
TRAPPIST-1 is an ultracool dwarf star, 10 times smaller and 2.5 times cooler than our own sun. In fact, it's more comparable to Jupiter than to the sun.
Even though the TRAPPIST-1 planets are Earth-like, the system is definitely an alien one. It's not clear what the likelihood of life might be in such a system.
• How close is too close?
The planets of this system orbit in super tight circles around their sun. The entire system is barely bigger than the distance between the sun and Mercury. This is what lets them stay warm by their star's dim light, but it also puts them at risk.
Solar flares could damage their atmospheres (if they have atmospheres), radiation could blast away any nascent life (if life even emerged).
Michael Gillon, the lead author of the study, said that TRAPPIST-1 is a relatively quiet star, and the level of radiation the planets probably receive doesn't look totally hostile to life. But it's still something to worry about.
• The planets don't have days and nights.
Because the planets are close to the sun, and to one another, the astronomers believe that they are "tidally locked." This is what happens when the amount of time it takes a body to orbit matches the length of one rotation on its axis.
The result is that the same side of the body always faces the object it orbits around. The moon is tidally locked with Earth, which is why we always see the same face of our satellite when we look up at night.
For the planets of the TRAPPIST-1 system, this means that one side of each body is constantly blasted with their sun's heat, while the other sides are perpetually in darkness.
This . . . doesn't sound very homey. It could create huge temperature gradients that drive powerful winds. It could mean that half of each planet freezes while the other half burns.
• We can't see the planets directly.
Scientists were able to detect the planets using the "transiting method," in which they use tiny dips in light from the star caused by the planets passing across its face.
With the Hubble Space Telescope, the soon-to-launch James Webb Space Telescope, and ground-based observatories, astronomers can also analyze the light that passes through the atmospheres of the planets to figure out what molecules those atmospheres contain.
We won't be able to directly image the solar system with current technology, Gillon said. Because the planets are so close to their star, they will be impossible to distinguish amid its glare.
• Just because some of the planets are in the habitable zone doesn't mean that they're actually habitable, let alone inhabited.
The "habitable zone" is kind of a squishy concept. Astronomers define it as the range of orbits around a given star at which planets are warm enough to sustain liquid water on their surface. But it's all theoretical.
Scientists assume that a middling distance from the sun and liquid water make a planet habitable because they're so essential to life on Earth. But there are ways for bodies to hold water even if they're far away from their stars.
Just look at Europa, the moon of Jupiter that is thought to have a vast, salty ocean beneath its icy surface. Plus, lots of liquid water isn't necessarily a good thing.
In a paper published in the journal Science in 2013, astrophysicist Sara Seager pointed out that water is a greenhouse gas - too much of it too close to a star could trap heat on a planet and turn it into something like Venus.
Besides, a planet requires a lot more than water and light to be livable. Scientists think Mars once had water, but when its internal dynamo broke down it lost its atmosphere and became the frozen desert we know today.
Truly habitable planets probably need strong magnetic fields to protect their inhabitants from radiation and fierce solar winds.
Amy Barr Mlinar, a senior scientist at the Planetary Science Institute, pointed out that, in some ways, life is a requirement for life.
"Geology and life have worked together to make the Earth habitable," she said. Without photosynthetic organisms to create oxygen, fungi to recycle waste products, bacteria to fix nitrogen in the soil, our planet would be unrecognizable.
• Even with all these caveats, the planets are really, really exciting.
"There are other worlds out there just like the Earth that have some commonalities with the Earth and we can imagine them," NASA's Thomas Zurbuchen said at a news conference Wednesday.
"The question, 'Are we alone out here?' is being answered as we speak."
There are 100 billion stars in our galaxy, and it's thought that about 15 percent of them are ultracool dwarfs like TRAPPIST-1.
If even a fraction of those stars host multiple planet systems, and even a fraction of those have terrestrial planets in the habitable zone, there could potentially be millions of rocky worlds waiting for us to explore.
"The discovery gives us a hint that finding a second Earth is not just a matter of if, but when," Zurbuchen said.
But we don't need to wait to find other Earth-like exoplanets. The TRAPPIST-1 system has offered up seven such worlds, and they're right within our sights.
All scientists have to do now is point their telescopes at it and look.
