On Aug. 24 scientists announced the discovery of the closest Earth-sized exoplanet ever found.
There's a lot of reasons to get excited about this new planet called Proxima b. It's orbiting around a star called Proxima Centauri just four light-years away from Earth, making Proxima b our next-door neighbor. It's also roughly Earth-sized and orbiting in the habitable zone around its star — the sweet spot that might allow liquid water to exist on its surface.
Those characteristics alone mean Proxima b could be habitable.
But there's also a ton that we don't know about this world. Even though it's our neighbor on a cosmic scale, it's mind-bogglingly far away on a human scale, and that makes it hard to study.
Here's what we still need to figure out before we can tell if Proxima b is really habitable:
Is the surface protected from stellar radiation?
Proxima Centauri is a red dwarf star a fraction of the size of the sun, but it spits out about the same amount of X-rays, according to National Geographic.
The problem with that is Proxima b orbits just 7.5 million kilometers away from Proxima Centauri. For reference, our solar system's innermost planet Mercury never gets closer than about 46 million kilometers from the sun.
That means Proxima b has a front row seat to dangerous bursts of stellar radiation.
We have no idea if Proxima b has any protective layer like a magnetic field that is strong enough to shield it from all that radiation.
What is its atmosphere made of?
Scientists first spotted the planet because its gravitational pull was tugging ever so slightly on Proxima Centauri and causing the star to wobble almost imperceptibly.
Now scientists are trying to catch the shadow of the backlit planet as it crosses in front of Proxima Centauri.
When scientists catch a planet transiting across a star like this, the outline of the planet's atmosphere is illuminated and they can study its chemical composition.
Unfortunately there's a very low chance that Proxima b is lined up in a way that would allow us to see it transit across Proxima Centauri from Earth, National Geographic reported. So for now, we don't have a good way to study the planet's atmosphere.
What is a day on Proxima b like?
There's strong evidence that Proxima b is actually tidally locked, meaning one side is always facing Proxima Centauri and one is always facing away. That means one side of the planet is in perpetual daylight, and the other in perpetual night. Any life there would have to have adapted to those conditions.
Even if Proxima b isn't habitable, it might have a sibling that is.
During their observations, the researchers detected a stray signal that wasn't caused by Proxima b. It could be a sign of another larger planet orbiting the star. It's possible the stray signal will disappear in follow-up observations, but a Proxima b sibling "cannot yet be ruled out," according to the study.
Ultimately, we'll probably have to visit Proxima b to determine whether it's habitable or not.
The research program Starshot is already working on a way to launch a space probe to Proxima Centauri within the next generation. Interstellar exploration could be closer than we think, and Proxima b will probably be our first stop.