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Closest exoplanet Proxima b remains mysterious, in the absence of direct observation

Astronomers have been observing Proxima Centauri, the closest star to our own, essentially non-stop ever since a small rocky exoplanet was found in orbit several months ago. We didn’t know much about the planet, called simply Proxima b. But the hope was that it would “transit” in front of its host star, allowing us to get a better look at it. However, new observations indicate that’s not likely to happen. The best we can do right now is make some educated guesses about conditions on Proxima b.

Proxima b is so interesting for scientists because it’s literally the closest potentially Earth-like planet, at just over four light years away. The planet orbits its star at a distance of just 4.6 million miles, which is only 10% the distance from Mercury to the sun. However, Proxima Centauri is a much smaller, cooler red dwarf. That means Proxima b is in the range where liquid water could exist on the surface. Of course, the actual existence of water on Proxima b depends on a great many factors. In our own solar system, Earth, Mars, and Venus are all in the habitable zone (sometimes called the Goldilocks zone), but only Earth is conducive to life as we know it.

If we could watch Proxima b pass in front of its star, we’d be able to pin down its size (and thus density) and atmospheric composition. For that to happen, we need to be looking at Proxima Centauri’s solar plane almost edge-on, and scientists estimated the likelihood of that at around 1.5%. David Kipping at Columbia University spent about 40 days in 2014 and 2015 observing Proxima Centauri. After the announcement of a planet, he and his team went back and combed through that data in search of a transit. Combined with newer observations, there’s no evidence that Proxima b passes between its star and Earth.

That leaves us with much less to go on as we attempt to characterize this exoplanet. According to researchers at France’s CNRS, there’s at least a possibility that Proxima b has liquid oceans on the surface. They simulated Proxima b based on what we know of its mass and distance from Proxima Centauri. If its composition is like other rocky planets, at the smaller end of plausible diameters, it could have water coverage similar to Earth. If it’s much larger and less dense, there could be a global 200km deep ocean. Of course, nothing precludes it from having toxic clouds of sulfuric acid like Venus.

There are still a lot of unanswered questions about Proxima b, but hopefully future instruments will be able to answer some of them. In particular, the Webb Telescope could help astronomers zero in on Proxima b, even if it doesn’t transit Proxima Centauri.

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