Not just another exoplanet: why the discovery of Proxima d is so exciting

The discovery of exoplanets – planets orbiting a star other than our sun – is not news. Researchers have discovered several thousand in our Milky Way galaxy. But finding an exoplanet close to home is still something special. Proxima D is one of those. Its discovery means that Proxima Centauri, the closest star to our sun, has its own planetary system. Exciting, because it would be the only system we have little chance of getting into.

At the moment, one of the best ways to get information about our neighboring planets is a very sophisticated telescope instrument in Chile. And this tool — the Echelle Spectrograph for Rocky Exoplanet and Stable Spectroscopic Observations, or ESPRESSO — recently gave astronomers a big boost: a potential new planet in our backyard. After all, by cosmic standards, Proxima Centauri is right next door. At 4.2 light-years away, it is the closest star to our sun, making the planets around Proxima the closest planets in the universe.

Of course we can’t even visit this close, at least not with current technology. 40 trillion (40,000,000,000,000,000,000) kilometers is nothing. But if people embark on an interstellar journey, Proxima Centauri is where they will go. Researchers are already thinking deeply about what it would take to get to the cosmic neighborhood where the star resides, starting with tiny robots that can fly faster than conventional spacecraft and can cut the journey from thousands of years to just decades.

We can’t see them, but we know they’re there

If the data is correct, the discovery means that Proxima Centauri has its own planetary system, in which worlds are fermented from the remnants of its star’s gas and dust, just like our planets. In 2016, astronomers found Proxima b, an Earth-sized planet orbiting in the star’s habitable zone, and have since confirmed its existence. They’re still working on confirming Proxima c, the largest planet they discovered in 2019. Now, for now, they’re excited about this new candidate planet, which they’re calling Proxima d, which seems like a better possibility than Proxima c.

Astronomers can’t actually see any of the Proxima planets, but they can detect signs of their presence by observing the movements of their stars. As the planets rotate, their gravity pulls on Proxima Centauri, causing the star to shake slightly. João Faria, an astrophysicist at the Instituto de Astrofísica e Ciências do Espaço, in Portugal, and his colleagues studied Proxima Centauri in this way, trying to look at Proxima b to confirm its existence, when they found a sudden wobble they thought came from another, smaller planet.

On the one hand, it’s always day and on the other, it’s always night

According to the data, Proxima d has a mass of about a quarter of the mass of Earth and orbits outside the habitable zone of Proxima Centauri, locked in a special formation around this star. On one side of the planet it is always day and on the other side it is always night. Proxima d takes only five days to orbit its star, and given its proximity, the planet likely has no atmosphere. It will be boiled away in the day and frozen at night.

It is now up to other astronomers to analyze the available ESPRESSO data and also make their own observations of the possible planet. To learn more about this potential planet, and especially its composition, astronomers must watch Proxima d transit its star and examine how much starlight the planet blocks. It is very likely that there are more planets orbiting Proxima Centauri that are currently evading detection.

Alpha Centauri A & B is more attractive

Proxima Centauri is actually part of a triple galaxy known as Alpha Centauri. The orbits of Alpha Centauri A and B are close to each other as Proxima orbits farther away. Astronomers have also tried to discover planets around stars A and B, but so far they have not been successful. Alpha Centauri A and B are particularly attractive in the search for extraterrestrial life because they are similar to our Sun. On the other hand, Proxima is a red dwarf star – smaller, fainter and prone to frequent solar flares that emit radiation. Red dwarfs sanitize the surfaces of the planets around them, so this is bad news for the habitability of those planets.

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