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Proxima b, the closest exoplanet we know, may be even more Earth-like than we thought

This artist's impression shows what the surface of the alien planet Proxima b might look like.  (Image credit: ESO/M. Kornmesser)

The closest alien planet to our solar system is even more Earth-like than scientists had thought, new observations suggest.

In a new study, an international team of researchers found that the minimum possible mass for Proxima b, which lies just 4.2 light-years from Earth, is just 17% more massive than our planet. 

Previously, scientists thought that this exoplanet, which lies in the habitable zone of its star, harbored a minimum of about 1.3 Earth masses. The new measurement indicates that Proxima b could be even more like our home planet, at least in size, than previous observations led scientists to think. 

The research team studied Proxima b using the Echelle Spectrograph for Rocky Exoplanet and Stable Spectroscopic Observations, or ESPRESSO for short.  ESPRESSO is a Swiss spectrograph that is currently mounted on the European Southern Observatory's (ESO) Very Large Telescope in Chile. Spectrographs observe objects and split the light coming from those objects into the wavelengths that make it up so that researchers can study the object in closer detail. 

Related: Proxima b: Complete Coverage of the Exoplanet Discovery

Proxima b was first detected four years ago by an older spectrograph, HARPS ("High Accuracy Radial Velocity Planet Searcher"), which is installed on a scope at ESO's La Silla Observatory in Chile  But with these newer observations, scientists have an updated, ultra-precise view of the planet. 

"We were already very happy with the performance of HARPS, which has been responsible for discovering hundreds of exoplanets over the last 17 years," study co-author Francesco Pepe, an astronomy professor at the University of Geneva in Switzerland and the person in charge of ESPRESSO, said in a statement. "We're really pleased that ESPRESSO can produce even better measurements, and it's gratifying and [a] just reward for the teamwork lasting nearly 10 years."

"ESPRESSO has made it possible to measure the mass of the planet with a precision of over one-tenth of the mass of Earth," Michel Mayor, a Swiss astrophysicist who won the Nobel Prize for Physics in 2019 and helped to develop a new type of spectrograph called Elodie, who was not an author on this study, said in the same statement. "It's completely unheard of."

An alien planet 

So what's the deal with this Earth-sized planet? Proxima b is "one of the most interesting planets known in the solar neighborhood," Alejandro Suarez Mascareño, the lead author on this study, said in the same statement. 

This strange alien planet orbits Proxima Centauri, the closest star to our sun. Because the planet orbits right in the middle of its star's habitable zone, it's possible that liquid water — and potentially even life — could exist there. Because of its Earth-like mass, scientists believe that, not only could liquid water exist on Proxima b, it could also be a rocky, terrestrial planet similar to Earth. 

But Proxima b orbits around a star that, while close to our solar system, is also much dimmer, and much less massive than our sun. Researchers think that the exoplanet is tidally locked and in synchronous rotation with its star, meaning that one side is always facing the star and one is always facing away: a light side and a dark side. 

In addition, it's unclear if, Proxima b has an atmosphere. The planet lies very close to its star, completing one orbit every 11 Earth days. So, some researchers think that radiation coming from Proxima Centauri might have stripped away Proxima b's air, making it impossible for the alien planet's surface to hold onto liquid water. As scientists continue to study this system with new and better technology, we will be able to better understand what it's really like on Proxima b. 

This new study was published May 26 to the preprint server arXiv and accepted to the journal Astronomy & Astrophysics. 

Editor's Note: A previous version of this article stated that researchers had pinpointed Proxima b's mass. Instead, they changed the minimum possible mass for the alien planet. 

Email Chelsea Gohd at or follow her on Twitter @chelsea_gohd. Follow us on Twitter @Spacedotcom and on Facebook.

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  • rod
    FYI. Proxima b has been in the news a number of times. Here is a statement indicating the host star likely produces 400x more flares on the planet than our Sun does on Earth. ESPRESSO confirms the presence of an Earth around the nearest star, "Although Proxima b is about 20 times closer to its star than the Earth is to the Sun, it receives comparable energy, so that its surface temperature could mean that water (if there is any) is in liquid form in places and might, therefore, harbour life. Having said that, although Proxima b is an ideal candidate for biomarker research, there is still a long way to go before we can suggest that life has been able to develop on its surface. In fact, the Proxima star is an active red dwarf that bombards its planet with X rays, receiving about 400 times more than the Earth. "Is there an atmosphere that protects the planet from these deadly rays?" says Christophe Lovis, a researcher in UNIGE's Astronomy Department and responsible for ESPRESSO's scientific performance and data processing. "And if this atmosphere exists, does it contain the chemical elements that promote the development of life (oxygen, for example)? How long have these favourable conditions existed? We're going to tackle all these questions, especially with the help of future instruments like the RISTRETTO spectrometer, which we're going to build specially to detect the light emitted by Proxima b, and HIRES, which will be installed on the future ELT 39 m giant telescope that the European Southern Observatory (ESO) is building in Chile." shows the surface temperature is 216K, -57C. Very cold :)
  • ChrisA
    Oxygen in the atmosphere is NOT needed to promote life. The Earth did not have oxygen in its atmosphere until about 1.5 billion years AFTER life got started. Life did just fine with a CO2 atmosphere for over a billion years.

    In fact, biologists refer to the development of plants as "The Oxygen Catastrophe" These new organisms called "plants gave off a poison gas call oxygen that killed most (yes most) existing life. We descended from those few who could tolerate this new poison. Later animals evolved to make use of this very reactive gas but O2 came later and life had to adapt.

    A planet does NOT need a protective atmosphere for life. Life can live 100 feet underground in aquafers ad use chemical energy. It wil not be as energetic are we are but still life.

    My guess is that almost all life in the universe is anaerobic bacteria that lives underground.
  • rod
    FYI. This exoplanet site shows Proxima b surface temperature may be 234 K or -39C,
  • rod
    Oops, here is the link,
    Today we have two very good exoplanet sites that I use and review regularly. and shows 4266 confirmed exoplanets, exoplanetarchive site shows 4158. None are reported as showing life of any kind exist on them, surface, in the atmosphere if they have an atmosphere, or below the surface. None are listed as confirmed with life presently. Based upon the scientific method, the Earth has life and a fossil record. The experiments of Louis Pasteur show the fossil record is a record of biogenesis at work throughout Earth history (Precambrian, Cambrian explosion, Cenozoic) that also features massive death and extinction too. The plant record in the fossil record is a good example of biogenesis at work and massive burial processes at work and death too.
  • In my opinion with these planets being so close to their host star, They are All Tidally locked like our moon one side facing the star all the time. With this being the case and as we know the Eartg creates its Much Needed Van Allen Belt of protection from Sun by Rotating with an Iron core that is Molten. Without spin no Electromagnetism and Planet gets destroyed by Radiation Everytime.
  • Lovethrust
    scott said:
    In my opinion with these planets being so close to their host star, They are All Tidally locked like our moon one side facing the star all the time. With this being the case and as we know the Eartg creates its Much Needed Van Allen Belt of protection from Sun by Rotating with an Iron core that is Molten. Without spin no Electromagnetism and Planet gets destroyed by Radiation Everytime.
    I take it you are talking about life and not the planet itself. Even if true (which though seemingly probable is by no means certain) this would still not rule out oceans full of life or land life adapted to extreme radiation.