Proxima d: a tiny exoplanet neighbor next door
Today we publish the detection of a new planetary neighbor. In fact, in this case, it is the neighbor that we have door to door. We have found with a high probability the existence of a new planet in the Proxima Centauri planetary system.
Credit: ESO/L. Calçada
Proxima Centauri is the smallest star of the three that make up the Alpha Centauri triple system. This means that in the closest stellar neighbor to the Sun, not one but three stars reside. Two of them (Alpha Centauri A and B) are similar to our star while the third, Proxima Centauri, is a much smaller star, with a size and mass around 10% of that of the Sun. The three stars dance in the cosmos orbiting each other in an eternal waltz.
In Proxima Centauri, the detection of a planet in its habitable zone (known as Proxima b) was published in 2016 in a study led by Guillem Anglada-Escudé, which represented a great technological challenge. It turned out that the closest star to our Solar System also harbored a rocky Earth-like planet in its habitable zone. The implications of this discovery are very relevant at the astrobiological and philosophical level. The star is “only” 4 light-years away (about 36 billion kilometers, that is “36” followed by 12 zeros: 36,000,000,000,000 km!), so there could be the possibility of being able to visit (although in a very, very distant future) an extrasolar planet, and also with the capacity to harbor liquid water on its surface.
This planet was confirmed independently by us using the ESPRESSO instrument (located in the Atacama desert, Chile) in a work led by Alejandro Suárez-Mascareño, from the Canary Islands Institute of Astrophysics (IAC). At the same time, the detection of a second planet, Proxima c, further away from the star and with an orbital period of more than 5 years, was published. Although in this case the planet could not yet be confirmed and even today it remains a planetary candidate.
What we present today in the journal Astronomy & Astrophysics, led by João Faria (researcher at the Institute of Astrophysics and Space Sciences in Portugal) is the detection of a third planet in our stellar neighbor: Proxima d. This planet is also one of the lightest detected to date with only 25% of the mass of Earth, about three times the mass of Mars. The detection has only been possible thanks to the intense observation of the star, again with the ESPRESSO instrument of the European Southern Observatory. Thanks to this instrument, we have been able to measure a subtle wobble of the star Proxima Centauri of only 39 centimeters per second. That is, we are seeing that this star moves in tiny circles at the speed of a turtle.
This has been a true technological challenge, since the signals from the other two planets and the star’s own noise due to its magnetic activity are superimposed on this signal. But the ESPRESSO instrument is able to reach these precisions thanks to unprecedented thermal stability. This spectrograph is located in a completely isolated chamber in which the temperature does not vary by more than one thousandth of a degree. Thanks to this we can achieve precision of up to 10 centimeters per second, opening the door to the detection of planetary systems similar to ours. The detection of “Proxima d” tells us that the closest star to ours is not alone but hosts another planetary system that also appears to be a small scale of our Solar System. One more example that planets abound in the Universe and that technological progress allows us to delve deeper into the search for these small worlds, which are most likely the most abundant in the cosmos.