One of the Lightest Planets Ever Discovered Could Help Us Find More Habitable Worlds

Astronomers have discovered one of the lightest planets ever found and it's orbiting the star right next door to our own, just over four light-years away from Earth.

Confirmation of Proxima d is evidence that a sensitive exoplanet detection method that measures the "wobble" a planet causes around its star can spot smaller planets like Earth that are potentially habitable.

With a mass a quarter of that of Earth, the planet is one of the lightest exoplanets ever discovered. The extrasolar planet, or exoplanet, orbits Proxima Centauri at a distance of 2.5 million miles, 10 times closer to its star than the closest planet to the sun, Mercury. This proximity means that the planet takes just five Earth days to orbit its host star, a red dwarf that is smaller and cooler than the sun.

Researcher at the Institute of Astrophysics and Space Sciences, Portugal, and lead author of a study published in the journal Astronomy & Astrophysics João Faria told Newsweek: "This detection strengthens our belief that low-mass planets, like Earth that may be habitable, are quite common in our galaxy. It also proves that we now have the technical capacity to find them using state-of-the-art instruments.

"At this point, we cannot say much about the atmosphere of the planet or even if it has an atmosphere. But from the measurement of the very low mass, we can conclude that this planet likely has a rocky composition, like the Earth and Mars."

Proxima d isn't alone in orbiting Proxima Centauri, this is the third planet discovered orbiting the closest star to the sun. This new exoplanet is too close to its star to be in the habitable zone—defined as the region around a star where water can exist as a liquid.

Of the other two planets around Proxima Centauri, Proxima b a planet with a mass similar to that of Earth, which orbits the stars every 11 days, is within this habitable zone, also sometimes called the "Goldilocks zone" because conditions here are "not too hot, not too cold, but just right" for liquid water, a vital ingredient for life.

Further out from the star is Proxima c, classed a super-Earth, with a mass seven times that of our planet, which takes five Earth years to complete an orbit of Proxima c.

Proxima b was discovered in 2016 following observations with the HARPS (High Accuracy Radial velocity Planet Searcher) spectrograph on the ESO 3.6-meter telescope at La Silla in Chile. Confirmation delivered in 2020 by the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) instrument that is part of the Very Large Telescope (VLT) located in the Atacama Desert region of Northern Chile.

It was at this time during this investigation that astronomers also picked up a weak signal that seemed to correspond to a planet with a five-day orbit of Proxima Centauri, one of three stars in the Alpha Centauri system.

Follow-up investigations with ESPRESSO confirmed the signal was caused by a tiny planet and not the result of changes in the red dwarf itself.

Though small in mass, this is a big breakthrough for ESPRESSO and the radial velocity method that it uses to detect exoplanets, as Proxima d is the lightest planet ever spotted with this technique.

The radial velocity method uses small "wobbles" in a star's motion caused by the gravitational influence of planets that orbit it.

Faria explained to Newsweek: "The radial velocity technique is an indirect planet detection method which measures the gravitational pull of the planets on their host star. We observe the star itself, not the planets, and see it moving back and forth while the planet completes one orbit.

"From this periodic variation in the star's velocity (hence the name), we can measure the orbital period, or long it takes for the planet to complete one orbit, as well as the mass of the planet."

These perturbations are usually extremely small but with a planet as light as Proxima d, the effect is so small that the planet only causes Proxima Centauri to move back and forth at about 40 centimeters (around 15 inches) per second.

ESPRESSO instrument scientist, Pedro Figueira, explained: "This achievement is extremely important. It shows that the radial velocity technique has the potential to unveil a population of light planets, like our own, that are expected to be the most abundant in our galaxy and that can potentially host life as we know it."

In order to learn more about Proxima d and its fellow exoplanets, the researchers will aim to observe the planet as it crosses in front of its host star.

University of Geneva Department of Astronomy researcher Baptiste Lavie explained: "We need to know if these planets hold an atmosphere. So we need to see if we can observe their transits.

"If they don't transit, which is possible, we are already developing new techniques to detect an eventual atmosphere. For example, the observatory of Geneva is building an instrument called RISTRETTO that will allow us to detect the light reflected by Proxima b, which will tell us about the presence or not of an atmosphere."

Proxima d
An artist's impression of Proxima d. The newly discovered exoplanet four light-years from Earth is one of the lightest exoplanets ever discovered. L. Calçada/ESO