At just over four light-years away, the low-mass star Proxima Centauri is practically next door to us, cosmically speaking. It is known to host two exoplanets, but recent research using the European Southern Observatory’s Very Large Telescope (ESO’s VLT) has shown that these two planets may have a baby brother, in the form of one of the lightest exoplanets ever found.
The newly discovered planet, called Proxima d, orbits extremely close to its star at just 2.5 million miles away — less than one-tenth of the distance between Mercury and the sun. It is so close that it takes just five days to complete an orbit, meaning it is too close to be in the habitable zone (where liquid water could be present on its surface).
The planet is just a quarter of Earth’s mass, making it extremely light by exoplanet standards. “The discovery shows that our closest stellar neighbor seems to be packed with interesting new worlds, within reach of further study and future exploration,” said lead author of the study João Faria in a statement.
The tiny mass of the exoplanet made it hard to spot, so after initial observations with the ESO’s 3.6-meter telescope, the researchers turned to the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) instrument on the VLT. “After obtaining new observations, we were able to confirm this signal as a new planet candidate,” Faria says. “I was excited by the challenge of detecting such a small signal and, by doing so, discovering an exoplanet so close to Earth.”
Many exoplanets are discovered using the transit method, in which astronomers look for small dips in the brightness of a star caused when a planet passes between the star and us. But this exoplanet was discovered using a different method called the radial velocity technique, looking for tiny wobbles in the movements of a star caused by the gravity of a passing planet. Because the gravitational wobble caused by a light planet like Proxima d is so small, traditionally the radial velocity method has been used primarily to search for larger planets.
“This achievement is extremely important,” said Pedro Figueira, ESPRESSO instrument scientist at ESO in Chile. “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.”
The research is published in the journal Astronomy & Astrophysics.