So far, two planets have been known to exist around our nearest neighboring star, Proxima Centauri. Now astronomers have tracked down a third satellite of this red dwarf, which is only a good four light-years away. The exoplanet, identified by the tiny wobbles of its star, has just over a quarter of the mass of Earth, making it the lightest exoplanet ever detected using this method. The planet, christened Proxima Centauri d, is the innermost of the now three planets in this nearby system and takes only around 5.1 days to orbit the star. It is orbiting just too far in for the star’s habitable zone.
The red dwarf Proxima Centauri is the closest stellar neighbor to our sun. At a distance of just over four light-years, this could be the first system to be visited by an interstellar spacecraft in the future. All the more exciting was the discovery that this nearby red dwarf also has two planets in addition to a dust disk. The first planet, Proxima Centauri b, was discovered in 2016 and orbits its parent star in the habitable zone – this super-Earth with 1.3 times Earth’s mass could therefore have a habitable climate. Then, in 2020, new observations confirmed the existence of a second planet orbiting further out. Proxima Centauri c is about seven times the mass of Earth and takes more than five years to complete one orbit. It is therefore too cold to be habitable.
Suspicious fluctuations
During the observations in 2020, astronomers around Proxima Centauri noticed another, albeit still unclear, signal. This became apparent when they took another look at the system with the new, high-resolution ESPRESSO spectrograph on the Very Large Telescope of the European Southern Observatory (ESO). This is particularly well suited to detecting the tiny shifts in the light spectrum caused by the gravitational effects of planets on the movement of their star. However, because the nearby red dwarf shows strong bursts of radiation, which also affect its rotation speed, it is difficult to distinguish such disturbances caused by stellar activity from real planetary signals. That is why a team led by João Faria from the Institute for Astrophysics and Space Research in Portugal observed Proxima Centauri more closely in 2021 with the ESPRESSO instrument. In total, they were able to evaluate 117 spectral measurements of the radial velocity that had been carried out between 2019 and 2021.
The analyzes of these measurement data showed that there is another noticeable fluctuation in addition to the signal from the planet Proxima b, which occurs with a period of around eleven days. This is repeated every 5.12 days. After matching stellar activity to show that this signal could not have come from mere bursts of radiation from the star, astronomers conclude that it must be a planet. “After receiving new observations, we were able to confirm this signal as a new planet candidate,” says Faria. Accordingly, our nearest neighboring star has another, third planet. The planet, named Proxima d, takes a good five days to complete one orbit and orbits the red dwarf at a distance of about four million kilometers, which is less than a tenth of the distance of Mercury from the sun. This moves the planet too far in for its star’s habitable zone. Astronomers estimate its mean surface temperature at around 86 degrees Celsius.
One of the lightest of its kind
“The discovery shows that our nearest stellar neighbor appears to be full of interesting new worlds that are within reach of further study and future exploration,” says Faria. The measurements show that the newly discovered planet probably weighs only around a quarter of the Earth’s mass. This makes it the lightest exoplanet around Proxima Centauri and also the lightest exoplanet ever detected using the radial velocity technique. The effect of Proxima d’s gravity is so small that it rocks its star back and forth at only about 40 centimeters per second. That it was possible to detect such a small variation is extremely significant, says co-author Pedro Figueira from ESO. “This shows that the radial velocity technique has the potential to discover light planets like our own, which are believed to be the most common in our galaxy and which may potentially host life as we know it.”
Exactly how large Proxima d is and whether the innermost planet of our neighboring star has an atmosphere cannot be determined from the measurement data obtained so far. However, the research team hopes that future observations will be able to see the small planet in transit – the passage in front of its star. From the planet’s dimming of the starlight and the change in the light spectrum, astronomers could then get more information about the new member of the planetary family around Proxima Centauri.
Source: João Faria (Instituto de Astrofísica e Ciências do Espaço, Porto) et al., Astronomy & Astrophysics, doi: 10.1051/0004-6361/202142337