Proxima Centauri has a third planet

So far it is known that there are two planets around our nearest neighbor star, Proxima Centauri. Now, astronomers have tracked down this red dwarf’s third satellite, which is only a good four light-years away. The exoplanet, identified by the tiny wobble of its star, has just over a quarter of the mass of the Earth, making it the lightest exoplanet ever detected using this method. The planet, christened Proxima Centauri d, is the innermost of the three presently planets in this nearby system, and it only takes about 5.1 days to orbit the star. It orbits too far for a zone to be inhabited by a star.

The red dwarf Proxima Centauri is the closest stellar neighbor of our Sun. At just over four light-years away, this could be the first system to visit an interstellar spacecraft in the future. The discovery that this nearby red dwarf had two planets in addition to the dust disk was all the more exciting. The first planet, Proxima Centauri b, was discovered in 2016 and orbits its parent star in a habitable zone – this 1.3 Earth-mass super-Earth may therefore have a friendly climate. Then, in 2020, new observations confirmed the existence of a second planet orbiting further. Proxima Centauri c is about seven times the mass of Earth, and it takes over five years to complete one orbit. Therefore it is too cold to be habitable.

Suspicious fluctuations

During their 2020 observations, astronomers around Proxima Centauri noticed another, though still unclear, signal. This became apparent when they re-examined the system with the new high-resolution ESPRESSO spectrograph on the European Southern Observatory’s (ESO) Very Large Telescope. It is especially well suited to detecting slight shifts in the light spectrum caused by the gravitational effects of planets on the motion of their stars. However, since a nearby red dwarf shows strong flares of radiation that also affect its rotation speed, it is difficult to distinguish such disturbances caused by star activity from actual planetary signals. Therefore, a team led by João Faria from the Institute of Astrophysics and Space Research in Portugal observed Proxima Centauri in more detail in 2021 with the ESPRESSO instrument. In total, they were able to evaluate 117 spectral radial velocity measurements carried out in 2019-2021.

Analyzes of these measurement data showed that, in addition to the signal from the planet Proxima b, which occurs with a period of about eleven days, there is another noticeable fluctuation. This is repeated every 5.12 days. After matching the activities of the stars to show that the signal could not have come from ordinary flares of the star’s radiation, astronomers concluded that it must be a planet. “After receiving the new observations, we were able to confirm this signal as a candidate for a new planet,” says Faria. Accordingly, our nearest neighboring star has a different, third planet. The planet, dubbed Proxima d, takes a good five days to complete one orbit and orbits the red dwarf at a distance of about four million kilometers, or less than a tenth of the distance of Mercury from the Sun. This places a planet too far away in the habitable zone of its star. Astronomers estimate its average surface temperature to be around 86 degrees Celsius.

One of the lightest of its kind

“The discovery shows that our closest stellar neighbor appears to be full of interesting new worlds that are within the scope of further study and exploration,” says Faria. Measurements show that the newly discovered planet probably weighs only about a quarter of the mass of the Earth. This makes it the lightest exoplanet around Proxima Centauri and also the lightest exoplanet ever detected using the radial velocity technique. The influence of Proxima d’s gravity is so slight that it rocks its star back and forth at about 40 centimeters per second. That such little variability was detected is extremely important, says co-author Pedro Figueira of ESO. “This shows that the radial velocity technique has the potential to discover light planets like ours, which are considered the most common in our galaxy and which can potentially lead life as we know it.”

What exactly is Proxima di or 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 allow us to see a small planet in transit – the passage ahead of its star. Based on the star’s fading light and the change in the light spectrum, astronomers could learn more about a new member of the planet 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

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