Shifts in starlight from Proxima Centauri reveal its third planet — a world orbiting within the star’s habitable zone.
By Davide Castelvecchi
Astronomers have discovered a third planet orbiting Proxima Centauri, the star closest to the Sun. Dubbed Proxima Centauri d, the newly spotted world is probably a bit smaller than Earth, and well within the habitable zone of its host star — meaning that it could have oceans of liquid water that can potentially harbour life.
“It’s showing that the nearest star probably has a very rich planetary system,” says Guillem Anglada-Escudé, an astronomer at the Institute for Space Sciences in Barcelona, Spain, who led the team that in 2016 discovered the first planet to be seen orbiting Proxima Centauri.
Astronomer João Faria and his collaborators detected Proxima Centauri d by measuring tiny shifts in the spectrum of the star’s light as the planet’s gravity pulled it during orbit. The team used a state-of-the art spectrograph called ESPRESSO at the Very Large Telescope, a system of four 8.2-metre telescopes at the European Southern Observatory in Cerro Paranal, Chile. The results were published on 10 February in Astronomy & Astrophysics1.
This ‘wobble’ technique looks for changes in the star’s motion along the line of sight from Earth, and ESPRESSO can detect variations of just 10 centimeters per second. The total effect of the planet’s orbit, which lasts only five days, is about 40 cm/s, says Faria. “I knew that ESPRESSO could do this, but it was still surprised to see it showing up,” says Faria, who is at the Institute of Astrophysics and Space Sciences in Porto, Portugal.
To find the wobble, the team made more than 100 observations of Proxima Centauri’s spectrum over two years. ESPRESSO is kept in a special room at the observatory, inside a tank that keeps its pressure and temperature constant. This ensures that observations are consistent and repeatable over years. ESPRESSO can measure the wavelength of spectral lines with a precision of 10–5 angstrom, or one-ten-thousandth the diameter of a hydrogen atom, Faria says.
Provisional planet
So far, researchers consider Proxima Centauri d only a ‘planet candidate’: astronomers traditionally wait for independent confirmation before officially inducting a new world into their catalogues. But the ESPRESSO team has high confidence in its detection, says Faria. From its effects on the star’s spectrum, the team estimates that the planet is probably smaller than Earth, but no less than 26% of its mass.
ESPRESSO was built mainly to search for extrasolar planets, as well as to study light from extremely bright but distant objects called quasars. The most exciting thing about the latest discovery is that it shows the instrument works as advertised, says Anglada-Escudé. “ESPRESSO is the new instrument which everyone wants to use and play with,” he says.
Proxima Centauri has a special place in astronomers’ hearts, Anglada-Escudé adds. “It always has a little bit of mystique, being the closest one.”
“It is fascinating to know that our Sun’s nearest stellar neighbour is the host to three small planets,” says Elisa Quintana, an astrophysicist at NASA’s Goddard Space Flight Center…
