Second Proxima Centauri planet confirmed in 25-year-old Hubble data

The starry sky above ESO’s La Sill Observatory showing the double star Alpha Centauri AB and Proxima Centauri, the closest star to the Sun. Proxima Centauri is now known to host two exoplanets. Image: Y. Beletsky (LCO)/ESO/ESA/NASA/M. Zamani

The red dwarf Proxima Centauri, just 4.2 lights years away, is the Sun’s nearest stellar neighbour. It was discovered in 1915 and 101 years later, astronomers at MacDonald Observatory found a planet orbiting the star every 11.2 days at a distance of about 7.5 million kilometres (4.5 million miles).

Earlier this year, a team led by Maria Damasso of Italy’s National Institute for Astrophysics (INAF), announced they might have found a second planet orbiting much farther out, a tentative conclusion based on Proxima Centauri’s motion as the star is tugged back and forth by the gravity of one, and possibly two, exoplanets.

The data suggested the planet, dubbed Proxima Centauri c, orbits the red dwarf every 1,907 days in an orbit about 1.5 times larger than Earth’s orbit around the Sun.

Now, University of Texas astronomer Fritz Benedict and research partner Barbara MacArthur, using astrometry data he collected from the Hubble Space Telescope 25 years ago, have confirmed the presence of Proxima Centauri c.

He then combined the Hubble data with more recent observations by Raffaele Grafton of INAF using the SPHERE instrument with the European Southern Observatory’s Very Large Telescope to refine the mass of the newly confirmed world: about seven times the mass of Earth.

“Basically, this is a story of how old data can be very useful when you get new information,” Benedict said. “It’s also a story of how hard it is to retire if you’re an astronomer, because this is fun stuff to do!”