The second exoplanet recently detected orbiting the closest neighboring star to our solar system–Proxima Centauri–appears to be surrounded by a series of rings, just as planet Saturn in our own solar system.
In 2016, astronomers working at the European Southern Observatory (ESO) confirmed the existence of a rocky planet around the nearest stellar neighbor: Proxima Centauri. The discovery of this exoplanet (Proxima b) was momentous for many reasons, including the fact that it was similar to Earth in size and its orbit was within the habitable zone, which means that there could be liquid water on its surface, and life as we know it (in this case alien life) could have developed on the planet. Earlier this year, using the radial velocity method, the Instituto Nazionale di Astrofisica (INAF) in Italy found a second exoplanet (Proxima c) orbiting the same star. Now, based on the distance between the two planets, another team led by INAF attempted to observe this new planet using the Direct Image Method. And while they were not entirely successful, their observations raise the possibility that this planet has a ring system around it.
The new study, which recently appeared in the journal Astronomy & Astrophysics, saw scientists rely on data obtained by the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument on ESO’s Very Large Telescope (VLT). This extreme adaptive optics and coronagraphic system is dedicated to the characterization of exoplanet systems at optical and near-infrared wavelengths.
For years, SPHERE has been revealing the existence of protoplanetary disks around distant stars, something that is extremely difficult to do with conventional optics. However, this particular set of data was collected during the SpHere INfrared survey for Exoplanets (SHINE), where the instrument was used to image 600 nearby stars in the near-infrared spectrum.
The data obtained by scientists was unprecedented.
Relying on SPHERE’s high contrast and high angular resolution, the purpose of this survey was to identify new planetary systems and explore how they were created. One such system was Proxima Centauri, a low-mass M-type (red dwarf) star located just 4.25 light-years from Earth. At the time of the survey, the existence of Proxima c was not yet known. Like Proxima b, Proxima c was discovered using the Radial Velocity method (also known as Doppler Spectroscopy). This consists of measuring the movement of a star from one side to the other (or “wobble”) to determine if the gravitational influence of a system of planets acts on it. However, the team was confident that if Proxima c produced a large enough infrared signal, SPHERE would have detected it.
Unfortunately, the SPHERE data did not reveal any clear indication of Proxima c. What they found was a candidate signal that had strong noise and an orbital plane orientation that fit well with an earlier image taken with the Atacama Large Millimeter / submillimeter Array (ALMA). They also noted that their orbital position and movement were not consistent with what was observed by ESA’s Gaia mission.
Finally, they discovered that the candidate had an unexpectedly high apparent brightness (flux) for a planet orbiting a red dwarf star. Because of this, the team was unable to say for sure if what they observed was really Proxima c.
However, this last element raised another possibility that the team had to consider, that the unusual shine may be the result of a circumplanetary material. In other words, they theorize that the brightness could be caused by a ring system around Proxima c, which would be radiating additional light in the infrared spectrum and contributing to the overall brightness spotted by the astronomers.
“We searched for a counterpart in SPHERE images acquired during four years through the SHINE survey. In order to account for the expected large orbital motion of the planet, we used a method that assumes the circular orbit obtained from radial velocities and exploits the sequence of observations acquired close to quadrature in the orbit. We checked this with a more general approach that considers keplerian motion, K-stacker,” explained Raffael Gratton of the Astronomical Observatory of Padova.
The new find makes Proxima c a prime target for follow-up studies using radial velocity measurements, near-infrared imaging, and other methods. In addition, next-generation telescopes, such as the Thirty Meter Telescope (TMT), Giant Magellan Telescope (GMT), and ESO’s Extremely Large Telescope (ELT), will be suitable for direct imaging studies of Proxima Centauri.