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In 2016, astronomers working for the European Southern Observatory (ESO) confirmed the existence of a terrestrial planet around Earth’s closest stellar neighbor: Proxima Centauri. The discovery of this nearby extrasolar planet (Proxima b) caused no emotion shortage because, in addition to being similar in size to Earth, it was discovered to orbit within the star’s habitable zone (HZ).
Thanks to a team led by INAF, a second exoplanet (a super Earth) was found around Proxima Centauri earlier this year using the Radial Speed Method. Based on the separation between the two planets, another team led by INAF attempted to observe this planet using the Direct Image Method. While not entirely successful, his observations raise the possibility that this planet has a ring system around it, just like Saturn.
For the sake of his study, which recently appeared in the magazine Astronomy and astrophysics, the team relied 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 four-year SpHere INfrared survey for the Exoplanets survey (SHINE), where SPHERE was used to image 600 nearby stars in the near-infrared spectrum.
Relying on SPHERE’s high contrast and high angular resolution, the purpose of this survey was to characterize new planetary systems and explore how they formed. One such system was Proxima Centauri, a low-mass M-type (red dwarf) star located just 4.25 light years from our Solar System. At the time of the survey, which ran from a, 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 planet system acts on it. However, the team was confident that if Proxima c produced a large enough infrared signal, SPHERE would have detected it.
As the team, led by Raffael Gratton of the Padua Astronomical Observatory, explained their methods in their study:
“We looked for a counterpart in SPHERE images acquired over four years through the SHINE survey. To account for the large orbital motion expected from the planet, we use a method that assumes the circular orbit obtained from the radial velocities and exploits the sequence of observations acquired near the quadrature in the orbit. We verify this with a more general approach that considers the Keplerian movement, K-stacker. ”
Unfortunately, the SPHERE data did not reveal any clear detection of Proxima c. What they found was a candidate signal that had a strong signal-to-noise ratio and that the orientation of its orbital plane fitted well with an earlier image taken with the Atacama Large Millimeter / submillimeter Array‘s (ALMA).
However, they also noted that their orbital position and movement (also known as an astrometric signal) were not consistent with what ESA observed Gaia mission. Last but not least, they discovered that the candidate had an unexpectedly high apparent brightness (also known as flux) from a planet orbiting a red dwarf star. Because of this, the team was unable to confidently say whether 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. As they explain:
“In this case, we imagine a conspicuous ring system or the production of dust by collisions within a swarm of satellites, or the evaporation of dust that increases the luminosity of the planet. This would be unusual for extrasolar planets, with Fomalhaut b, for which there is no dynamic mass determination, as the only other possible example. “
This 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), the Giant Magellan Telescope (GMT), and ESO’s Extremely Large Telescope (ELT), will be suitable for direct imaging studies of this system to detect Proxima C.
Also, if astronomers manage to confirm that the candidate seen here was Proxima c, Breakthrough Starshot will likely want to take part in the action! For years, this organization has been working toward the goal of sending a gram-scale wafer to the Alpha Centauri system using directed energy propulsion. Since the discovery of Proxima b, there has also been talk of doing a Proxima Centauri flyby.
This spacecraft could not only get a close-up view of Proxima b, but it could also go through Proxima c and get some snapshots of the planet and its (possible) ring system. Regardless, if the team’s findings are confirmed, it will be the first time that direct images of a discovered planet have been made from radial velocity measurements and the second time that reflections of circumplanetary material have occurred (after Fomalhaut b).
In any case, these results could have significant implications for future studies and the characterization of Proxima Centauri.
Further reading: arXiv
