I’m keeping an eye on the recent attention being paid to Proxima Centauri c, the putative planet whose image may have been spotted by careful analysis of data from the SPHERE (Spectro-Polarimetric High-Contrast Exoplanet Research) imager mounted on the European Southern Observatory’s Very Large Telescope. A detection by direct imaging of a planet found first by radial velocity methods would be a unique event, and the fact that this might be a planet in the nearest star system to our own makes the story even more interesting.
I hasten to add that this is not Proxima b, the intriguing planet in the star’s habitable zone, but the much larger candidate world, likely a mini-Neptune, that has been identified but not yet confirmed. Proxima Centauri c could use a follow-up to establish its identity, and this direct imaging work would fit the bill if it holds up. But for now, the planet is still a candidate rather than a known world. From the paper:
While we are not able to provide a firm detection of Proxima c, we found a possible candidate that has a rather low probability of being a false alarm. If our direct NIR/optical detection of Proxima c is confirmed (and the comparison with early Gaia results indicates that we should take it with extreme caution), it would be the first optical counterpart of a planet discovered from radial velocities. A dedicated survey to look for RV planets with SPHERE lead to non-detections (Zurlo et al. 2018b).
But we’re not far enough along to spend much time on this in these pages — a great deal of follow-up work will be needed to nail down what is at best an unlikely catch. I call it that because it strains credulity to believe that we would find a planet whose mass suggests a world far less bright than this one is (if indeed it is a planet), with a luminosity that demands something like a huge ring system to explain it. Other explanations will need to be ruled out, and that’s going to take time. The radial velocity work on Proxima Centauri c points to a world of a minimum six Earth masses, orbiting 1.5 AU out. But if this direct imaging work has indeed identified (and confirmed) Proxima c, it is a planet with a most unusual makeup:
If real, the detected object (contrast of about 16-17 mag in the H-band) is clearly too bright to be the RV [radial velocity] planet seen due to its intrinsic emission; it should then be circumplanetary material shining through reflected star-light. In this case we envision either a conspicuous ring system (Arnold & Schneider 2004), or dust production by collisions within a swarm of satellites (Kennedy & Wyatt 2011; Tamayo 2014), or evaporation of dust boosting the planet luminosity (see e.g. Wang & Dai 2019). This would be unusual for extrasolar planets, with Fomalhaut b (Kalas et al. 2008), for which there is no dynamical mass determination, as the only other possible example. Proxima c candidate is then ideal for follow-up with RVs observations, near IR imaging, polarimetry, and millimetric observations.
So good for Raffaele Gratton (INAF – Osservatorio Astronomico di Padova, Italy) and colleagues for pursuing this investigation and for suggesting the numerous ways it can be approached with various follow-up methods. And kudos to Mario Damasso (Astrophysical Observatory of Turin) as well. Damasso was lead author of the discovery paper on Proxima Centauri c, and it was he who suggested to Gratton that the SPHERE instrument might just be able to detect it.
Now we have a wait on our hands before we have anything definitive. At this point we have a possible detection that is tantalizing but definitely no more than tentative. Meanwhile, here’s a figure from the paper that gives an idea what Gratton and team are talking about.
Image: This is Figure 2 from the paper. The SPHERE images were acquired during four years through a survey called SHINE, and as the authors note, “We did not obtain a clear detection.” The figure caption in the paper reads like this: Fig. 2. Individual S/N maps for the five 2018 epochs. From left to right: Top row: MJD 58222, 58227, 58244; bottom row: 58257, 58288. The candidate counterpart of Proxima c is circled. Note the presence of some bright background sources not subtracted from the individual images. However, they move rapidly due to the large proper motion of Proxima, so that they are not as clear in the median image of Figure 1. The colour bar is the S/N. S/N detection is at S/N=2.2 (MJD 58222), 3.4 (MJD 58227), 5.9 (MJD 58244), 1.2 (MJD=58257), and 4.1 (MJD58288). Credit: Gratton et al.
The paper is Gratton et al., “Searching for the near infrared counterpart of Proxima c using multi-epoch high contrast SPHERE data at VLT,” accepted at Astronomy & Astrophysics (abstract).