Proxima Centauri Dust Indicates a Complicated System

Just how elaborate is the planetary system around the nearest star? It’s a question rendered more interesting this morning by the news that the ALMA Observatory in Chile has now detected dust in the system in an area one to four times as far from Proxima Centauri as the Earth is from the Sun. Moreover, there are signs of what may be an outer dust belt, an indication that while we have already discovered Proxima Centauri b, we are looking at a system in which cold particles and debris that could have formed other planets continue to accompany the star.

Image: This artist’s impression shows how the newly discovered belts of dust around the closest star to the Solar System, Proxima Centauri, may look. ALMA observations revealed the glow coming from cold dust in a region between one to four times as far from Proxima Centauri as the Earth is from the Sun. The data also hint at the presence of an even cooler outer dust belt and indicate the presence of an elaborate planetary system. These structures are similar to the much larger belts in the Solar System and are also expected to be made from particles of rock and ice that failed to form planets. Note that this sketch is not to scale — to make Proxima b clearly visible it has been shown further from the star and larger than it is in reality. Credit: ESO/M. Kornmesser.

It would surprise no one if Guillem Anglada-Escudé were the lead author of this work, as Anglada-Escudé led the effort that discovered Proxima b. But in this case, the Guillem Anglada who led the dust work, based at the Instituto de Astrofísica de Andalucía, Granada, Spain, simply shares Anglada-Escudé’s name — the two are not related. Further complicating matters is the fact that Proxima b discoverer Anglada-Escudé is a co-author of the paper on dust.

Image: This picture combines a view of the southern skies over the ESO 3.6-metre telescope at the La Silla Observatory in Chile with images of the stars Proxima Centauri (lower-right) and the double star Alpha Centauri AB (lower-left) from the NASA/ESA Hubble Space Telescope. Proxima Centauri is the closest star to the Solar System and is orbited by the planet Proxima b, which was discovered using the HARPS instrument on the ESO 3.6-metre telescope. Credit: Y. Beletsky (LCO)/ESO/ESA/NASA/M. Zamani.

Says lead author Anglada:

“The dust around Proxima is important because, following the discovery of the terrestrial planet Proxima b, it’s the first indication of the presence of an elaborate planetary system, and not just a single planet, around the star closest to our Sun.”

Given Proxima Centauri’s age, at least as old as the Solar System and perhaps older, we’re probably detecting residual dust from regions something like the Kuiper Belt and main asteroid belts in our own system. Such dust can also produce zodiacal light, which can be seen in our own system as sunlight is scattered by interplanetary dust, a faint but observable phenomenon.

Image: This image of the sky around the bright star Alpha Centauri AB also shows the much fainter red dwarf star, Proxima Centauri, the closest star to the Solar System. The picture was created from pictures forming part of the Digitized Sky Survey 2. The blue halo around Alpha Centauri AB is an artifact of the photographic process, the star is really pale yellow in colour like the Sun. Credit: Digitized Sky Survey 2. Acknowledgement: Davide De Martin/Mahdi Zamani.

The total mass of the inner belt appears to be about 1/100th of Earth’s mass, at a temperature of about -230 degrees Celsius, roughly the temperature in the Solar System’s Kuiper Belt. The inner belt appears to extend a few hundred million kilometres from Proxima Centauri, and could be quite a useful find if it helps us estimate the inclination of the Proxima Centauri system. The dust is assumed to be elliptical given the tilt of what is probably a circular ring around the star. Such a determination would offer us a way to tighten mass estimates of Proxima b.

And are there other planets waiting to be discovered here? Anglada again:

“This result suggests that Proxima Centauri may have a multiple planet system with a rich history of interactions that resulted in the formation of a dust belt. Further study may also provide information that might point to the locations of as yet unidentified additional planets.”

The paper speculates interestingly about a “compact source of 1.3 mm emission at a projected distance of ∼ 1.2” SE from the star…” which may or may not be an artifact in the equipment, but if real could be anything from a background galaxy (the authors consider this remote) to a Saturn-mass planet with a temperature around 1000 K. The problem: No radial velocity signature of such a planet has yet been found, although if it were there, it should be detectable.

Image: Figure 4 from the paper. Sketch (not to scale) of the proposed components in the Proxima Centauri planetary system. Question marks indicate marginally detected features. Credit: Anglada et al.

The paper is Anglada et al., “ALMA Discovery of Dust Belts Around Proxima Centauri,” accepted at Astrophysical Journal Letters (preprint).