Tantalizing new observations hint that there's a lot more going on in the Proxima Centauri system — the one that hosts the nearest exoplanet to Earth — than meets the eye.
In 2016 astronomers announced the discovery of Proxima b, a planet orbiting the nearest star to our Sun, Proxima Centauri. It turns out that that announcement was just the beginning of cool news from this system: new observations suggest Proxima b might not be alone.
A team of astronomers led by Guillem Anglada* (Institute of Astrophysics of Andalucía, Spain), studied Proxima Centauri using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. ALMA’s high resolution and sensitivity at millimeter wavelengths make it ideal for spotting the glow of cold dust around nearby stars. Anglada and his team pointed ALMA at Proxima Centauri for more than 20 hours, revealing the presence of a dusty ring around the star. The observations also hint at the existence of additional rings outside the first — and perhaps even another planet. The results will appear in The Astophysical Journal Letters.
Because Proxima Centauri is a smaller, dimmer star, its system is more compact. Proxima b circles the star at 0.05 astronomical units (a.u., the average distance between Earth and the Sun) — for reference, Mercury orbits the Sun at 0.39 a.u. The dusty ring lies well beyond that, extending from 1 to 4 a.u.
The Proxima ring is similar in some ways to the Kuiper Belt, a cold, dusty belt in the far reaches of our solar system (beyond 40 a.u.) that contains a fraction of Earth’s mass. While the Kuiper belt is well known for larger members such as Pluto and Eris, it also contains fine grains, ground down through collisions over billions of years. The dust ALMA observed around Proxima Centauri is composed of similar small grains. The average temperature and total mass of the Proxima ring is also about the same as our Kuiper Belt.
The Kuiper Belt might represent a planet that never formed, as gravitational interactions with Neptune constantly stirred the material. Thus, the existence of a dusty belt around Proxima Centauri suggests that there might be more planets wandering around this dim star.
“The dust around Proxima is important,” Anglada says. “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.”
The team notes tantalizing hints of additional features in the system. The most speculative of these, but also the most intriguing, is the possible discovery of an asymmetry in the dust belt, with an average distance of 1.6 a.u. The authors suggest this could be another planet embedded within the dust. It might even have its own Saturn-like ring system.
The astronomers also found hints of a second, colder belt of dust, about 30 a.u. away from the star. Astronomers have detected similar cold dust rings around other stars using the Herschel telescope, but we’re still working on explaining what they might mean. The team also suggested that there might be a warm shroud of dust closer to the star at roughly ½ a.u.
More observations are needed to confirm these structures — these observations are only the beginning. “These first results show that ALMA can detect dust structures orbiting around Proxima,” says coauthor Pedro Amado (Institute of Astrophysics of Andalucía, Spain). “Further observations will give us a more detailed picture of Proxima's planetary system.”
Our cool stellar neighbor will be providing us with cool news for years to come!
* By coincidence, Guillem Anglada shares his name with Proxima b’s discoverer, Guillem Anglada-Escudé (Queen Mary University of London), who coauthor the current study.