Planetary Rings: Dwarf Planet Edition

In the early morning hours of January 21, 2017, many professional and amateur astronomers were pointing their telescopes to a star in the constellation Boötes (The Herdsman) with the unwieldy name of URAT1 533-182543, a typical star that had not inspired any special interest up until that very moment. So what happened on that day to place the star in the spotlight? For about 2 minutes, that star was occulted by Haumea, one of the largest icy bodies in the Solar System orbiting beyond Neptune, and a so-called “dwarf planet,” like Pluto.

Occultations have been used for decades to study details of many small bodies in the Solar System. Conceptually, they are just like total solar eclipses, when lucky witnesses on Earth see the Moon temporarily blot out the entire Sun. In that case, the observers are in the deepest part of the Moon’s shadow. In an occultation, instead of the Moon, there is an asteroid or other object, and instead of the Sun, there is a distant star. These occultations usually last just a few seconds or minutes, and so require careful planning to actually see. Most often, occultations are used to independently find the sizes and shapes of asteroids, properties that can be very difficult to determine otherwise. This is especially true with the very distant objects beyond Neptune, known as “trans-Neptunian objects,” or TNOs. Haumea, in addition to being a dwarf planet, is one of these TNOs.

Twelve telescopes at 10 different observatories around Europe managed to witness Haumea’s blockage of the star. But in addition to measuring Haumea’s properties, the observers saw something unexpected: Haumea has a ring! The discovery of the first ring system around a dwarf planet is a reminder that there are many fascinating worlds to explore in the outskirts of the Solar System.

Estela Fernández-Valenzuela, soon to be a post-doctoral researcher at the Florida Space Institute at UCF, participated in the Haumea occultation project and is a coauthor of this discovery.

“I worked in the planning of the observational campaign. A few days in advance of the phenomenon, we predicted that the shadow of this occultation was going to cross Europe, so we sent out an announcement and coordinated the observations. After the successful detection, I worked on modeling the effects that the ring has on the variation of Haumea’s brightness. This information is needed to determine the orientation of the rotation axis of the body.”

The group of researchers, led by José Luis Ortiz from the Instituto de Astrofísica de Andalucía, Spain, found that Haumea’s ring lies in the same plane as its equator and as the orbit of its outer moon, Hi’iaka. The ring has a radius of about 2,287 kilometers and a width of 70 km, lying at about 1,000 km from Haumea’s surface. They also found that Haumea is larger than previously estimated – its largest axis is about 2300 km long – and this implies that it is not as reflective or as dense as originally thought.

More details can be found in the Nature publication: “The size, shape, density and ring of the dwarf planet Haumea from a stellar occultation,” Nature 550, 219–223 (URL: http://www.nature.com/nature/journal/v550/n7675/full/nature24051.html?foxtrotcallback=true). A video with an artist’s conception of the Haumea system is also available (URL: https://youtu.be/TeUegAEbGxo).