Thanks largely to the improvements in our instrumentation, we've started to get a picture of what the far reaches of our Solar System look like. Beyond the orbit of the outermost planet, there is a large collection of icy dwarf planets called trans-Neptunian objects, or TNOs. Pluto may have been the founding member of the TNOs, but there are now more than 1,200 known bodies in the list; only 270 of those have even been observed well enough to have their orbits characterized. It's a safe bet that there will be some surprises among them.

In a paper being released today, researchers are reporting on one such surprise, courtesy of the dwarf planet Haumea: it's got a ring. The observations that spotted the ring also suggest that Haumea is larger than we thought it was, which means that gravity hasn't yet pulled it into a stable, rounded shape. Awkwardly, this means that Haumea may not fit the definition of dwarf planet set down by astronomers.

Meet the dwarf

Haumea, named after a Hawaiian goddess, has a closest approach to the Sun of 35 Astronomical Units (AU, the typical distance between the Earth and Sun). For context, Neptune is typically about 30 AU. Due to its highly elliptical orbit, Haumea is over 50 AU at its farthest distance from the Sun. At those distances, not a lot of sunlight reaches it, so most observations of the dwarf planet involve just a few smeary pixels. But we have observed it enough to know that it's consistently associated with two smaller smears of pixels, its moons Hi'iaka and Namaka. The moons' orbits told us something about Haumea's mass.

The images also show that Haumea has an odd, potato-like shape, technically a "triaxial ellipsoid." To understand that term, think of a sphere: since it has the same radius in any direction, it really only has a single axis. Now squash that sphere down so that it's still rounded, but flatter. Now, the object has a short center-to-top radius and a long center-to-equator radius. It's biaxial. Now push in two opposing sides at the equator, forming the potato shape. This splits the equatorial axis in two, one for the long axis at the side, and a second for the short one. Hence, three axes, or triaxial.

We could tell this was the case because of relatively rapid changes in the amount of light reflected by Haumea, as a different fraction of its surface would be facing Earth depending on where it was in its rotation. This indicates that it completes a rotation in fewer than four hours.

But all of these measurements contained significant uncertainties, so there was plenty of space for further observations. An opportunity arose in January of this year when orbital calculations suggested that Haumea would be passing in front of the star URAT1 533−182543. So a large group of researchers arranged observations at a set of telescopes scattered across Europe.

That’s no moon

The idea behind the observations is that Haumea would block some or all of the light from the star as it passed between Earth and the star. The exact amount of light blocked would allow us to get new measurements of the dwarf planet's shape and possibly provide other details of its properties.

This definitely worked out. To begin with, every one of the dozen telescopes used saw the same pattern as the planet passed in front of the star: rather than a gradual dimming, the light cut out suddenly. This is an indication that Haumea lacks an atmosphere, which would have caused a more gradual dimming. The team behind the observations estimate that any gas surrounding Haumea is so sparse that the atmospheric pressure would be measured in nanobars (Earth's atmospheric pressure is about a bar).

Another notable feature in many of the observations is that the light dipped slightly before and after the planet passed in front of the star. This, the authors conclude, is the result of a ring that blocks about half the light that would otherwise pass through it. A couple of models are consistent with the timing of the slight dip in the star's light, but the researchers favor one that places the ring at Haumea's equator and between it and the moon Hi'iaka. This would make the ring 70km thick and place it about 1,000km above the planet's equator.

This places the ring at about a 3:1 resonance with the surface of the planet. This means that a given location in the ring orbits once for every three times the planet rotates. That's also close enough that Haumea's gravity will be strong enough to disrupt any moons that might otherwise form in the ring material.

The transit also revealed more about the shape of Haumea. Observations before and after suggested that the dwarf planet was in an orientation that would block the minimal amount of light. That provided a measure of the smallest axes; comparing that to past measurements in other orientations would provide a new estimate of the larger axes.

The new estimates suggest that Haumea is bigger than we'd thought, with its long axis being at least 2,322km. That, in turn, suggests its density is much lower, at about 1,900 kilograms per cubic meter. That value is consistent with those we've seen in other TNOs, making Haumea less of an outlier.

But those numbers may create some problems for Haumea's status as a dwarf planet. All planets, dwarf or otherwise, are defined by having sufficient gravity to force them into a stable, rounded shape. This state is called hydrostatic equilibrium. The new values for Haumea suggest that it may not have reached this point yet—it'll have to go through further reshaping and become more rounded. Under this definition, then, Haumea isn't a dwarf planet.

Of course the ring, the rapid rotation, and the odd shape of Haumea could all point to a possible impact. The problem with that idea is that Haumea isn't the only small body that has recently been spotted with a ring. A couple of bodies that have orbits that intersect those of the giant planets (called "centaurs") have recently been found to have small rings. These bodies are thought to originally have been TNOs that were brought inward by gravitational interactions with the giant planets of the outer Solar System.

So rings may be a common feature beyond the orbit of Neptune, and this one has nothing to do with Haumea's other odd features. It's impossible to tell at this point, but the ring's presence certainly makes the object—planet or not—worth further observation.

Nature, 2017. DOI: 10.1038/nature24051 (About DOIs).