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The size, shape, density and ring of the dwarf planet Haumea from a stellar occultation

Rejected submission by c0lo at 2017-10-12 05:16:17 from the those-are-chips-not-a-dwarf-planet dept.

artstechnica [], buzzfeed [] and perhaps others report on in relation with a Letter published by nature [] on a trans-Neptunian dwarf planet Haumea [] - 1/3rd of the size of Pluto, triaxial ellipsoid, has two moons and a ring, a quite eccentric orbit (between 30 and 50 AU), a short day (4 hours).

...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.

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 moon 70km thick and place it about 1,000km above the planet's equator.
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.

Original Submission