Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse
Object: Quaoar, a Kuiper belt misfit
Shape: Craggy, or eggy
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Pluto shouldn鈥檛 complain. It may no longer be a planet, but at least it gets to be the beloved king of the dwarfs. Life isn鈥檛 so simple for Quaoar, another ball of ice and rock adrift in the freezing fringes of the solar system.
It was once Pluto鈥檚 second in command, the second-largest object in the Kuiper belt, a ring of dwarf planets and other bodies beyond the orbit of Neptune. But new, bigger worlds kept turning up. Meanwhile, the size of Quaoar (pronounced 鈥渒wawar鈥�) was revised downwards thanks to improved measurements. The strange world was all but forgotten.
Now Quaoar may have lost its last remaining honour, as the densest object in the Kuiper belt. The latest revisions to its size, density and shape suggest that the neglected object has a lot more in common with its neighbours than we suspected.
That could be a good thing 鈥� its new, larger size potentially ups its eligibility for membership to the dwarf planet club, which was formed as a result of Pluto鈥檚 fall from grace. Except that Quaoar seems to be an ellipsoid, which could be a no-no 鈥� even dwarf planets must be spherical.
Screwy mountains
Named for a Native American creator-god, Quaoar orbits at about 6.5 billion kilometres from the sun. Its size puts it near the limit of what the Hubble Space Telescope can see, making it hard to tease out details. Previous work combed over Hubble鈥檚 blurry images and made models of Quaoar and its only moon, Weywot, based on the notion that both objects would look roughly like the moons of Uranus. That research hinted that Quaoar is about 900 kilometres wide and so dense that it may be mostly rock 鈥� unusual for the Kuiper belt, where most objects are mixtures of ice and dirt.
But infrared views from modern telescopes, such as the Herschel space telescope, and other observations showed that Quaoar鈥檚 surface composition is nothing like anything seen on the Uranian moons. So Felipe Braga-Ribas of the National Observatory in Rio de Janeiro, Brazil, and colleagues took a different tack.
In 2011 and 2012, multiple teams watched Quaoar cross in front of a star, making that star wink out for a short while. By carefully timing the observations and recording the changes in starlight, these occultations gave some of the most accurate measurements yet of distant Quaoar鈥檚 size and shape.
Eggy Quaoar
Braga-Ribas鈥檚 team calculates that Quaoar is actually 1138 kilometres wide 鈥� a bit bigger than the dwarf plant Ceres 鈥� and that it has a density of just 1.99 grams per cubic centimetre, which would make it more of an icy dirt-ball like Pluto.
But there鈥檚 a catch. The occultations make most sense if Quaoar is an elongated ellipsoid incorporating either a very large mountain or a deep crater. The trouble is that neither feature should last for long if the object is made of an ice-rock mixture. 鈥淲hen you take the times recorded at face value, Quaoar looks screwy 鈥� it鈥檚 completely unreasonable,鈥� says of the Herzberg Institute of Astrophysics in Victoria, British Columbia, Canada, who was not on the research team.
So the team also examined what it would take for a smoother shape to fit the data. Assuming slight timing errors that are mostly within expected limits, a featureless but rounder, egg-shaped ellipsoid could fit too.
Kuiper chemistry
Braga-Ribas鈥檚 team also reports no atmosphere on Quaoar. This is something that Fraser finds suspicious. 鈥淢ost of the large objects in the Kuiper belt are expected to have atmospheres of some sort,鈥� he says. 鈥淭hey have moderately volatile ices on their surfaces that are relatively warm; enough to produce slight, tenuous atmospheres.鈥�
In work submitted to the Astrophysical Journal Letters, Fraser and his colleagues present data from when Quaoar seemed to pass the edge of a star in mid-July, from the perspective of the . They rule out a nitrogen or carbon monoxide atmosphere, but they think a pure methane atmosphere is still possible, and that a fluffy, diffuse covering of methane would actually fit with Braga-Ribas鈥檚 occultation results. Either Quaoar does have a slim envelope of gas, or it is somehow defying our understanding of Kuiper belt chemistry.
What is clear from the various sightings is that Quaoar is not perfectly round. But when the International Astronomical Union (IAU) redefined planets, it also decided that dwarf planets need to be massive enough that their gravity makes them at least nearly round. So does Quaoar鈥檚 probable egg-shape exclude it from the club?
鈥淐eres isn鈥檛 perfectly round either. That would suggest the IAU definition may need to be re-examined,鈥� says Fraser. 鈥淚 would lean towards calling it a dwarf, and someone would have to convince me otherwise.鈥�
Journal reference: Astrophysical Journal, DOI:
