Discovery of mysterious dwarf planet beyond Neptune stuns astronomers

At this time, scientists who are theoretical experts in many different fields are trying to guess how the rings around the dwarf planet Quaoar exist.

Recent telescope data has revealed that a small planet in the far reaches of our solar system has a dense ring surrounding it. And scientists are struggling to explain why.

Simulation of a dwarf planet wandering in the Solar System.

Quaoar , sometimes referred to as a dwarf planet , is one of about 3,000 objects orbiting the Sun beyond Neptune and has a diameter of 1,110 km (710 mi) (the seventh largest dwarf planet, with Pluto and Eris ranking as the largest).

Observations of Quaoar taken between 2018 and 2021 revealed that the dwarf planet has a ring that is farther away than scientists previously thought, the European Space Agency said in a press release, after using ground-based telescopes and a new space-based instrument called the Cheops telescope to collect data.

By conventional reasoning, all the material that made up Quaoar's dense ring should have condensed and formed a small moon. But it didn't.

The European Space Agency said: 'Initial results suggest that the cold temperatures at Quaoar may play a role in preventing icy particles from sticking together but more investigation is needed.'

Roche limit exceeded

Before these new observations of Quaoar, most scientists believed that planets couldn't form rings beyond a certain distance. It's a widely accepted rule of celestial mechanics that material in orbit around a planet will form a spherical object (or moon) if it orbits far enough from the planet. But that moon will be torn apart if it moves closer than what's known as the 'Roche limit', a point at which the planet's tidal forces become stronger than the gravitational pull holding the moon together.

Quaoar's orbit is between Neptune and Pluto.

For example, all of Saturn's rings lie within the planet's Roche limit. What's puzzling about Quaoar, however, is that its rings lie outside the Roche limit, in the region where material that would otherwise form a moon would be.

'According to our observations, the classical idea that dense rings exist only inside the Roche limit of a celestial body must be radically revised,' said Giovanni Bruno of the INAF Astrophysical Observatory in Catania, Italy.

How to study a dwarf planet from a distance?

According to the ESA, the data collection that revealed Quaoar's puzzling rings was cause for celebration. Due to the planet's small size and distance from Earth, researchers wanted to observe it using a method called occultation. In this method, a planet is observed by waiting for it to be backlit by a sufficiently bright star.

According to the ESA, this can be an extremely difficult process because the telescope, planet and star all have to be aligned perfectly. The observation was made possible by the space agency's recent efforts to provide an unprecedentedly detailed map of the stars.

ESA also used the Cheops telescope, launched in 2019. Cheops typically studies exoplanets, or objects outside our solar system. But in this case, it set its sights on a closer target: Quaoar. Although it is in the solar system, its orbit is farther away from the center of the Sun than Neptune, which orbits at about 44 AU (1 AU is the average distance between Earth and the Sun).

'I was a little skeptical at first about the possibility of doing this with Cheops,' said Isabella Pagano, director of INAF's Catania Astrophysical Observatory. ' But it worked.' And according to ESA, the Cheops observation marks the first time one of the most distant planets in our solar system has been observed by a space-based telescope.

The researchers then compared the data collected by Cheops with observations from telescopes on Earth, leading to their surprising discovery.

'When we put everything together, we saw that the dip in brightness was not caused by Quaoar, but that it indicated the presence of material in a circular orbit around it, ' said Bruno Morgado, a professor at the Federal University of Rio de Janeiro in Brazil, who led the analysis. 'We knew we were seeing a ring around Quaoar . '

Quaoar was discovered by astronomer Chad Trujillo on June 5, 2002 , when he identified it in images obtained with the Samuel Oschin telescope at Palomar Observatory the night before. The discovery was sent to the Minor Planet Center on June 6, with Trujillo and colleague Michael Brown being credited with the discovery. At the time of discovery, Quaoar was located in the constellation Ophiuchus, with an apparent magnitude of 18.5. Its discovery was formally announced at a meeting of the American Astronomical Society on October 7, 2002.

Quaoar orbits at about 43.7 astronomical units (6.54×109km; 4.06×109mi) from the Sun with an orbital period of 288.8 years. Quaoar has a low orbital eccentricity of 0.0394, meaning its orbit is nearly circular. Its orbit is moderately inclined to the ecliptic at about 8 degrees, typical for the population of small classical Kuiper belt objects (KBOs) but exceptional among large KBOs. Quaoar is not significantly perturbed by Neptune, unlike Pluto, which is in a 2:3 orbital resonance with Neptune (Pluto orbits the Sun twice for every 3 orbits Neptune completes). Quaoar is the largest body classified as a cubewano, or classic Kuiper belt object, by both the Minor Planet Center and the Deep Ecliptic survey (although the dwarf planet Makemake, which is larger, is also classified as a cubewano).

Quaoar occasionally passes closer to the Sun than Pluto, as Pluto's apogee (the farthest distance from the Sun) lies beyond and below Quaoar's orbit. In 2008, Quaoar was only 14 AU from Pluto, making it the closest large object to Pluto in 2019. Quaoar's rotation period is uncertain, and two possible rotation periods for Quaoar have been proposed (8.64 hours or 17.68 hours). Based on rotational light curves of Quaoar observed between March and June 2003, its rotation period was measured to be 17.6788 hours.

Quaoar's albedo or reflectivity may be as low as 0.1, which is still much higher than the lower estimate of 0.04 for 20000 Varuna. This may indicate that fresh ice has disappeared from Quaoar's surface. The surface is moderately red, meaning that Quaoar is relatively more reflective in the red and near-infrared spectrum than in the blue. The Kuiper belt objects Varuna and Ixion are also moderately red in the visible spectrum. Larger Kuiper belt objects are typically much brighter because they are covered in fresher ice and have higher albedos, and thus appear neutral in color. A 2006 model of internal heating through radioactive decay suggested that, unlike 90482 Orcus, Quaoar may not be able to sustain an internal water ocean at the core-mantle boundary.