The discovery of a new trans-Neptunian dwarf planet beyond Pluto - reported yesterday in Nature. - has planetary astronomers in a quandary. In particular how this planet (named Quaoar) could possibly be sporting a ring beyond what is called "the Roche limit". To review the basics, in 1850 the French astronomer Édouard Roche determined that if the constituent parts of a moon or satellite are held together only by their mutual gravitation, and if the smaller object has the same density as the planet, it will hold together so long as its distance is not less than 2.44 times the planet's radius. This is illustrated in the diagram below:
Here, the smaller object shown at far left is far enough away from the main body to hold together with its self-gravitation, and external tidal forces are at a minimum. In the next step - midway to the Roche limit- the tidal forces are evident, meaning that one side of the satellite is pulled with greater force than the other. At the Roche limit the satellite is torn asunder by tidal forces.
In the case of the planet Quaoar, discovered by a team using a powerful Earth-based telescope at La Palma in the Canary Islands, the quandary facing astronomers is shown below in an artist's rendition of the situation.
Note here that the ring for Quaoar (about half the size of Pluto) exists beyond its Roche limit. If the ring represents the disintegrated residue of a smaller satellite, how can that be? Right now astronomers have zero idea. If the smaller object is beyond the Roche limit it ought to still exist more or less in the form of the smaller object depicted in the topmost graphic. But it doesn't. It exists only as a ring of debris. To be specific, the ring is located at a distance of more than seven planetary radii or 2.8 times farther out than the Roche limit for Quaoar.
To be sure, astronomers cannot actually see the rings, they have to be inferred from photometric observations in what we call "stellar occultations.". The basic technique is similar to that from which we can infer the existence of an exoplanet based on changes in its central star's light curve when it passes in front of it, i.e.
In the above diagram when the planet passes in front of the star a dip in the brightness of the star is detected. In the case of Quaoar, four such stellar occultations (involving bright background stars) were used to infer the existence of the ring. The study's authors observed four such events involving Quoar between 2018 and 2021. In each case, observations by the HiPERCAM telescope showed a sharp dip in starlight as Quaoar passed in front of the star. Interestingly, two smaller dips - before and after- were also observed, from which astronomers deduced a ring system. In the words of Matthew Tiscareno, an astrodynamicist at the SETI Institute (quoted in yesterday's Wall Street Journal):
"If you see this kind of a symmetric signature, that indicates the presence of a ring."
To be sure, the existence of the ring around Quaoar challenges the entire existing theory of ring formation and sends planetary astronomers back to their 'drawing boards'. In respect of confirmation of the Quaoar ring an international team of nearly 60 researchers from more than a dozen countries used telescopes on Earth and in space to help confirm the existence.
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