An object on the outer edges of the solar system may have a large moon orbiting it at an unusually close distance. The find could help explain how such binary objects evolved.
Jos茅-Luis Ortiz at the Institute of Astrophysics of Andalusia in Spain and his colleagues observed the previously discovered object, known as 2002 TC302, in January 2018 when it eclipsed a distant star, casting a shadow on Earth and allowing its properties to be studied.
From 12 separate observations, the researchers deduced that the object was likely to be about 500 kilometres across. But they also found something very unusual: what appears to be a large moon about 200 kilometres across orbiting less than 2000 kilometres from it.
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This distance is around five times the height of the International Space Station above Earth, meaning the moon would loom large in 2002 TC302鈥檚 sky.
The object is one of many trans-Neptunian objects (TNOs), and orbits the sun at an average distance of 55 times the distance between Earth and the sun.
鈥淭here are approximately a hundred satellites already discovered around TNOs, but all of them are relatively far from their main bodies,鈥� says Ortiz. 鈥淲e do not know other objects orbiting this close for an object this big.鈥�
Objects made of two bodies that touch each another, known as contact binaries, may be abundant in the solar system 鈥� one example is the two-lobed object Arrokoth, which was visited by NASA鈥檚 New Horizons spacecraft in early 2019.
But finding a non-touching binary like 2002 TC302 and its moon orbiting so closely would be a first, providing some useful information about these dual systems.
It may suggest a lot of TNOs were born with a large amount of angular momentum, spinning themselves into pieces to create satellites. Such close satellites could also cause large tidal interactions with the parent body, producing wobbles in the orbit that could reveal details about the bodies鈥� interiors, says Ortiz.
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