In the far reaches of the Solar System, past the orbit of Neptune, things start getting trickier and trickier to see. Directly imaging small objects out in the darkness of the Kuiper Belt - where Pluto resides - is really difficult, which makes a recent discovery all the more exciting.
If you know where something is, you can observe it by waiting for it to pass in front of distant stars. This is called occultation, and astronomers use it to study all sorts of trans-Neptunian objects.
But when astronomers used occultation in 2018 to study one such object they've been watching for nearly two decades, they found something really unexpected - a chonk of a moon, relative to the body it is orbiting. A study describing their findings has now been accepted into Astronomy & Astrophysics, and was first covered by Jonathan O'Callaghan over at New Scientist.
The object caught sporting this moon is probable dwarf planet (84522) 2002 TC302. It was first discovered in 2002, after which it was also identified in earlier observations.
Between 2000 and 2018, astronomers collected at least 126 observations of the object across a variety of wavelengths (including the Hubble Space Telescope); using this information, they calculated the potential dwarf planet's orbit, size, and colour.
That's pretty awesome. It means 2002 TC302 almost meets the requirements for a dwarf planet - it's in orbit around the Sun (but not another planet); it hasn't cleared its orbital neighbourhood; and it must have enough mass to achieve hydrostatic equilibrium, or a round shape.
But we're not quite sure. When predictions of its orbit pointed to an occultation event on 28 January 2018, observatories around Europe pointed their eyes at 2002 TC302's neighbourhood to try and figure out its physical properties, such as size and shape.
Telescopes in Italy, France, Slovenia and Switzerland made 12 positive detections of the occultation event, as well as four negative detections. This produced the best observation of a trans-Neptunian object we've obtained to date, the researchers said.
Adding these together allowed the researchers to obtain a new, more accurate measurement of the object's diameter: 500 kilometres (311 miles).
So, how to account for the missing 84 kilometres calculated from the other observations? Well, there's a really interesting answer to that. If 2002 TC302 had a moon around 200 kilometres (124 miles) in diameter, and just 2,000 kilometres (1,243 miles) from the probable dwarf planet, it could produce the signal that other astronomers interpreted as a slightly larger 2002 TC302.
This is crazy close. The Moon, for context, is 384,400 kilometres (238,900 miles) from Earth (on average). At such a close proximity, 2002 TC302's satellite would be extremely hard to image - not even the Hubble Space Telescope images taken in 2005 would be able to resolve it individually.
If the potential dwarf planet really has a satellite, that can help us learn things about the early Solar System. Stuff in the Kuiper Belt has changed very little since the Solar System formed, and as such, these objects are considered time capsules.
Two objects extremely close together could help us to better understand close interactions when the Solar System was forming. Since the planets are thought to have formed via accretion - more and more stuff sticking together - this could be an important clue as to how smaller bodies grow.
An object of similar interest is Arrokoth, the weird snowman-shaped rock visited by the New Horizons probe in 2015. The data provided by that flyby showed us that planetary accretion may be a more gentle process than we thought.
2002 TC302 is a lot bigger than Arrokoth, but it could be at a later stage of the process - which would be really useful in piecing together the stages in which it happens. At any rate, it's clear that we should probably look at it a bit more and try to figure out what its deal is. Exciting!
The research has been accepted into Astronomy & Astrophysics, and is available on arXiv.