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Astronomers have discovered a huge and previously unknown object entering our solar system that will reach the orbit of Saturn in 2031. It is possibly the largest body from the outer reaches of our solar system ever found to make such a close approach to the sun.

Known as 2014 UN271, it is estimated to be between 100 and 370 kilometres across. The object was spotted by the Dark Energy Survey (DES), a project using the Victor M. Blanco Telescope in Chile that was originally designed to study dark energy, but was recently also used to look for objects in the outer solar system.

at the University of Pennsylvania led a team that used six years of DES data to find 800 bodies beyond the orbit of Neptune, called trans-Neptunian objects. But UN271 jumped out of the data for its size and the calculations that show how far in towards the sun it will fly.

鈥淭his is the coolest thing we鈥檝e found,鈥� says Bernardinelli.

We know of many asteroids and comets that have flown in from the outer solar system, but most tend to be up to a few kilometres wide. The size of UN271 is particularly unusual.

While it isn鈥檛 large enough to be a dwarf planet, it could be a minor planet or 鈥減ossibly a mega comet鈥�, says at the University of Regina in Canada.

Its orbit is also interesting. It is currently about 20 times further out than Earth鈥檚 distance from the sun 鈥� known as one astronomical unit (AU) 鈥� but in January 2031 it will reach a distance of just 10.9 AU, close enough for astronomers to make some basic studies of the object. Saturn, by comparison, orbits at an average of 9.5 AU.

At its furthest, UN271 travels out to a distance of 40,000 AU, or about 0.6 light years, on an orbital path that takes 3 million years to complete, making its discovery partly down to very fortunate timing.

It isn鈥檛 clear if the object has ever flown into the solar system before, or if we may have caught it on its first passage after it was pushed inwards. 鈥淚t seems like it鈥檚 an old object鈥� and has done so, based on simulations of its trajectory, says Bernardinelli. 鈥淏ut this is not set in stone yet.鈥�

So far, our views of UN271 are very low resolution, but as it makes its way inward we could look for cometary activity such as melting ices and the appearance of a tail. It is also unclear if it will be spherical like other small bodies, such as Pluto or Ceres, or more irregular.

鈥淲e haven鈥檛 really seen any 100-kilometre [outer solar system] objects close up,鈥� says Lawler. 鈥淢aybe it鈥檒l be some totally bizarre shape. I鈥檓 really hoping it has rings.鈥�

UN271 could be a particularly good target for a spacecraft to fly by and take images up close. Such a mission, which would need to launch in the next six or seven years to reach it in time, is logistically 鈥渄oable鈥�, says at the European Space Agency.

While there are expected to be many more such objects awaiting discovery in the outer solar system 鈥� particularly during the 10-year survey of the region planned at the Vera Rubin Observatory in Chile from 2023 鈥� it is unclear how unique UN271 is and how rare an opportunity this might be.

鈥淢aybe this is the only one that鈥檚 this big and will come in this close,鈥� says Lawler. 鈥淲e don鈥檛 really know.鈥�

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