Simulated structure of Planet Nine: a rocky core surrounded by ice and gas. – Esther Linder / Christoph Mordasini / University of Bern

A mysterious planet in the far reaches of the Solar System is probably 10 times as massive as Earth and aglow with infrared radiation, but has a frigid temperature of -226 ºC, a simulation suggests.

Astrophysicists Esther Linder and Christoph Mordasini from Switzerland's University of Bern modelled the likely structure of "Planet Nine", a hypothesised but as-yet unfound planet in the outer reaches of the Solar System. Their paper will be published in the journal Astronomy & Astrophysics.

“With our study, candidate Planet Nine is now more than a simple point mass,” Mordasini says. "It takes shape having physical properties."

The Solar System does not end with dwarf planet Pluto. One definition of the edge of the Solar System is its "termination shock" – the point where the solar wind, a stream of particles from the Sun, can reach. This is around 80-200 times the distance between the Sun and Earth.

Some scientists estimate the Sun's gravitational effects can extend two light-years into space, and anything inside that radius is technically within the Solar System.

Pluto orbits an average 40 times the distance between the Sun and Earth. So there's plenty of room beyond for other planets to roam, and it's no surprise planetary scientists have suspected at least one more out there.

Because the Sun's heat is all but dissipated that far out, any planets should be rocky and/or icy. So Linder and Mordasini assumed Planet Nine to be a smaller version of Uranus or Neptune, with a rocky core made of iron and silicate surrounded by a thick layer of ice and a sheath of hydrogen and helium.

They simulated a number of planetary masses at different distances from the Sun, keeping in mind it couldn't be too big or close, as we'd probably have spotted it by now.

They found the most likely size is 10 times the mass of Earth, or 3.7 times the radius. Its orbit would be 700 times the distance between the Earth and Sun.

In visible light, it would be chilly and dim, receiving fewer photons from the Sun to heat it up and bounce off. But given the age of the Solar System – around 4.6 billion years – they calculated Planet Nine would still be cooling. As it cools, they say, it would throw off radiation in the form of infrared light.

“This means that the planet’s emission is dominated by the cooling of its core, otherwise, the temperature would only be 10 Kelvin [-263 ºC],” explains Linder.

And because it'd be oozing infrared radiation, a telescope such as the Large Synoptic Survey Telescope being built in Chile, powerful enough to pick out faint emissions in the sky, is our best bet for finding it.

“That is an exciting perspective,” they write.

Further reading:

The search for planets beyond Pluto