Far beyond Neptune, in the darkest depths of the solar system where the temperatures hover near absolute zero, lies a small roundish body made up of ice and rock. Lit dimly by the Sun 13.5 billion kilometers away, it’s so faint it has avoided detection by Earthly telescopes for all of history.

Until now. A team of astronomers has now seen its feeble light, and have taken its measure [link to journal paper].

Called 2017 OF201 (designating the year of the observations in which it was first seen and its order in the list of minor bodies discovered in the two-week period of the second half of July), it was found in observations of the sky taken with the Dark Energy Camera Legacy Survey, a survey of the sky taken using the Dark Energy Camera mounted on a 4–meter telescope in Chile. The camera’s observations are designed to look at distant galaxies, to get a handle on the dark energy accelerating the cosmic expansion. However, it also captures solar system objects, and these images can be used to look for ones never before seen.

They saw 2017 OF201 in ten such observations from 2014 to 2018. It appeared to move against the background stars over that time, and, using formulae that were first determined four centuries ago by Kepler, the astronomers calculated the orbit of the object. They could then predict where it was in the past, and found that it had been seen in nine further observations by other telescopes. This gave them a baseline of about 7 years of observations, which allowed them to refine the orbit even more.

2017 OF201 is a Trans-Neptunian Object, or TNO, one of perhaps trillions of ice and rock bodies orbiting the Sun past Neptune. But it’s what’s called an extreme TNO due to its orbit. Its path around the Sun is extremely elliptical (for any mathophiles, it has an eccentricity of 0.95, which is phenomenally elongated) that takes it as close to the Sun as 6.8 billion kilometers but as staggeringly far out as 240 billion kilometers! That’s over 50 times farther from the Sun than Neptune… and it takes nearly 25,000 years to complete one circuit.

Given its distance and a good estimate of how much light it reflects from the Sun, the astronomers conclude it’s roughly 700 kilometers across. That’s decently big! It’s larger than Vesta in the asteroid belt, and about a third the diameter of Pluto.

The only reason it was seen at all is because it happens to be near perihelion right now, the point in its orbit when it’s closest to the Sun, so it’s at its brightest. Even then it’s at a magnitude of 23 — the faintest star you can see by eye is about 6 million times brighter. It’s lucky anyone spotted it! As it pulls away from the Sun it’ll fade severely. As an aside: you may know that the brightness of an object drops as the square of the distance (the inverse square law); so, from 2017 OF201 the Sun will be 100 times dimmer when it’s ten times farther away. But we are on Earth, and the sunlight the TNO reflects will also drop in strength by the square of the distance as it heads toward Earth. So, for us, the brightness drops by the distance to the fourth power. By the time it’s ten times its current distance from us it will be one-ten-thousandth as bright as it is now!

Which brings up an interesting point. It’s only bright enough for us to detect in our current surveys for about 0.5% of its orbit. That implies, assuming there are more objects on similar orbits and we miss them 99.5% of the time, that the total mass of such objects adds up to about 1% of Earth’s mass. That’s more than all the mass in the asteroid belt, meaning there is a lot of stuff out there we have yet to find.

Its orbit has other implications, too.

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