New dwarf planet in the outer solar system

What’s round, orbits the sun and resides in the heavily-populated parts of our solar system such as the asteroid belt or the Kuiper belt? It’s a dwarf planet, and astronomers have just discovered a new one.

Designated 2015 RR245 (based on specific rules), the dwarf planet lies in the solar system’s icy regions beyond the orbit of Neptune.

It was found using the Canada-France-Hawaii Telescope on Maunakea, Hawaii as part of the ongoing Outer Solar System Origins Survey (OSSOS). Since the survey began in 2013, the team has located more than 500 new trans-Neptunian objects but at 700km across, this new discovery is the largest one so far.

2015 RR245 has a 700 year orbit that is highly elliptical. At its most distant the dwarf planet is more than 18 billion km (or 120 astronomical units) from the sun.

Slowly it is making a move towards its closest approach to the sun. At present it is nine billion km (or 60 astronomical units) away. It’ll take 80 years to reach its closest point, albeit still a chilly five billion km (or 34 astronomical units) from the sun.

Astronomers will be studying the dwarf planet in detail to improve their estimates of its size. Size is determined by the object’s brightness which is dependent on the object’s albedo or how easily it reflects sunlight.

If it’s nice and shiny, then 2015 RR245 could well be smaller in size. Or perhaps its surface is dark and dull then estimates of its size would need to swell.

When compared to the officially recognised dwarf planets – Pluto, Eris, Haumea and Makemake – this new dwarf planet is about two to three times smaller. According to a statement from the Canada-France-Hawaii Telescope, 2015 RR245 is the 18th largest object in the Kuiper Belt.

Size Matters

When it comes to dwarf planets, a critical measure is that the object must be round. This is what distinguishes a dwarf planet from among the millions of raggedy-shaped asteroids in the asteroid belt and the hundreds of thousands of objects thought to make up the Kuiper belt.

Furthermore, the definition of roundness is grounded in physics. Rather than choose an arbitrary size definition (such as 1,000km in diameter for example), roundness implies that the object must have enough mass so that under its own gravity it can form a spherical shape.

Ceres is the only object in the asteroid belt known to be round and therefore a dwarf planet. Made of rigid, rocky material it has a diameter of about 900km and a mass of around 900 billion billion kg.

But trans-Neptunian objects are made of weaker stuff. Their icy interiors are more easily shaped by gravity and so require much less force to obtain a spherical shape. The lower size-limit for an icy object to be a dwarf planet is around 320km with a mass of only 1,000kg.

This certainly stands true when looking at the icy moons that orbit Saturn. The smallest icy object known to be round is Saturn’s moon Mimas, at 400km across. Smaller moons, with diameters around 200km, are not round (see astronomer Mike Brown’s excellent description here).

The numbers game

Officially there are four dwarf planets beyond Neptune but unofficially there are many more. Mike Brown, who discovered the dwarf planet Eris, keeps a list of unofficial dwarf planets.

On that list are ten trans-Neptunian objects which are nearly certainly dwarf planets.

Extend the list to include fainter objects that have diameters of perhaps 600km or more, and another 27 potential dwarf planets (not counting 2015 RR245) are included.

What about fainter still? The uncertainties increase but potentially another 51 dwarf planets could be added to the mix, if they are icy and larger than 500km, as observations currently suggest.

In all, that would be 88 dwarf planets beyond Neptune and 2015 RR245 brings that count to 89. There truly is a lot still to explore within our solar system.