Pluto is the largest known member of the Kuiper Belt – a wide zone beyond the orbit of Neptune, the Sun’s most remote major planet. But Pluto isn’t the most massive member of the belt. That distinction goes to Eris – a fellow dwarf planet that may be a lot like Pluto.

Eris was discovered almost 20 years ago. It wasn’t noticed earlier because it’s a long way from the Sun – almost three times Pluto’s distance. At that range, it looks quite faint. And it creeps along against the background of stars, so it took a while to realize that it’s a member of the solar system.

Eris is a tiny bit smaller than Pluto. But it’s a good bit more massive. That means it’s denser than Pluto – it has a higher ratio of rock to ice. But like Pluto, it’s probably still geologically active.

One indication of that is that the surface of Eris is almost pure white – it reflects almost all of the sunlight that strikes it. That suggests that fresh ice is erupting from its interior, repaving the surface.

Another indication is the chemistry of methane ice at the surface. Observations by Webb Space Telescope showed that the methane probably formed from chemical reactions between water and carbon inside the little world. Such material must be constantly renewed – oozing to the surface through cracks or holes in the icy surface of Eris – the heaviest of the Sun’s dwarf planets.

We’ll talk about another dwarf planet tomorrow.

Script by Damond Benningfield