Astronomers have identified a new dwarf planet beyond Pluto's orbit, carrying the provisional designation ED138. If the International Astronomical Union recognises it, it becomes the Solar System's sixth dwarf planet.
It was found by Matthew Holman, a lecturer in Harvard University's Department of Astronomy and a senior astrophysicist at the Smithsonian, analysing data from the telescope at the Vera C. Rubin Observatory in Chile.
The measurements
ED138 is roughly 46 astronomical units from the Sun — that is, 46 times the Earth-Sun distance, placing it well beyond Pluto.
Its estimated diameter is 600 to 1,500km. The range is wide because size is inferred from brightness, and brightness depends on how reflective the surface is, which nobody yet knows. Even at the bottom of that range it is larger than Ceres, the smallest body currently recognised as a dwarf planet.
Why nobody saw it before
This is the genuinely interesting part, and it is not about the object being faint.
ED138 sits in the southern sky, which makes it very hard to observe from the northern hemisphere, where a great deal of historical sky surveying has been done.
Worse, it lies in the same plane as the Milky Way. Looking for a slow-moving dot there means looking through the densest part of our own galaxy — a field so crowded with stars that a moving object is extremely difficult to pick out. Astronomers have long accepted that this band of sky hides things. ED138 is a demonstration of exactly what.
What makes it scientifically useful
Michele Bannister, an associate professor at the University of Canterbury in New Zealand, said ED138 is scientifically very interesting because it is a large trans-Neptunian object. Other telescopes are expected to begin work on it now.
Large distant objects are the most informative kind, because their orbits record the gravitational history of the outer Solar System — where the giant planets moved, and what they scattered on the way.
Why "dwarf planet" is a precise term, not a diminutive
The definition was settled at a 2006 conference in Prague, and it has three parts. A planet orbits the Sun; it has enough gravity to pull itself into a nearly round shape; and it has cleared its orbital neighbourhood of other bodies.
A dwarf planet satisfies the first two and fails the third. That is precisely what happened to Pluto — it is round, it orbits the Sun, and it shares its region with a great deal of other material.
So ED138 being called a dwarf planet is not a comment on whether it is impressive. It is a statement about how crowded its neighbourhood is, which is itself one of the things astronomers want to measure.
Why this observatory changes the arithmetic
ED138 was found in data from the Vera C. Rubin Observatory, and that detail is close to being the real story. Rubin is built to photograph the entire visible southern sky repeatedly, night after night, rather than to stare at chosen targets.
Finding a distant Solar System object requires exactly that: the same patch of sky, imaged again and again, so that the handful of dots which have shifted can be separated from the millions that have not. One deep image of a crowded field tells you almost nothing. A hundred of them tell you what is moving.
Which is why astronomers expect ED138 to be an early entry on a long list rather than a rarity. The outer Solar System has not suddenly acquired new residents. We have acquired the instrument that can pick them out of the crowd.
