How big is Gliese 229B?

Gliese 229B is a pair of brown dwarfs, objects too massive to be planets but too small to shine like stars. Each of the two is about 120,000 kilometers (75,000 miles) across, a little smaller than Jupiter. That figure is the width of each one's visible surface, estimated from its light. In 2024 astronomers found that it is two objects, not one.

Size

A brown dwarf never gathers enough mass to sustain the nuclear fusion in its core that makes a star shine. Without that source of energy, it slowly cools and fades.

The widths of the two brown dwarfs, Gliese 229Ba and Gliese 229Bb, are estimates. A 2024 study worked them out by fitting computer models to the pair's combined light, observed with the James Webb Space Telescope. It found radii of about 0.81 and 0.85 times Jupiter's, so the two are about 116,000 and 122,000 kilometers across.

Fitted as if it came from a single object, the same light gave a width of about 1.17 times Jupiter's, about 167,000 kilometers. The study noted that this is unusually large for a single brown dwarf of that mass and age. It was another clue that Gliese 229B is really two objects.

Size comparisons

Each brown dwarf is a little narrower than Jupiter, but about 34–38 times as massive.
Each is about as wide as Saturn, and about nine Earths lined up side by side would span either one.
The two orbit each other about 6.3 million kilometers apart, about 16 times the distance from Earth to the Moon. If each brown dwarf were shrunk to the size of a basketball, they would be about 13 meters (42 feet) apart.
Wolf 359, a nearby red dwarf star, has only about three times the mass of either brown dwarf, but it is about 1.7 times as wide.

The first brown dwarf of its kind

Gliese 229B orbits Gliese 229A, a red dwarf star with a little more than half the Sun's mass, about 19 light-years from Earth. A light-year is the distance light travels in one year, about 9.46 trillion kilometers. Astronomers from Caltech and Johns Hopkins University found Gliese 229B in October 1994, using the 60-inch (1.5-meter) telescope at Palomar Observatory in California with an image-sharpening device.

The discovery was announced in 1995, after follow-up observations confirmed that the object orbits Gliese 229A. A spectrum from Palomar's 200-inch (5-meter) Hale Telescope showed methane in its atmosphere, like that of Jupiter. That was clear evidence that it was not a star. It was the first brown dwarf found with methane, and the Hubble Space Telescope took a confirming image.

One object becomes two

Gliese 229B circles its star on a very stretched orbit, on average about 33 times as far out as Earth is from the Sun. Tracking that orbit showed that Gliese 229B has about 71 times the mass of Jupiter. That was a puzzle, because models predicted that a single brown dwarf that heavy would shine at least 2 to 6 times as brightly as Gliese 229B does.

In 2024 two teams solved the puzzle. One, using the European Southern Observatory's Very Large Telescope in Chile, split Gliese 229B into two brown dwarfs, with about 38 and 34 times Jupiter's mass. They circle each other every 12.1 days. The other team, using the Keck Observatory, found evidence of the pair from changes in how fast Gliese 229B moves toward and away from Earth.