Triton, the largest of Neptune’s eight known satellites, was discovered just a month after the planet it orbits. It is unlike most of the other known satellites in the solar system, but may be remarkably similar to the planet Pluto, giving us a glimpse into the only outer planet not yet visited.

Triton was discovered on Oct. 10, 1846. by British astronomer William Lassell. Just a week earlier, Lassell had spotted what looked like a ring around the newly discovered planet Neptune. Attempting to confirm this, he instead found that the ring was caused by distortion from his new telescope. However, he also spotted what looked like a satellite; this satellite, later named Triton, was quite real.

Triton is the only known satellite that moves in a retrograde direction (opposite the rotation of the planet it orbits). It moves in a tilted, circular orbit, and takes about 5.875 days to orbit Neptune, at a distance of about 205,000 miles above the planet’s cloud tops. Triton’s landscape is dotted with volcanic structures, similar to geysers, that emit nitrogen gas. Within this gas are fine particles that are carried to an altitude of one to five miles, and then swept downward onto Triton‘s surface. It is believed that small particles of nitrogen ice may form thin clouds a few miles above Triton.

Triton is the coldest known object in the Solar System, with a surface temperature of -391 degrees Fahrenheit. At about 500 miles above Triton’s surface, the temperature is just -290 degrees Fahrenheit. Triton has an extremely thin atmosphere, which may extend as much as 500 miles above its surface. It is also highly dense, with a mean density of about 2.066 grams per cubic centimeter (the density of water is 1.0 gram per cubic centimeter). This, along with its retrograde orbit, suggests to some scientists that Triton may be a captured object, pulled into Neptune’s orbit billions of years ago.

Though Triton is unlike many of the other icy satellites we know about, it does appear to have a similar size, density, chemical makeup, and structure as the planet Pluto. Therefore, the more we learn about Triton, the more we may understand about Pluto, one of our most distant, and therefore most unknown, planetary neighbors.