4 Planets in Our Solar System You've Probably Never Heard Of

An artist’s concept of Makemake. The dwarf planet is 870 miles across and was discovered in 2005. Image credit: NASA

On Monday, June 11, astronomers announced the discovery of a new dwarf planet beyond the orbit of Neptune. Temporarily named 2015 RR245 (scientists will eventually think of something snappier), not much is known about the trans-Neptunian object. "It's either small and shiny, or large and dull," said Michele Bannister of the University of Victoria, who is part of the discovery team.

Until humans build more powerful telescopes, this might be the last dwarf planet in our solar system to be observed for quite some time. But that's OK. There are four other already discovered planets orbiting our Sun that you might not know about (and several others that might one day be classified as such—especially 2007 OR10). Here they are.  

1. ERIS, THE PLUTO DEMOTER IN THE SCATTERED DISC

Artist's concept of Eris. Image credit: NASA/JPL-Caltech

When Mike Brown and his team at Palomar Observatory in San Diego discovered Eris in 2005, Pluto's categorization as a planet proper was suddenly brought up for debate. Eris was believed to be larger than Pluto (it's not, as New Horizons later revealed) and is certainly more massive, and astronomers were unhappy with the idea of dozens of planets orbiting our Sun when they could just redefine "planet," drop Pluto, and keep things much simpler. The New Horizons data has only intensified the debate. Pluto has clouds! Anyway, it's not like Pluto blew up Alderaan-style when it was re-categorized. It's still there, and on a long enough timeline, its scientific classification will be refined and improved. Not even Earth's status as a planet may be all that safe.

Eris is located in the Scattered Disc, a region of the solar system beyond the Kuiper Belt, which is itself a region of space beyond Neptune. Eris has one known moon, called Dysnomia (who is in Greek mythology the daughter of Eris). If you were to stand on Eris, you would almost certainly die, though before freezing to death you would have a hard time identifying the Sun. From that distance, the Sun appears only as a really bright star.

2. HAUMEA, THE HAWAII-INSPIRED PLUMP CIGAR

An artist's concept of Haumea and its moons, Hi'iaka and Namaka. Image credit: NASA

Located in the Kuiper Belt, Haumea is another Mike Brown discovery. It was observed in 2003 and announced in 2005. Haumea is one of the fastest-spinning objects in the solar system, and as a result, it has a distinct oblong shape (NASA describes it as being shaped like a "plump cigar"). The dwarf planet is named for a Hawaiian goddess, and its moons, Hiʻiaka and Namaka, are named for her daughters. Hiʻiaka is the patron goddess of the island of Hawaii (and of hula dancers), while Namaka is a water spirit. (Haumea's early, unofficial name among its discoverers was Santa, and its moons were Rudolph and Blitzen.) A single Haumean orbit takes 285 Earth years. Its surface albedo (or reflectivity) is suggestive of crystalline water ice—and a lot of it. The dwarf planet is as bright as snow, and as much as 80 percent of its surface might be covered in ice.

3. MAKEMAKE, THE EASTER BUNNY IN THE KUIPER BELT

You've probably already guessed who discovered Makemake (seen at top in an artist's rendition), which is located in the Kuiper Belt and classified as a dwarf planet. Pronounced "mockey-mockey," the world has a 22.5-hour day similar to Earth's, but its year is 310 of ours. It also no atmosphere to speak of. This lack of an atmosphere was determined when Makemake crossed in front of a star, and surprised scientists, who expected it to be more Pluto-like given its midway proximity between Pluto (which has an atmosphere) and Eris (which does not).

Earlier this year, scientists using the Hubble Space Telescope discovered a dark moon in orbit around Makemake. They're calling the moon S/2015 (136472) 1. (It was observed by the telescope in April 2015.) For its part, Makemake is named for a god in Easter Island's Rapa Mui mythology. (The Kuiper Belt object's nickname among its discovery team was Easter Bunny.)

4. CERES, THE WHITE-SPOTTED PERPLEXER

NASA/JPL-Caltech/UCLA/MPS/DLR/IDA, converted from Nature Publishing Group press’s YouTube channel

OK, you might have heard of Ceres; we wrote about it being fully mapped just a few months ago. Despite this mapping, many puzzles remain. Ceres is a dwarf planet in the Asteroid Belt whose vexing white spots have enthralled scientists and the public alike since 2015. (In the end, those spots turned out not to be extraterrestrial homing beacons but salts, possibly caused by water beneath the planet's surface.)

Ceres is the largest object in the Asteroid Belt and has been well explored by the Dawn spacecraft. Last week, NASA decided to extend Dawn's mission and see what happens as the ever-mysterious Ceres reaches perihelion—that is, the closest point in its elliptical orbit that it comes to the Sun. The Dawn team had hoped to leave Ceres's orbit and send the spacecraft to Adeona, another object in the Asteroid Belt. Dawn is already the first spacecraft to orbit two separate celestial bodies (Vesta and Ceres), and Adeona would have set the bar even higher for some future upstart orbit-happy spacecraft. For what it's worth, Mike Brown didn't discover Ceres, but that's likely only because Guisseppe Piazzi, who spotted it first, was born two centuries earlier.