10 Facts About the Dwarf Planet Eris

An artist's rendering of the dwarf planet Eris
An artist's rendering of the dwarf planet Eris
ESO/L. Calçada and Nick Risinger (skysurvey.org) // CC BY 4.0

Far beyond the orbit of Pluto exists a celestial body that’s a little smaller, a little colder, and a little denser—the dwarf planet Eris. In Greek mythology, Eris is the goddess of strife, and never was there a more appropriately named body in the solar system. When astronomer Mike Brown of Caltech and his team discovered Eris in 2005, the finding set off a chain reaction that would see the planetary status of Pluto called into question. Here are 10 things you might not know about Eris.

1. An Eridian day is just slightly longer than Earth's.

If you were an astronaut, you wouldn’t find an Eridian day, at 25.9 hours, too disconcerting. This compares favorably with, say, Venus, whose day lasts 5832 hours (admittedly, it's an outlier). An Eridian year is a bit longer than what we're used to, with the dwarf planet completing an orbit of the Sun every 557 Earth years. And that orbit is not along the relatively flat plane with the orbits of most of the other planets of the solar system. Imagine your elementary school solar system model of planets on wires around a light bulb: Instead of a path neatly aligned with the other planets, Eris’s orbit is tilted at a 44 degree angle.

2. Eris was once thought to be bigger than Pluto.

After Eris’s discovery, the best measurements then available placed it as slightly larger than Pluto, with a radius of 722 miles. But after the initial spacecraft reconnaissance of Pluto by New Horizons in 2015, Pluto’s ranking as the ninth-largest planetary object orbiting the Sun was restored; it is now known to have a radius of 736 miles. In comparison, Earth’s Moon has a radius of 1079 miles. Ganymede, Callisto, Io, and Europa (Jupiter’s largest moons), Titan (Saturn’s largest moon), and Triton (Neptune’s largest moon) are also bigger than Pluto. On the other hand, Eris is 34 percent denser than Pluto.

3. Eris is responsible for the big debate over the definition of "planet."

When Brown’s team discovered Eris, it was initially hailed as either the 10th planet of the solar system, or a big problem for scientists who like nicely ordered celestial objects. The discovery of Eris came on the heels of the discoveries of Sedna and Quaoar, both beyond the orbit of Neptune. Astronomers were looking at the possibility of a dozen planets in the solar system or more, because—based on these three—who knew how many Pluto-sized bodies were out there? The International Astronomical Union eventually defined a planet in our solar system as something that has achieved hydrostatic equilibrium (in other words, it's round), orbits the Sun, and has “cleared its neighborhood” (i.e., is gravitationally dominant in its orbit). Yet the debate continues [PDF].

4. It has its own moon.

Eris has a moon called Dysnomia that circles the dwarf planet every 16 days. In Greek mythology, Dysnomia is the name of one of Eris’s daughters and means “anarchy.”

5. Initially, Eris was called Xena.

Before it was called Eris, it was called 2003 UB313 (a provisional designation by the International Astronomical Union). But before that, Brown’s team of astronomers named it Xena—yes, of Warrior Princess fame. “We always wanted to name something Xena,” Brown told The New York Times in 2005 after the discovery. Among Brown’s colleagues, Dysnomia was called Gabrielle, who was, of course, Xena’s sidekick.

6. Its surface is like Pluto's heart.

The primary way to analyze the composition of the surface of a celestial body is through spectroscopy, which is basically looking at an object and seeing how much light comes back at you as a function of wavelength. Many materials have characteristic absorptions of light at certain frequencies, and so less light will come back to you at that frequency.

“Eris has very, very strong methane ice absorption bands,” Will Grundy, a planetary scientist at Lowell Observatory and a member of the New Horizons team, tells Mental Floss. “They are much stronger than Pluto’s, and of course we’ve seen methane all over the place on Pluto, so I think it’ll be more ubiquitous on Eris’s surface.” The implication is that Eris is more than just a dead ice rock in space, because methane degrades very quickly in a space environment, darkening and forming heavier hydrocarbons. “The fact that it’s bright and covered with methane ice says it’s refreshing its surface relatively rapidly, and there are any number of ways it can do that. One is the methane just periodically sublimates underneath the atmosphere and then re-condenses somewhere else, just sort of painting on top of whatever dark stuff that forms,” Grundy says.

7. Pluto data enriches our understanding of Eris.

Pluto data returned from the New Horizons spacecraft give scientists new ideas about the processes that might be at work on Eris. “One of the things the Pluto flyby showed us that nobody really talked about, even in wild speculations, was something like Sputnik Planitia: this big, bright, teardrop-shaped region on the encounter hemisphere. Volatile ices there are trapped in a deep basin and they are just convectively overturning, like a simmering pot of soup,” Grundy says.

That process might be happening writ large on Eris. It might be, in a sense, an ice lava lamp planet. “I’ve called it a Sputnik planet,” Grundy says, “but nature is much more clever than scientists at coming up with new ways of doing things with the same old ingredients. Who knows, we might get there and find out it’s doing something completely different than Pluto was doing to refresh its surface. The real lesson is that activity on a lot of different timescales is possible, even on a tiny little planet that’s at frigid temperatures, far away from the sun.”

8. Its neighborhood is a potential gold mine of information.

In comparative planetology, scientists use planets to understand other planets. By studying Venus, which is similar to Earth in terms of size, mass, and basic composition, scientists can better understand how our planet operates and evolved. The objects in Eris’s celestial neighborhood work the same way. “The Kuiper Belt”—a region rich in rocky and icy objects beyond Neptune’s orbit—“is an incredibly rich environment for comparative planetology because there are so just many of these tiny planets out there,” Grundy says. “It’s going to take a while to discover them all, let alone explore them all, but that’s what is exciting about it.” The New Horizons data from Pluto are helping planetary scientists develop models to tease out the secrets of Eris.

9. Geologists could learn a lot, too.

“If you work out the surface area of, say, objects there that are bigger than 100 kilometers, based on extrapolation, the Kuiper Belt has more solid geology surface area than of all of the planets in the solar system—including the terrestrial planets—combined,” Grundy explains, adding that it holds true even if you wanted to include the ocean floor on Earth. “If you like geology—and especially if you like exotic, cryogenic temperature geology—this is the place to explore, and there’s just so much territory to explore out there.”

10. A mission to Eris will take a while.

It took New Horizons, one of the fastest spacecrafts ever built, nine years to get to Pluto. Eris is currently three times farther from the Sun than Pluto (though due to a highly elliptical orbit, this number changes), so if a mission is ever approved, don’t expect to find out how it all ends. “It takes decades to pull something like that together, so if you want to be around to see the results, you’ve got to start young,” Grundy says. A possible future Kuiper Belt mission might be part of a flyby mission to Uranus or Neptune, after which the spacecraft would continue into that region of space. It will be a very long time before technology allows an Earth-centric telescope—in space or otherwise—to take pictures of the geology of Eris.

How to See the Full Sturgeon Moon on Thursday

Brook Mitchell, Stringer/Getty Images
Brook Mitchell, Stringer/Getty Images

The full moon of every month has a special nickname. Some—like September's harvest moon, December's cold moon, and May's flower moon—have obvious connections to their seasons, while other names are harder to decode. August's sturgeon moon is an example of the latter. It may not be the prettiest lunar title in The Old Farmer's Almanac, but that doesn't mean the event itself on August 15, 2019 won't be a spectacular sight to behold.

What is a Full Sturgeon Moon?

The first (and normally the only) full moon that occurs in August is called a sturgeon moon. The name may have originated with Native American tribes living around the Great Lakes in the Midwest and Lake Champlain in New England. These bodies of water contain lake sturgeon, a species of freshwater fish that grows up to 6.5 feet in length and can live 55 years or longer. August's full moon was dubbed the sturgeon moon to reflect its harvesting season. This full moon is sometimes called the green corn moon, the grain moon, and the blackberry moon for similar reasons.

When to See the Full Sturgeon Moon

On Thursday, August 15, the full sturgeon moon will be highly visible around sunrise and sunset. The satellite will be 99.9 percent illuminated by the sun when it sets Thursday morning at 5:57 a.m EDT—just nine minutes before dawn. On the West Coast, the setting moon will coincide perfectly with the rising sun at 6:15 a.m. PDT.

If you aren't interested in getting out of bed early to catch the sturgeon moon, wait until Thursday evening to look to the horizon. Twenty-seven minutes after sunset, the full moon will rise on the East Coast at 8:21 p.m. EDT. On the West Coast it rises at 8:10 p.m. PDT, 30 minutes after the sun sets.

The moon generally looks bigger and brighter when it's near the horizon, so twilight and dawn are ideal times to catch the spectacle. But it's worth taking another peek at the sky closer to midnight Thursday night; the Perseid meteor shower is currently active, and though the light of the moon may wash them out, you're most likely to spot a shooting star in the late night and early morning hours.

A Full Harvest Moon Is Coming in September

suerob/iStock via Getty Images
suerob/iStock via Getty Images

The Old Farmer's Almanac lists a special name for every month's full moon, from January's wolf moon to December's cold moon. Even if you're just a casual astronomy fan, you've likely heard the name of September's full moon. The harvest moon is the full moon that falls closest to the fall equinox, and it's associated with festivals celebrating the arrival of autumn. Here's what you need to know before catching the event this year.

What is a harvest moon?

You may have heard that the harvest moon is special because it appears larger and darker in the night sky. This may be true depending on what time of night you look at it, but these features are not unique to the harvest moon.

Throughout the year, the moon rises on average 50 minutes later each night than it did the night before. This window shrinks in the days surrounding the fall equinox. In mid-latitudes, the moon will rise over the horizon only 25 minutes to 30 minutes later night after night. This means the moonrise will occur around sunset several evenings in a row.

So what does this mean for the harvest moon? If you're already watching the sunset and you catch the moonrise at the same time, it will appear bigger than usual thanks to something called the moon illusion. It may also take on an orange-y hue because you're gazing at it through the thick filter of the Earth's atmosphere, which absorbs blue light and projects red light. So if you've only seen the full harvest moon around sunset, you may think it always looks especially big and orange, while in reality, any full moon will look that way when it's just above the horizon.

When to See the Harvest Moon

This year, the harvest moon will be visible the night of Saturday, September 14—about a week before the fall equinox on September 23. The moon will reach its fullest state at 12:33 a.m. ET—but if you're still convinced it's not a true harvest moon without that pumpkin-orange color, you can look for it at moonrise at 7:33 p.m. on September 13.

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