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Planetary News: Trans-Neptunian Objects (2007)

Measurement of Eris's Mass Reignites Planetary Debate

By Amir Alexander
June 19, 2007
Eris and it moon Dysnomia as viewed by the Keck observatory
Click to enlarge >
Eris and it moon Dysnomia as viewed by the Keck observatory
Credit: The Keck Observatory

Eris,the giant Kuiper belt object (KBO) named for the Greek goddess of discord, is staying true to its name and once more stirring up the community of planetary scientists. On June 15 Eris discoverer Michael Brown of Caltech and his graduate student Emily Schaller announced that Eris is 27% more massive than Pluto, the previous “king” of the outer reaches of the solar system. “This was Pluto’s last chance to be the biggest thing found so far in the Kuiper belt,” Brown said; “these new results show that it’s second place at best for Pluto.”

Brown’s words were unlikely to calm the storm that has surrounded Eris ever since the object’s measured dimensions were announced in July of 2005, when it was still known by the moniker “2003 UB313.” At around 2400 kilometers (1500 miles), Eris’s diameter is 100 kilometers (60 miles) greater than Pluto’s, a fact that thoroughly undermined the traditional “nine planet” picture of our solar system. Clearly, if Pluto is a planet, then the larger Eris must be one too, and other giant KBO’s should be considered strong candidates for the same status. Such a designation would effectively replace the nine Solar planets with ten, with more objects likely to join the family soon. But if Eris is not a planet, then the smaller Pluto should not be one either, and the solar system has only eight planets at the most.

The debate between the two views came to a head at the International Astronomical Union (IAU) meeting which took place in Prague in August 2006. On the one side were those who wanted to preserve Pluto’s planetary status, and advocated an open-ended number of planets in the solar system, including both Pluto and Eris. On the other side were those who wished to maintain a clear distinction between “true” planets and the many smaller objects in the solar system. According to them there are only eight planets in the solar system, and the rest – including Pluto and Eris – should receive other designations.

In the end, after a tumultuous meeting, those championing an exclusive club of eight planets came out on top. A planet, according to the final decision, must not only be spherical (i.e. in “hydrostatic equilibrium”) and orbiting the Sun, but should also have cleared its region of space from smaller orbiting objects. Pluto and Eris, which orbit in the debris-rich Kuiper belt, fail this latter criterion, and are consequently relegated to a newly defined category of “dwarf planets.” The only true planets of the solar system according to the IAU vote are the eight traditional ones –Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Many astronomers nevertheless remain uneasy about the result of the IAU meeting, and one of the chief critics is Alan Stern of NASA headquarters, who is Principal Investigator on the New Horizons mission, currently on its way to Pluto and the Kuiper belt. Stern argues that the IAU definition was contrived to artificially limit the number of planets in the solar system. “Arbitrarily limiting the number of planets makes no scientific sense” he said. “We don’t have upper limits for the number of mountains, the number of rivers, or the number of stars – why should we have a limit for the number of planets?” A proper definition of an astronomical object, he argued, should hold true regardless of how many such objects there are or where they are located. The fact that Eris and Pluto happen to be located in the Kuiper belt, Stern said, should not exclude them from Planetary status.

“We don’t have upper limits for the number of mountains, the number of rivers, or the number of stars – why should we have a limit for the number of planets?”

--Alan Stern

Stern also took issue with the IAU’s authority to decide scientific matters by vote, saying that “scientific issues should be decided on scientific grounds.” “Voting on science sends the wrong message to the public” he added, “by presenting scientific truth as an arbitrary matter that can be decided by majority opinion.” In accordance with Stern’s position, the New Horizons team continue to refer to their spacecraft’s destination as “the ninth planet.”

But as the debate raged on over whether Pluto and Eris should count as true planets, the scientific investigation of these amazing objects at the far edge of the solar system continued apace. Whereas Eris’s size had been determined through direct measurement of telescopic images, its mass was obtained by observing the orbit of its moon, Dysnomia. In Newtonian celestial mechanics the mass of a massive object, such as the Earth or the Sun, can be determined from the orbital characteristics of smaller objects circling it. For example, the mass of the Sun can be calculated from the fact that Earth orbits it every 365 days at an average distance of 150 million kilometers (93 million miles). Similarly, if Dysnomia’s orbital period and average distance from Eris were known, then Eris’s mass would easily follow.

To obtain data on Dysnomia’s orbit Brown made use of the most powerful telescopes available. On December 3, 2005, and again on August 30, 2006, the Hubble space telescope pointed its lens at Eris and took several images; at the same time, on four nights in late August, 2006, Brown had images of Eris taken with the giant Keck telescope in Hawaii, using corrective optics system to compensate for atmospheric interference. Taken together the observations of Hubble and Keck indicated that Dysnomia orbits Eris every 16 days in a near circular orbit, at an average distance of 37,000 kilometers (23,000 miles).

Eris and Dysnomia as viewed by the Hubble Space Telescope
Click to enlarge >
Eris and Dysnomia as viewed by the Hubble Space Telescope
The image was taken on August 30, 2006. Credit: NASA, ESA, and M. Brown (California Institute of Technology)

From this, Brown and Schaller could calculate Eris’s mass, which comes to 16.6 billion trillion kilograms (36.9 billion trillion pounds), or 1.27 times the mass of Pluto. From this they calculated Eris’s density as 2.3 grams per centimeter cube, which is close to Pluto’s density of 2.03 grams per centimeter cube and also to that of other large KBO’s.

According to Brown and Schaller, Dysnomia’s near circular orbit strongly suggests that it was created by a massive impact on the body of Eris, probably in the early days of the solar system. Other moons with near circular orbits, such as the Earth’s moon and Pluto’s Charon were also created when a massive impact threw large amounts of debris into orbit, which ultimately coalesced to form a moon. The other possibility is that Dysnomia was originally an independent KBO that was captured by Eris, but such captured moons typically have highly elongated, rather than circular, orbits.

While emphasizing that Eris is both larger and more massive than Pluto, Brown is also quick to point out the strong similarities between the two. With a near-identical diameter, similar density, and a moon created by ancient impact, the two dwarf planets are as similar as any two objects in the solar system. “Pluto and Eris are essentially twins” said Brown, “except that Eris is slightly the pudgier of the two.”

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