Eris: The Forgotten Planet

Eris

When Michael Brown announced on February 28, 2004, the discovery of Sednar, an object believed to be about 2,000 km large orbiting between the Kuiper Belt and the interior of the Oort Cloud, it seemed to everyone that the Solar System had finally found it. His tenth planet. But the affair was short-lived... Sednar's diameter, however attractive the planetoid, was soon reduced and the number of planets returned to what it had been before.

Only 517 days passed, and on July 29, 2005, Mike Brown announced the discovery of yet another massive object, tentatively designated 2003 UB313. The approximate diameter is even 2400 km, slightly larger than Pluto, and at the time, it seemed clear that the newcomer had all the qualifications to be crowned the tenth planet of the solar system, the first found outside the orbit of Pluto! Then it was known how it happened. Not only was Eris—as it was later definitively called—never considered a planet, Pluto also lost its planet status.

So that instead of increasing the number of planets decreases from nine to eight. Pluto, though demoted, was still on everyone's lips and in everyone's heart, but Eris, missing the train that would bring it notoriety, was soon almost forgotten... perhaps because it was considered one of the many planets that would continue in the future Kuiper. Discovered in the belt. Since 2005, indeed, nothing remotely comparable has been found in that remote area.

And Eris is still the ninth largest object to orbit the Sun – the largest discovered since 1847, when Triton, Neptune's largest moon, was discovered. In our opinion, on the other hand, Eris deserves much more attention than it has received so far... we'll probably have to go all the way to the Oort Cloud to find anything bigger... and maybe even there we won't find an object more worthy of the planet title. Let's keep its memory alive and get to know it a little closer, shall we?

Eris Discovery

Eris was discovered by astronomer Michael Brown and his colleagues Chad Trujillo and David Rabinowitz on January 5, 2005, from images taken on October 21, 2003. Why the long delay? How did they notice it sooner? Well, the reason is simple. The team spent several years scanning the sky for the largest trans-Neptunian objects and discovered several of them—such as Quar, Orcus, and Sedna, for example—using automated search software that analyzed all the images taken. This dramatically speeds up the process of object discovery and allows them to survey a wider area of the sky.

However, the team decided that the software would exclude all objects moving at angular speeds greater than 1.5 arcseconds per hour: inconsistent with their estimated distances. To get an idea of how small such a shift is, remember that the Moon's angular diameter is about 1800 arc seconds. But after manually discovering Sedna, a decidedly slow-moving object, Brown and his colleagues realized that the software would miss its presence, so they decided to retest, reanalyzing all the images they had collected in the previous months.

A transneptunian that moved very slowly compared to the background star and thus was ignored in the first stage of the analysis. The discovery was announced on July 29, 2005, the same day as Makemake's discovery and just two days after Haumea's. Further observations were then made to determine Eris' initial orbit, and on September 10, 2005, further analysis with the 10-meter Keck Telescope in Hawaii also revealed the presence of a satellite, later known as Dysnomia.

Dysnomia

Dysnomia is about 700 km in diameter, the second largest of the dwarf planet's moons after Pluto's Charon, and it orbits Eris at a distance of 37,000 km in about 16 days. At that moment, Brown held an object of considerable diameter, even a large moon..

It seemed that nothing could stop Eris from finally claiming the title of the tenth planet. But instead, all hell broke loose, and its discovery essentially marked the beginning of the end for Pluto as well. In 2006, very sadly, Pluto and Eris were given the comforting title of dwarf planets; A division that now includes Ceres, Haumea, and Mecme. Orbit: The highly eccentric and inclined Eris orbits the Sun in a highly elliptical orbit with an orbital period of 559 years.

In fact, its maximum distance from the Sun at aphelion is 98 AU, while its minimum, at perihelion, is only 38 AU. It orbits the Sun, completing one revolution every 25.9 hours. In the past Eris reached perihelion around 1699 and aphelion around 1977; In the future, it will return to perihelion around December 2257. Unlike the eight planets, whose orbits are roughly in the same plane as Earth's, Eris' orbit is highly inclined: about 44 degrees.

This is why, despite its fair brilliance, it was never seen before 2003; In fact, searches for large outer solar system objects tend to focus on the plane of the planets, where most celestial bodies are located. Currently, Eris is the most distant object known in our solar system, if we ignore long-period comets. In theory, the isolated object Sedna, which we met when discussing transneptunian objects, is far from the Sun, with a semi-major axis of about 524 AU. However, currently, Sedna is closer to the Sun along part of its orbit, moving to 84 AU and will reach perihelion around 2075-2076. Due to the high inclination of its orbit, Eris crosses several constellations in the traditional zodiac and is now in the constellation Cetus.

Size Matters!

Since its discovery, the diameter of Eris has been gradually refined and reduced. In 2003, based on images from the Hubble Space Telescope, the diameter of Eris was estimated to be 2397 km. For this, the classical method of using Eris' distance, its albedo and its apparent brightness was used. Albedo is an astronomical term for the reflective power of a celestial body's surface. Minimum albedo is 0 when no fraction of light is reflected.

In the first case one is a completely white object, the other is a completely black object. Eris has an estimated albedo of 0.96, higher than any other large body in the Solar System except Enceladus, Saturn's sixth largest moon. Such high reflectivity is thought to be caused by surface ice, which grows when—as it does now—Eris is in an orbit away from the Sun. Comparing the diameter of Eris to Pluto, which was then estimated to be 2370 km, Eris appeared to be slightly larger than Pluto, considering the best case tolerances.

It is very difficult to tell which object is bigger, Pluto or Eris. Pluto's atmosphere complicates this assessment by interfering with measurements of its hard surface. In 2007, the Spitzer Space Telescope was used to refine the dwarf planet's diameter, and measurements indicated a diameter of 2600 km. This has never been seen before for an object so distant from our solar system. The results cast serious doubt on previous measurements, in particular, the Spitzer Space Telescope. Astronomical data, when Eris passes in front of a star, indicates a diameter of only 2326 km. In contrast, in 2015 the New Horizons probe accurately measured Pluto's diameter at 2376 km.

At present, then, excluding other combinations always possible, Pluto is confirmed as the largest dwarf planet, albeit by a narrow margin compared to Eris.

Mass Matters

But mass matters! Determining the mass of an isolated object like Eris with sufficient approximations is certainly difficult. Fortunately, the discovery of Dysnomia has addressed the root of the problem. In astronomy, knowing a satellite's orbital period and its distance from the parent planet is sufficient to easily calculate the satellite's mass via Kepler's third law... Eris is 27 percent greater than Pluto. From which it follows that the density of Eris must be greater than that of Pluto. In fact, if we consider the diameter determined during stellar acquisition, we get a density of 2.5 grams per cubic centimeter for Eris... Pluto has a density of 1.75 grams per cubic centimeter (for comparison, significantly higher than the density of water) 1 , while our planet averages density 5.5). So Pluto and Eris appear to have the same size but different densities; Their structure must therefore be somewhat different. Eris is probably more rocky and composed of denser material than Pluto. Some astronomers suggest a composition of about 70 percent rock and 30 percent ice. But of course, these are nothing more than guesses.

The Surface and The Atmosphere

The Surface and The Atmosphere of Eris

The surface temperature, due to a distance from the Sun that can vary considerably, varies between -240° and -216°C. Whether telescopes are on the ground or in space, no surface feature can be observed directly from Earth. To learn more, therefore, it is necessary to proceed with indirect measurements such as spectroscopic analysis.

Spectra were obtained on January 25, 2005 at the 8 m Gemini North Telescope in Hawaii, and infrared spectral analysis revealed the presence of methane at the surface. Further analysis indicated an abundance of carbon monoxide and nitrogen. It is similar to Pluto. When Eris approaches its perihelion, sublimation processes will be applied causing gases to escape the surface and form a thin atmosphere.

Then, as Eris exits its perihelion, the surface temperature will continue to decrease until it reaches its lowest aphelion. Falling temperatures will enable re-sublimation, meaning gases in the atmosphere will freeze and return to the surface. Sublimation processes may be possible at this temperature because methane is highly volatile. This, however, raises another question. How can we explain the observed abundance of methane if it is so volatile and sublimation can occur?

Two possibilities are conceivable. First, Eris is always at such a great distance from the Sun that it is cold enough not to trigger sublimation. Second, there may be an internal source for methane that replenishes gas escaping from the surface. We can only notice that its color is different from Pluto and Triton. While Pluto and Triton show a reddish hue, Eris appears to be simply gray like other Kuiper Belt objects.

Future Research

The extreme distance of Eris, which is the largest object not visited by a spacecraft, combined with its small size, makes the dwarf planet difficult to observe without powerful telescopes and advanced technology. A real possibility, which may come to fruition soon, is that Eris could be observed by the James Webb Space Telescope, which is expected to enter its operational phase in these days. Regarding exploration by probes, in the 2010s, there were several studies for follow-on missions to explore the Kuiper Belt, with Eris being evaluated as a candidate.

It was calculated that a flyby mission to Eris using a Jupiter gravity assist would take 25 years, based on a launch date of April 3, 2032 or April 7, 2044. Eris will be 92 or 90 AU from the Sun when the spacecraft arrives. But for now these are just projects. To this day, Eris is still a distant dream for us.