Eris, the forgotten planet
When Michael Brown announced on Feb. 28, 2004, the discovery of Sedna, an object believed to be
nearly 2,000 km large orbiting between the Kuiper belt and the innermost part of the Oort Cloud,
it seemed to everyone that the solar system had finally found its "tenth planet."
But the thing lasted very little... Sedna’s diameter, however interesting planetoid, was
soon reduced and the count of the planets returned to be the same as before.
Only 517 days passed, and on July 29, 2005, still Mike Brown announced the discovery of another
large object, provisionally designated 2003 UB313. The estimated diameter was even 2400 km, slightly
larger than Pluto's, and at that point, it seemed evident that the newcomer
had all the qualifications to be crowned "the tenth planet in the solar system"....
the first one found beyond Pluto's orbit! Then it became known how the thing turned out.
Not only was Eris-as it was later definitively called-never considered a "planet," but Pluto
also lost its planetary status...so that the planets instead of increasing in number,
even decreased, from nine to eight. Pluto, despite the downgrade, still
remained on everyone's lips and in everyone's hearts, but Eris, having missed the train
that would bring it notoriety was soon almost forgotten... probably because it was considered
just one of many planetoids that would continue to be discovered in the Kuiper belt in the future.
Wong prediction... Since 2005, in fact, nothing remotely comparable has been found in those
remote regions. And Eris is still to this day the ninth most massive object to revolve around the
Sun--the largest ever discovered since 1847, when Triton, Neptune's great moon, was found.
In our opinion, Eris, on the other hand, deserves much more attention than it has received so far...
To find something even bigger will probably have to go as far as the Oort Cloud...and
perhaps even there we will not find an object more deserving of the title of "planet".
Let's keep its memory alive and get to know it a little closer, shall we?
“Roll intro”
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 come they didn't notice it sooner?
Well, the reason is simple. The team had been systematically
scanning the sky for the largest trans-Neptunian objects for several years and had discovered
several of them-such as Quaoar, Orcus, and Sedna, for example-using automatic search software that
analyzed all the images taken. This dramatically accelerated the process of object discovery
and allowed them to cover a wider region of the sky with their survey.
However, the team had decided to have the software exclude all objects moving at an angular speed
greater than 1.5 arcseconds per hour: not compatible with their presumed distance.
To get an idea of how small such a shift is, keep in mind that the angular diameter
of the Moon is about 1800 arc seconds. But after manually discovering Sedna,
a decidedly slow-moving object, Brown and his colleagues realized that the software
would ignore its presence, so they decided to reexamine to reanalyze all the images
they had collected over the preceding months. And it was only in this way that they were able
to find the object that would later become Eris. A transneptunian that moved very slowly relative
to the background stars and had thus been ignored during the first phase of analysis.
The discovery was announced on July 29, 2005, the same day as Makemake's
discovery and only two days after Haumea's. Further observations were then made to determine
the preliminary orbit of Eris, and on September 10, 2005, a subsequent analysis with the 10-meter
Keck telescope in Hawaii also revealed the presence of a satellite, later named Dysnomia.
Dysnomia is about 700 km in diameter, making it the second-largest of the dwarf planet moons
after Pluto's Charon, and it orbits Eris in nearly 16 days at a distance of 37,000 km.
At that point, Brown was holding an object of considerable diameter,
accompanied even by a large moon... It seemed that nothing could stop Eris from
finally claiming the title of "tenth planet..." But instead, all hell broke loose,
and its discovery basically marked the beginning of the end for Pluto as well.
During 2006, very sadly, Pluto and Eris were given the consolatory title of "dwarf planet";
a category that now also includes Ceres, Haumea, and Makemake.
The orbit: very eccentric and inclined
Eris orbits the Sun in a very 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. As it orbits the Sun, it completes one rotation every 25.9 hours.
In the past Eris arrived at perihelion around the year 1699, and at aphelion around 1977;
in the future, it will return to perihelion around December 2257.
Unlike the eight planets, whose orbits all lie roughly in the same plane as Earth's,
Eris' orbit is highly inclined: about 44 degrees.
This is why, despite its fair brightness, it had never been sighted before 2003;
in fact, searches for large outer Solar System objects tend to focus on the plane
of the ecliptic, where most celestial bodies lie. Currently, Eris is the most distant object known
in our Solar System, if we disregard long-period comets. In theory, the detached object Sedna,
which we met while discussing transneptunian objects, is farther away from the Sun,
having a semi-major axis of about 524 AU. However, currently, Sedna is moving along a part of its
orbit closer to the Sun, at 84 AU, and will arrive at perihelion around 2075-2076.
Due to the high inclination of its orbit, Eris crosses only a few constellations
of the traditional Zodiac and is now in the constellation of Cetus.
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Size matters! Since its discovery,
Eris' diameter has been gradually refined and reduced. In 2003, considering images of
the Hubble Space Telescope, Eris' diameter was estimated to be 2397 km. For this, the
classical approach of using the distance to Eris, its albedo, and its apparent brightness was used.
Albedo is a term in astronomy for the reflective power of a celestial body's surface.
The maximum albedo is 1, when all incident light is reflected. The minimum albedo is 0
when no fraction of the light is reflected. The first case is that of a perfectly white object,
the other of a perfectly black object. Eris has an estimated albedo of 0.96, higher
than any other large body in the Solar System except Saturn's sixth-largest moon, Enceladus.
Such high reflectivity is hypothesized to be due to surface ices,
which are on the rise when-as at this time-Eris is in an orbital position far from the Sun.
Comparing the diameter of Eris with that of Pluto, which at the time was estimated at 2370
km, Eris appeared to be slightly larger than Pluto considering the best case of tolerances.
It is very difficult to say which object is larger, Pluto or Eris. Pluto's atmosphere further
complicates this assessment which interferes with measurements of its solid surface.
In 2007, the Spitzer Space Telescope was used to refine the dwarf planet's diameter, and the
measurements indicated a diameter of 2600 km. In 2010, a rare event occurred, a stellar
occultation. Never before had an occultation for such a distant object in our solar system
been observed. The results cast serious doubts on previous measurements,
in particular, those of the Spitzer Space Telescope. Data obtained from
the occultation during which Eris passed in front of a star, indicated a diameter of only 2326 km.
By contrast, in 2015 the New Horizons probe accurately measured Pluto's diameter to 2376 km.
At present, then, barring other adjustments that are always possible,
Pluto is confirmed as the largest dwarf planet, albeit by a very small margin compared to Eris.
But mass also matters! Determining the mass of an
isolated object like Eris with sufficient approximation is definitely difficult.
Fortunately, the discovery of Dysnomia has solved the problem at its root.
In astronomy, it is indeed sufficient to know the orbital period of a satellite and its distance
from the parent planet to easily determine the mass of the latter through Kepler's third law...
A simple calculation then made it possible to find that the mass of Eris
is 27 percent greater than that of Pluto. From which it followed that the density of
Eris must be greater than that of Pluto. In fact, if we take into account the diameter
determined during the stellar occultation, we obtain for Eris a density of 2.5 grams
per cubic centimeter ... significantly higher than Pluto's density of 1.75 grams per cubic centimeter
(for comparison, the density of water is 1, while the average density of our planet is 5.5).
Pluto and Eris, therefore, seemed to have similar sizes but different densities; their composition
must therefore have been somewhat different. Eris is probably composed of much rockier
and denser material than Pluto. Some astronomers suggest a composition
of about 70 percent rock and 30 percent ice. But of course, these are only guesses.
The surface and the atmosphere The temperature of the surface, as a
result of a distance from the Sun that can vary considerably, varies between -240° and -216°C.
No surface features can be observed directly from Earth, regardless of whether the telescopes
are on the ground or in space. To know more, it is, therefore, necessary to proceed with
indirect measurements, such as spectral analysis. On January 25, 2005 spectra were obtained
at the 8 meters Gemini North Telescope in Hawaii, and analysis of the infrared spectrum
revealed the presence of methane on the surface. Further analysis indicated abundances of carbon
monoxide and nitrogen. This is similar to Pluto. When Eris approaches its perihelion, sublimation
processes will be enforced causing gases to escape from the surface and forming a thin atmosphere.
Then, when Eris is departing from its perihelion, the surface temperature will continuously decrease
until it reaches its minimum at the aphelion. The falling temperatures will enable re-sublimation,
i.e. the gases in the atmosphere will freeze out and fall back onto the surface.
Sublimation processes may be possible at these temperatures as methane is highly volatile.
This, however, raises another question. How can we explain the observed abundances of methane
if it is so volatile and sublimation may occur? Two possibilities are conceivable. First,
Eris has always resided at such large distances to the Sun where it is cold enough
not to trigger sublimation. Secondly, there may be an internal source for methane which
replenishes the gas escaping the surface. No further details are known about Eris’ surface.
We can only observe that its color is different from Pluto and Triton. Whereas Pluto and Triton
show a reddish color, Eris appears to be simply grey like many other objects of the Kuiper Belt.
Future research The extreme distance of Eris, which
is the largest object that has not been visited by a spacecraft, combined with its small size, makes
it difficult to observe the dwarf planet without powerful telescopes and advanced technology.
One real possibility, which could come to fruition soon, is that Eris could be observed by the James
Webb Space Telescope, which is expected to enter its operational phase these days.
Regarding exploration by probes, In the 2010s, there were multiple studies for
follow-on missions to explore the Kuiper belt, among which Eris was evaluated as a candidate.
It was calculated that a flyby mission to Eris would take 25 years using a Jupiter gravity
assist, based on launch dates of April 3, 2032, or April 7, 2044. Eris would 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. And probably it will still be for many years.