The discovery of 2003 UB313 Eris, the 10th planet largest known dwarf planet

Discovery images of the dwarf planet Eris. The three images were taken 1 1/2 hours apart on the night of October 21st, 2003.

The Eris can be seen very slowly moving across the sky over the course of 3 hours.

Eris, the largest dwarf planet known, was discovered in an ongoing survey at Palomar Observatory's Samuel Oschin telescope by astronomers Mike Brown (Caltech), Chad Trujillo (Gemini Observatory), and David Rabinowitz (Yale University). We officially suggested the name on 6 September 2006, and it was accepted and announced on 13 September 2006. In Greek mythology, Eris is the goddess of warfare and strife. She stirs up jealousy and envy to cause fighting and anger among men. At the wedding of Peleus and Thetis, the parents of the Greek hero Achilles, all the gods with the exception of Eris were invited, and, enraged at her exclusion, she spitefully caused a quarrel among the goddesses that led to the Trojan war. In the astronomical world, Eris s tirred up a great deal of trouble among the international astronomical community when the question of its proper designation led to a raucous meeting of the IAU in Prague. At the end of the conference, IAU members voted to demote Pluto and Eris to dwarf-planet status, leaving the solar system with only eight planets .

The satellite of Eris has received the offical name Dysnomia, who in Greek mythology is Eris' daughter and the demon spirit of lawlessness. As Dysnomia is a bit of a mouthful, we tend to simply call the satellite Dy, for short.

As promised for the past year, the name Xena (and satellite Gabrielle) were simply placeholders while awaiting the IAU's decision on how an official name was to be proposed. As that process dragged on, however, many people got to know Xena and Gabrielle as the real names of these objects and are sad to see them change. We admit to some sadness ourselves. We used the names for almost two years now and are having a hard time swtiching. But for those who miss Xena, look for the obvious nod in the new name of the moon of Eris.

Artists concept of the view from Eris with Dysnomia in the background, looking back towards the distant sun. Credit: Robert Hurt (IPAC)

What is it?

This new dwarf planet (see the now out of date "What makes a planet? " below) is the largest object found in orbit around the sun since the discovery of Neptune and its moon Triton in 1846. It is larger than Pluto, discovered in 1930. Like Pluto, the new dwarf planet is a member of the Kuiper belt, a swarm of icy bodies beyond Neptune in orbit around the sun. Until this discovery Pluto was frequently described as "the largest Kuiper belt object" in addition to being a dwarf planet. Pluto is now the second largest Kuiper belt object, while this is the largest currently known.

Where is it?

The dwarf planet is the most distant object ever seen in orbit around the sun, even more distant than Sedna. the planetoid discovered almost 2 years ago. It is almost 10 billion miles from the sun and more than 3 times more distant than the next closest planet, Pluto and takes more than twice as long to orbit the sun as Pluto.

A view of the solar system from the north down. The four circles show the orbits of Jupiter, Saturn, Uranus, and Neptune. The yellow dot in the center is the sun. The earth, if it were shown, would be inside the yellow dot representing the sun. The orbits of the two outermost planets, along with their current positions, are also shown. If you are worried because the sun appears to not be the focus of the orbital ellipse you are very observant! But it is just a projection effect. The see the full 3D orbit go to this very nice web page

The dwarf planet can be seen using very high-end amateur equipment, but you need to know where to look. The best way to find precise coordinates (of this planet, or any other body in the solar system) is with JPL's horizons system. Click on "select target" and then enter "2003 UB313" under small bodies.

The orbit of the new dwarf planet is even more eccentric than that of Pluto. Pluto moves from 30 to 50 times the sun-earth distance over its 250 year orbit, while the new planet moves from 38 to 97 times the sun-earth distance over its 560 year orbit.

How big is it?

Usually when we first discover distant objects in the outer solar system we don't know for sure how large they are. Why not? Because all we see is a dot of light, like the picture at the top of the page. This dot of light is sunlight reflected off the surface of the planet (interestingly the sunlight takes almost a day to get out to the planet, reflect off of it, and get back to the earth!), but we don't know if the object is bright because it is large or if it is bright because it is highly reflective or both.

When an object is too far away to directly see how big it is, astronomers use an indirect method instead where they measure the heat coming from the object. If we wanted to measure the size of a fire, for example, we could do it by measuring the total amount of heat coming from the fire. The temperature of the flames in a match and a bonfire are essentially the same, but a bonfire emits much more heat because it is much bigger. The same is true of distant planets. Because we know how far away the planet is we have a pretty good idea of the surface temperature (a frosty 405 degrees below zero!), thus when we measure the total heat we can tell how big the object is. Unfortunately, the new planet is so far away and so cold that our first attempt at measuring the heat, using the Spitzer Space Telescope, could not detect the heat output. This fact tells us that the object must be smaller than about 3300 km.

In the meantime, observations have been made by a group from the University of Bonn from the 30-meter IRAM telescope. This telescope, like Spitzer, measures the heat output. IRAM measures the heat output in a region of the spectrum where much less heat is given off, but IRAM is a much larger telescope than Spitzer. The observations were successful in finally detecting the heat of Eris. From the amount of heat measured they determined that Eris has a diameter of 3000 +/- 400 km. A very nice discussion of the measurement and what the uncertainties mean can be found at the press release web page.

The newest size measurement comes from the Hubble Space Telescope. While for most telescopes the planet is too small to be seen as anything other than a dot of light, HST can (just barely) directly measure how big across it is. The measurement is extremely hard, however, even for HST, because even HST distorts light a little bit as it goes through the telescope, and we needed to be sure that we were measuring the actual size of the planet, rather than being fooled by distortion. So we waited until Eris was very close to a star and then snapped a series of 28 pictures and carefully went back and forth comparing the star and the planet. In the end, we determined that Eris t is 2400 +/- 100 km across.

The best ever picture from the Hubble Space Telescope, as unimpressive as it is (since Eris is so so so so so far away) looks like this:

When we initially guessed how big Erist was, we thought it was likely a bit larger, because we guessed that it probably reflected the same amount of sunlight as Pluto (about 60%). But this new size measurement tells us that the planet reflects considerably more sunlight than Pluto (86 +/- 7%). For more on this see below on what Eris is made out of.

The new HST measurement makes it sound like the previous measurement was "wrong," but it was not! All measurements in science are subject to uncertainty, and the group from Bonn carefully stated what their uncertainty was, just as we have with the new measurement. The

What is on the surface of Eris?

We study the composition of distant objects by looking at sunlight reflected off of them. The sunlight reflected off the surface of the earth, for example, shows distinct signatures of the oxygen in earth's atmosphere, of photosynthetic plants, and of abundant water, among other things. We have been using the Gemini Observatory on Mauna Kea, Hawaii to study the light reflected from the surface of Eris, and have found that the dwarf planet looks remarkably similar to Pluto. A comparison of the two is shown below, where we show the amount of sunlight reflected in near infrared light. This type of light, just beyond what is visible to the human eye, is most sensitive to the types of ices expected on surfaces in the outer solar system.

The plot above compares the amount of infrared sunlight of different colors ("wavelength") reflected from the new planet with the amount of sunlight reflected from Pluto. The dips in the amount of sunlight at 1.15, 1.35, 1.7, and 2.3 um are a characteristic signature of a surface covered with solid frozen methane (natural gas). Both Pluto and Eris show these signatures. At the very low temperatures of Pluto and Eris, methane, which is in gaseous form on the earth, is frozen solid. The interior of Eris, like the interior of Pluto, is likely a mixture of rock and ice.

Pluto and the new dwarf planet are not completely identical, however. While Pluto's surface is moderately red, the new dwarf planet appears almost white, and while Pluto has a mottled-looking surface which reflects on average 60% of the sunlight which hits it, the new planet appears essentially uniform and reflects 86% (+/- 7%) of the light that hits it. These characteristics were not at all expected. In fact, Eris reflects more sunlight from its surface than any body in the solar system other than Saturn's moon Enceladus. which has active geysers continuously coating the surface in fresh frost. We can't think of any source of heat for Eris that would cause similar geysers. So what is happening?

We think that the bright surface and uniform white coloring of the planet both have the same cause. Right now the planet is as far away from the sun as it ever gets, and thus as cold as it ever gets. At this distance from the sun even the planet's atmosphere is frozen solid. (In fact if the earth were brought that far away from the sun its atmosphere would freeze solid, too!). In 280 years the planet will be the closest it ever gets -- a factor of almost 2.6 times closer. The absolute temperature on the planet will rise over the next 280 years by a factor of 1.6 (which is the square root of 2.6). The current temperature of (a quite cold) 405 degrees below zero will be but a distant memory at this point when the temperatures will be a balmy 360 degrees below zero. While both of these temperatures seem frigid beyond imagination, to methane and nitrogen (the likely components of the atmosphere of the planet), the difference between the two is the difference between frozen solid and evaporating into the atmosphere.

In this hypothesis, then, Eris is bright and uniform because the atmosphere that it used to have (280 years ago at its peak) is now frozen solid to the ground, giving a bright shining coating to whatever type of mottled surface used to be there. The whole atmosphere is now probably only a few inches thick.

This whole process repeats itself over and over and over with the dwarf planet's orbital period of 580 years.

For comparison, the relative temperature change on the new planet is equivalent to the earth's average temperature changing

from about 60 degrees F to about 360 degrees F ever 6 months. No other planet in the solar system -- dwarf or otherwise -- goes through temperature swings nearly as extreme as this!

What is Eris made out of?

While we can only see the surface of the dwarf planet, we have some educated guesses about the interior. Pluto, we know, has a density about midway between ice and rock, thus we think that it is made of about half and half ice and rock on the inside. The new planet, being about the same size and the same surface composition as Pluto, is probably close to the same. We used to suspect that all objects out in the Kuiper belt are the same on the inside but recent measurements suggest a very wide variety! For this reason, we are quite anxious to measure the actual density of the planet itself. Such a measurement is possible by measuring the mass of the planet by looking at the way its moon goes around it and then dividing this mass by the volume (which we know because we know the size). We need more observations of the moon to accurately determine its orbit, however, so we don't think we will know the answer until the end of the year.

How was Eris found?

We have been conducting an ongoing survey of the outer solar system using the Palomar QUEST camera and the Samuel Oschin Telescope at Palomar Observatory in Southern California. This survey has been operating since the fall of 2001, with the switch to the QUEST camera happening in the summer of 2003. To date we have found around 80 bright Kuiper belt objects.

To find objects, we take three pictures of a small region of the night sky over three hours and look for something that moves. The many billions of stars and galaxies visible in the sky appear stationary, while satellites, planets, asteroids, and comets appear to move. The image below shows the three frames taken the night of October 21st, 2003 where we found the new planet. Can you find the moving object?

The area of sky shown here is approximately 0.015% of the amount of sky that we look at every night, but even though we survey vast regions of the sky per night, it is still going to take us about 5 years to look at all of the sky visible from Palomar Observatory.

Happily for us (and our families) much of the work is done by computers. The telescope is robotically controlled and sends its data to Pasadena every morning where it is searched by a bank of 10 computers at Caltech. Each morning the computers find approximately 100 potentially-moving objects that a human has to look at. The vast majority are some flaw in the camera and are not real solar system objects, but, occasionally, as seen above, a real object makes its presence known.

Because the new dwarf planet is so far away it is moving slower than most of the objects that we find. It is moving so slowly, in fact, that our computers didn't notice it the first time around! We began a special reanalysis a year later to specifically look for very distant objects. This reanalysis found the new planet at 11:20AM PST on January 5th 2005, almost 1 1/2 years after the initial data were obtained. Note that initial reports suggested that the discovery date was January 8th. We apologize for the mistake; it was caused because of the craziness surrounding the first day of announcement. We didn't have time to check our notes and apparently our memories are not as good as they used to be.

What is the real name going to be?

this part is obviously out of date. the answer to the question? Eris

When a new object is discovered the International Astronomical Union (IAU) gives it a temporary designation based on the date it was first seen. Thus 2003 UB313 can be decoded to tell you that the data from which the object was discovered was obtained in the second half of October 2003. Next, depending on what the object is, the discoverers propose a certain type of permanent name.

Interestingly, there are no actual rules for how to name a planet (presumably because no one expected there to be more). All of the other planets are named for Greek or Roman gods, so an obvious suggestion is to attempt to find such a name for the new planet. Unfortunately, most of the Greek or Roman god names (particularly those associated with creation, which tend to be the major gods) were used back when the first asteroids were being discovered. If a name is already taken by an asteroid, the IAU would not allow that name to be used again. One such particularly apt name would have been Persephone. In Greek mythology Persephone is the (forcibly abducted) wife of Hades (Roman Pluto) who spends six months each year underground close to Hades. The new planet is on an orbit that could be described in similar terms; half of the time it is in the vicinity of Pluto and half of the time much further away. Sadly, the name Persephone was used in 1895 as a name for the 399th known asteroid. The perhaps more appropriate Roman version of the name, Proserpina, was used even earlier for the 26th known asteroid. The same story can be told for almost any other Greek or Roman god of any consequence. One exception to this name depletion is the Roman god Vulcan (Greek Haphaestus), the god of fire. Astronomers have long reserved that term, however, for a once hypothetical (now known to be nonexistent) planet closer to the sun than Mercury (god of fire, near the sun, good name). We would not want to use such a name to describe such a cold body as our new planet!

Is this object really a planet or a dwarf planet? Is Pluto a planet? What makes a planet?

note that all of this is out of date as of August 2006!

Even after all of these years of debate on the subject of whether or not Pluto should be considered a planet, astronomers appear no closer to agreement. I wrote extensively about this at the time of the discovery of Sedna in March 2004. My thoughts have evolved since then, so it might be amusing to see what I said 1 1/2 years ago. I have been heavily influenced by writing a scientific review article this summer on the topic of "What is a planet?" with my colleague Gibor Basri at U.C. Berkeley who I thank for his insights. The main stumbling block in defining planets in our solar system is that, scientifically, it is quite clear that Pluto should certainly not be put in the same category as the other planets. Some astronomers have rather desperately attempted to concoct solutions which keep Pluto a planet, but none of these are at all satisfactory, as they also require calling dozens of other objects planets. While people are perhaps prepared to go from 9 to 10 planets when something previously unknown is discovered, it seems unlikely that many people would be happy if astronomers suddenly said "we just decided, in fact, that there are 23 planets, and we decided to let you know right now." There is no good scientific way to keep Pluto a planet without doing serious disservice to the remainder of the solar system.

Culturally, however, the idea that Pluto is a planet is enshrined in a million different ways, from plastic placemats depicting the solar system that include the nine planets, to official NASA web sites, to mnemonics that all school children learn to keep the nine planets straight, to U.S. postage stamps. Our culture has fully embraced the idea that Pluto is a planet and also fully embraced the idea that things like large asteroids and large Kuiper belt objects are not planets.

In my view scientists should not be trying to legislate an entirely new definition of the word "planet." They should be trying to determine what it means. To the vast majority of society, "planet" means those large objects we call Mercury through Pluto. We are then left with two cultural choices. (1) Draw the line at Pluto and say there are no more planets; or (2) Draw the line at Pluto and say only things bigger are planets. Both would be culturally acceptable, but to me only the second makes sense for what I think we mean when we say the word planet. In addition, the second continues to allow the possibility that exploration will find a few more planets, which is a much more exciting prospect than that suggested by the first possibility. We don't think the number of planets found by the current generation of researchers will be large. Maybe one or two more. But we think that letting future generations still have a shot at planet-finding is nice.

Astronomers tend to dislike this solution as it is clearly non-scientific. The best analogy I can come up with, though, is with the definition of the word "continent." The word sound like it should have some scientific definition, but clearly there is no way to construct a definition that somehow gets the 7 things we call continents to be singled out. Why is Europe called a separate continent? Only because of culture. You will never hear geologists engaged in a debate about the meaning of the word "continent" though. When geologists talk about the earth and its land masses they define precisely what they are talking about; they say "continental crust" or "continental drift" or "continental plates" but almost never "continent." Astronomers need to learn something from the geologists here and realize that there are a few things -- like continents and planets -- to which people have large emotional attachments, and they should not try to quash that attachment.

Thus, we declare that the new object, with a size larger than Pluto, is indeed a planet. A cultural planet, a historical planet. I will not argue that it is a scientific planet, because there is no good scientific definition which fits our solar system and our culture, and I have decided to let culture win this one. We scientists will continue our debates, but I hope we are generally ignored.

How was the planetary status be decided?

The above gives my personal view on how to resolve the planetary status. The official decision will come from the International Astronomical Union. We had hoped for a timely decision but we instead appear to be stuck in committee limbo. Here is the story, as best I can reconstruct it from the hints and rumors that I hear:

  • A special committee of the International Astronomical Union (IAU) was charged with determining "what is a planet."
  • Sometime around the end of 2005, this committee voted by a narrow margin for the "pluto and everything bigger" definition, or something close to it.
  • The exectutive committee of the IAU then decided to ask the Division of Planetary Sciences (DPS) of the American Astronomical Society to make a reccomendation.
  • The DPS asked their committee to look in to it.
  • The DPS committee decided to form a special committee.
  • The IAU decided to that it no longer wanted the DPS to look at the question
  • Nothing happened for a long time
  • During the summer of 2006 the IAU made a new committee that met for 2 days in Paris and came up with the "everything round is a planet" definition
  • The definition was met with heated opposition at the IAU General Assembly in Prague
  • The strict eight planet definition was agreed upon

What else is out there?

The last week of July 2005 was an exciting one for the outer solar system. In the course of two days the existence of three new objects was announced, and each object was brighter than all of the previously known objects in the Kuiper belt (with the exception of Pluto). With so many bright objects coming out at once it is hard to keep them all straight. Here is the quick score card:


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