Eris, the largest dwarf planet known, was discovered in an ongoing survey at

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

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This new dwarf planet (see the now out of date "What makes a planet?"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.



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.



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!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 thatobjects 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.

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?



What is the real name going to be?

this part is obviously out of date.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

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.

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?

What else is out there?

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:Whew.

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:



object

Eris 2003 EL61 2005 FY9 discoverers

Brown, Trujillo, Rabinowitz

Brown, Trujillo, Rabinowitz

Brown, Trujillo, Rabinowitz

size

2400 +/- 100 km (105% Pluto)

~3/4 Pluto

~3/4 Pluto

brightness

4th brightest Kuiper belt object (KBO)

3rd brightest KBO

2nd brightest KBO

(note that though we consider Pluto and Eris planets, they are also clearly members of the Kuiper belt, with Pluto the brightest member)

current distance

97 AU

52 AU

52 AU

(an AU is the distance from the earth to the sun)

orbital period

560 years

285 years

307 years

closest approach to sun

38 AU

35 AU

39 AU

furthest from sun

97 AU

52 AU

52 AU

tilt of orbit compared to planets

44 degrees

28 degrees

29 degrees

satellite?

yes!

yes! (two of them!)

no

surface composition

Pluto-like

water ice

Pluto-like

when visible

late summer, fall, early winter

later winter, spring, early summer



Here is where these extremely bright Kuiper belt objects are in the solar system these days:











What is the real story about the hasty announcement and the reports of "hacking"?



In mid-July 2005 short abstracts of scientific talks to be given at a meeting in September became available on the web (for example, here). We intended to talk about the object now known as 2003 EL61, which we had discovered around Christmas of 2004, and the abstracts were designed to whet the appetite of the scientists who were attending the meeting. In these abstracts we call the object a name that our software automatically assigned, K40506A (the first Kuiper belt object we discovered in data from 2004/05/06, May 6th). Using this name turns out to have been a very bad idea on our part! Unbeknownst to us, some of the telescopes that we had been using to study this object kept open records of who has been observing, where they have been observing, and what they have been observing (these detailed records have since been removed from the web). A two-second Google search of "K40506A" immediately reveals one of these observing records. A little playing around with web addresses then reveals even more records not initially Googleable. Ouch. Bad news for us. From the moment the abstracts became public anyone on the planet with a web connection, and a little curiosity about this "K40506A" object, and a knowledge of orbital dynamics could have found out where it was. Anyone on the planet with even a modest-sized telescope could then go find the object and claim a discovery as their own.



According to our web server logs, these observing logs were accessed on July 26, 2005 by a computer at the Instituto de Astrofisica in Spain. Less than two days after this computer accessed the observing logs, the same computer was used to send email officially claiming the discovery by P. Santos-Sanz and J.-L. Ortiz at the Instituto de Astrofisica (see detailed timeline here). At the time of the announcement we truly believed that they had no prior knowledge that we had been observing the object, and we truly believed that they had not used our data to make the announcement of the discovery, but other people found the coincidence suspicious.. Shortly after their announcement, however, we realized that all of our observing records -- including those about what is now known as 2003 UB313, the tenth planet -- were unexpectedly public, and made the decision to prematurely announce the discovery of 2003 UB313 that same afternoon by a press conference. We were unhappy about having to forgo normal scientific protocol and announce the discovery with no corresponding scientific paper, but under the circumstances we felt we had no choice.



It is worth asking: if the observing records were on a publicly accessible web site, is it wrong to look at them? The obvious answer is that there is nothing wrong with looking at information on any publicly accessible web site, just as there is nothing wrong with looking at books in a library. But the standards of scientific ethics are also clear: any information used from another source must be acknowledged and cited. One is not allowed to go to a library, find out about a discovery in a book, and then claim that discovery as your own with no mention of having read it in a book. One is not even allowed to first make a discovery and then go to the library and realize that someone else independently made the same discovery and then not acknowledge what you learned in the library. Such actions would be considered scientifically dishonesty. It is not clear from the timeline precisely what Ortiz and Santos-Sanz knew or how they used the web-based records. They were required by the standards of science, however, to acknowledge their use of our web-based records if they accessed them. The director of the IAA, Dr. Jose Carlos del Toro Iniesta has promised to investigate what precisely happened. We have confidence in Dr. del Toro Iniesta to clarify the situation and determine the appropriate actions.



Some have commented that the real fault here was our own for keeping the objects "secret." We are saddened by anti-scientific statements like these, and have already written extensively on why this rather bizarre accusation is spurious below. The community of scientists condemns scientists who announce their results publicly before publishing scientific papers. Regardless of the number of times these bizarre accusations are repeated, we will continue at all times to adhere to accepted scientific protocol.



Why does it take so long to announce these discoveries?



don't

it is now finally complete! see below

existence

existence

precisely

after

The scientific paper describing the discovery is finally done!

Soon after the announcement of the discovery of the new planet the suggestion slowly made its way around the internet that we, the discoverers, were somehow violating long standing scientific standards by keeping the existence of the planet "secret" for so long. This suggestion seemed so bizarre to us that we paid no attention at first, but, as with many things on the internet, it has been repeated enough times even reasonable people are starting to believe it. We would like to quickly dispell this odd misconception that no credible scientist would hold.One of the things that is so strange about this allegation is that it should also be made of every single scientific result that is published in a reputable scientific journal. In all such cases, scientists make discoveries, they verify their discoveries, they carefully document their discoveries, and they submit papers to scientific journals. What theydo is make discoveries and immediately hold press conferences to announce them (one need only think back to the cold fusion days to remember how thoroughly the scientific community condemns such behavior). Good science is a careful and deliberate process. The time from discovery to announcement in a scientific paper can be a couple of years. For all of our past discoveries, we have described the objects in scientific papers before publicly announcing the objects' existence, and we have made that announcement in under nine months. These papers allow other astronomers to verify, confirm, and critique the analysis we have done. Sadly, because we were forced to announce 2003 UB313 prematurely, we have still yet to complete the scientific paper describing this object (. We find this situation scientifically embarrassing and apologize to our colleagues who are reduced to learning about this new object from reading reports in the press. We are hard at work on this scientific paper, but, as we said above, good science is a careful and deliberate process and we are not yet through with our analysis. Our intent in all cases is to go from discovery to announcement in under nine months. We think that is a pretty fast pace.One could object to the above by noting that theof these objects is never in doubt, so why not just announce theimmediately upon discovery and continue observing to learn more? This way other astronomers could also study the new object. There are two reasons we don't do this. First, we have dedicated a substantial part of our careers to this surveyso that we can discover and have the first crack at studying the large objects in the outer solar system. The discovery itself contains little of scientific interest. Almost all of the science that we are interested in doing comes from studying the object in detaildiscovery. Announcing the existence of the objects and letting other astronomers get the first detailed observations of these objects would ruin the entire scientific point of spending so much effort on our survey. Some have argued that doing things this way "harms science" by not letting others make observations of the objects that we find. It is difficult to understand how a nine month delay in studying an object that no one would even know existed otherwise is in any way harmful to science!Many other types of astronomical surveys are done for precisely the same reasons. Astronomers survey the skies looking for ever higher redshift galaxies. When they find them they study them and write a scientific paper. When the paper comes out other astronomers learn of the distant galaxy and they too study it. Other astronomers cull large databases such as the 2MASS infrared survey to find rare objects like brown dwarves. When they find them they study them and write a scientific paper. When the paper comes out other astronomers learn of the brown dwarves and they study them in perhaps different ways. Still other astronomers look around nearby stars for the elusive signs of directly detectable extrasolar planets. When they find one they study it and write a scientific paper..... You get the point. This is the way that the entire field of astronomy -- and probably all of science -- works. It's a very effective system; people who put in the tremendous effort to find these rare objects are rewarded with getting to be the first to study them scientifically. Astronomers who are unwilling or unable to put in the effort to search for the objects still get to study them after a small delay.There is a second reason that we don't announce objects immediately, and that is because we feel a responsibility not just to our scientific colleagues but to the public. We know that these large objects that keep being found are likely to be the result of intensive interest by the public, and we would like to have the story as complete as possible before making an announcement. Consider, for example, the instantaneous Ortiz et al. announcement of the existence of 2003 EL61. Headlines in places like the BBC web site breathlessly exclaimed "new object may be twice the size of Pluto." But even at the time we knew that 2003 EL61 had a satellite and was only 30% the mass of Pluto. We quickly got the truth out, but just barely. Sadly, other interesting aspects of 2003 EL61 also got lost in the shuffle. No one got to hear that it rotates every 4 hours, faster than anything else known in the Kuiper belt. Or how that fast rotation causes it to be shaped like a cigar. Or how we use the existence of the satellite to calculate the mass. All of these are interesting things that would have let the public learn a bit more about the mysteries of physics and of the solar system. In the press you get one chance to tell the story. In the case of the instantaneous announcement of 2003 EL61 the story was simply "there is a big object out there." We are saddened by the lost opportunity to tell a richer scientific story and to have the public listen for just one day to a tale that included a bit of astronomy, a bit of physics, and a bit of detective story.Given that we do precisely what other astronomers do and that we are actually very prompt about making announcements, where did the crazy ideas that we should be announcing objects instantly come from? Interestingly, there is one area of astronomy in which instantaneous announcement is both expected and beneficial to all. In the study of rare, quickly changing objects, such as supernovae, gamma ray bursts, comets, and near earth asteroids, astronomers quickly disseminate their results so that as many people as possible can study the phenomenon before it disappears or changes completely. No one discovers a comet and keeps it to himself to study, because by the time the study was done the comet would be gone and no one else could study it ever again. The people initially suggesting that we were wrong to not announce our objects instantly are, for the most part, a small group of amateur astronomers who are familiar with comet and near earth asteroid observation protocols. We can only assume that this familiarity led them to their misconceptions. Kuiper belt objects are not quickly changing phenomena. Astronomers will be intensively studying Eris for a long time to come.We hope to discover a few more large objects in the outer solar system. When we do, we will do everything we can to learn as much as possible about them before we make their existence public, and we will try to make the announcement as complete and scientifically and publicly interesting as possible. We will take the chance -- as all scientists do -- that by taking the time to do the scientific job correctly someone else may beat us to the announcement, and if they do we will congratulate them heartily.As described in detail elsewhere, we were forced to announce the existence of Eris before we had finished a scientific paper describing the discovery. While announcing discoveries via press releases with no scientific paper is generally frowned upon by scientists (including us) our colleagues have been understanding of the unusual circumstances under which this happened. The scientific paper describing the discovery has just been submitted to the Astrophysical Journal. If you are curious what one of these papers looks like you can read the entire text. Now that the paper has been submitted to the journal, the journal will send it out for peer-review, where another scientist will carefully and critically read what we have read and help decide if the paper meets accepted scientific standards. In almost all cases, the reviewer will suggest at least some changes to the manuscript before the paper is finally accepted. This process helps ensure that published scientific papers are as accurate and complete as possible.As of now the review process is complete and the revised paper is published in the December 10th 2005 issue of the Astrophysical Journal Letters.You can read this page translated into Romanian here.