Object Bigger than Pluto Discovered, Called 10th Planet

Jan 27, 2020
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Eris.jpeg
Artist's rendering of Eris, announced in July 2005 by Mike Brown of Caltech. It is more massive than Pluto. The sun is in the background. (Image credit: NASA/JPL/Caltech)

By Robert Roy Britt
July 29, 2005

Astronomers have discovered an object in our solar system that is larger than Pluto. They are calling it the 10th planet, but already that claim is contested.

The new world's size is not at issue. But the very definition of planethood is.

Announcement made in haste after discoverer's web site hacked

If it's a planet, it is not No. 10, other astronomers say

Next up: Mars-sized objects?

Amateur astronomers can observe 2003 UB313

It is the first time an object so big has been found in our solar system since the discovery of Pluto 75 years ago.

The announcement, made today by Mike Brown of Caltech, came just hours after another newfound object, one slightly smaller than Pluto, was revealed in a very confusing day for astronomers and the media.

The new object, temporarily named 2003 UB313, is about three times as far from the Sun as is Pluto.

"It's definitely bigger than Pluto," said Brown, a professor of planetary astronomy. The object is round and could be up to twice as large as Pluto, Brown told reporters in a hastily called NASA-run teleconference Friday evening.

His best estimate is that it is 2,100 miles wide, about 1-1/2 times the diameter of Pluto.

One of many?

The object is inclined by a whopping 45 degrees to the main plane of the solar system, where most of the other planets orbit. That's why it eluded discovery: nobody was looking there until now, Brown said.

Some astronomers view it as a Kuiper Belt object and not a planet. The Kuiper Belt is a region of frozen objects beyond Neptune.

Pluto is called a Kuiper Belt object (KBO) by many astronomers. Brown himself has argued in the past for Pluto's demotion from planet status, because of its diminutive size and eccentric and inclined orbit.

But today he struck a different note.

"Pluto has been a planet for so long that the world is comfortable with that," Brown said in the teleconference. "It seems to me a logical extension that anything bigger than Pluto and farther out is a planet."

Offering additional justification, Brown said 2003 UB313 appears to be surfaced with methane ice, as is Pluto. That's not the case with other large Kuiper Belt objects, however.

"This object is in a class very much like Pluto," he said.

NASA effectively endorsed the idea in an official statement that referred to 2003 UB313 as the 10th planet.

Yet in recent years, a bevy of objects roughly half to three-fourths the size of Pluto have been found.

No definition for 'planet'

Brian Marsden, who runs the Minor Planet Center where data on objects like this are collected, says that if Pluto is a planet, then other round objects nearly as large as Pluto ought to be called planets. On that logic, 2003 UB313 would perhaps be a planet, but it would have to get in line behind a handful of others that were discovered previo

"I would not call it the 10th planet," Marsden told SPACE.com.

Alan Boss is a planet-formation theorist and an observational astronomer at the Carnegie institution for Space.

"To just call them planets does an injustice to the big guys in the solar system," Boss said in a telephone interview.

The very definition of what constitutes a planet is currently being debated by Boss and others in a working group of the International Astronomical Union. Boss said the group has not reached consensus after six months of discussion.

The debate actually stretches back more than five years and is rooted in the fact that astronomers have never had a definition for the word "planet," because the nine we knew seemed obvious.

"This discovery will likely re-ignite a healthy debate about what is and what is not a planet," Boss said.

Alan Stern, of the Southwest Research Institute and leader of NASA's New Horizons mission to Pluto, predicted in the early 1990s that there would be 1,000 Plutos out there. He has also contended, based on computer modeling, that there should be Mars-sized worlds hidden in the far corners of our solar system and even possibly other worlds as large as Earth.

In a telephone interview after Friday's announcement, Stern, who was not involved in the discovery, said he stands by those predictions and expects Mars-sized objects to be found within decades.

"I find this to be very satisfying," Stern said of 2003 UB313. "It's something we've been looking for for a long time."

Stern stopped short of calling it one of the greatest discoveries in astronomy, however, because he sees it as just one more of many findings of objects in this size range. Last year, for example, Brown's team found Sedna, which is about three-fourths as large as Pluto. Others include 2004 DW and Quaoar.

Stern sees the outer solar system as an attic full of undiscovered objects.

"Now we have the technology to see them," he said. "We're just barely scratching the surface."

Way out there

The new world is about 97 astronomical units, or some 9,021,000,000 miles, from the Sun. An astronomical unit is the distance between the Sun and Earth. It becomes the farthest-known object in the solar system, and the third brightest of the Kuiper belt objects.

It is colder than Pluto and "not a very pleasant place to be."

It was found using the Samuel Oschin Telescope at Palomar Observatory.

Backyard astronomers with large telescopes, some experience and a map may be able to spot 2003 UB313.

Brown said it will be a very exciting object to explore since professionals and amateurs both have access to it.

"It will be visible over the next six months and is currently almost directly overhead in the early-morning eastern sky, in the constellation Cetus," says Brown, who made the discovery with colleagues Chad Trujillo, of the Gemini Observatory, and David Rabinowitz, of Yale University, on Jan. 8.

The team had hoped to analyze the data further before announcing the planet but were forced to do so Friday evening because word had leaked out, Brown said.

"Somebody hacked our website," he said, and "they were planning to make [the data] public."

Brown and Trujillo first photographed the new planet with the 48-inch Samuel Oschin Telescope on Oct. 31, 2003. However, the object was so far away that its motion was not detected until they reanalyzed the data in January of this year. In the last seven months, the scientists have been studying the planet to better estimate its size and its motions.

Scientists infer the size of a solar-system object by its brightness and distance. The reflectiveness of the new planet is not known, however, which is why the estimate of its diameter ranges from one to two times the size of Pluto. But those constraints are well supported by the data, Brown said.

Next up: Mars-sized objects?

Alan Stern, of the Southwest Research Institute and leader of NASA's New Horizons mission to Pluto, predicted in the early 1990s that there would be 1,000 Plutos out there. He has also contended, based on computer modeling, that there should be Mars-sized worlds hidden in the far corners of our solar system and even possibly other worlds as large as Earth.

In a telephone interview after Friday's announcement, Stern, who was not involved in the discovery, said he stands by those predictions and expects Mars-sized objects to be found within decades.

"I find this to be very satisfying," Stern said of 2003 UB313. "It's something we've been looking for for a long time."

Stern stopped short of calling it one of the greatest discoveries in astronomy, however, because he sees it as just one more of many findings of objects in this size range. Last year, for example, Brown's team found Sedna, which is about three-fourths as large as Pluto. Others include 2004 DW and Quaoar.

Stern sees the outer solar system as an attic full of undiscovered objects.

"Now we have the technology to see them," he said. "We're just barely scratching the surface."

Estimating size

Scientists infer the size of a solar-system object by its brightness and distance. The reflectiveness of the new planet is not known, however, which is why the estimate of its diameter ranges from one to two times the size of Pluto. But those constraints are well supported by the data, Brown said.

"Even if it reflected 100 percent of the light reaching it, it would still be as big as Pluto," says Brown. "I'd say it's probably one and a half times the size of Pluto, but we're not sure yet of the final size. But we are 100 percent confident that this is the first object bigger than Pluto ever found in the outer solar system."

The upper size limit is constrained by results from the Spitzer Space Telescope, which records heat in the form of infrared light. Because the Spitzer can't detect the new planet, the overall diameter must be less twice Pluto's size, Brown said.

Brown has had a running bet for five years with a friend that an object larger than Pluto would be found by Jan. 1 this year. 2003 UB313 was spotted on Jan. 8.

"My first reaction was, 'aw, I lost the bet by seven days,'" he said.

Brown's team has submitted a name proposal to the International Astronomical Union and has chosen not to divulge it until that body makes a decision.

See: https://www.space.com/1373-object-bigger-pluto-discovered-called-10th-planet.html

^^

Emily Lakdawalla • Oct 02, 2005

A moon for the "10th planet," 2003 UB313

I was on the way out the door to a party last night when I checked my newsreader and was astonished to discover a report on the AP newswire that 2003 UB313, the "10th planet," has a moon! Here's the discovery image:

Eris moon.jpeg
The discovery of Dysnomia, the moon of Eris, from the W.M. Keck Observatory. Eris appears in the center,
while the moon is the small dot at the 3 o'clock position. Credit: M.E. Brown, W.M. Keck Observatory

kuiper objects moons.jpeg
Images of the four largest Kuiper belt objects from the Keck Observatory Laser Guide Star Adaptive Optics system. Satellites are seen
around all except for 2005 FY9. Without the adaptive optics system the images of the Kuiper belt objects would he
smeared out so much by the earth's atmosphere that the satellites would not be visible. Credit: M. Brown/Keck Observatory


The fact that 2003 UB313 has a moon is extremely convenient for scientists, because it will allow a reasonably accurate determination of the object's mass. Mass can tell you a lot about what the inside of a planet is made of -- in this case, ice or rock -- and allows another way of comparing this body with Pluto and 2003 EL61 and other Kuiper belt bodies, to see whether they are all the same and all different. You can learn more about the moon at Mike Brown's website.

It seems that 2003 UB313 is destined to have mucked-up publicity, though. Its discovery was announced in a hastily assembled press conference on a Friday evening. This new moon was announced late on a Saturday night. I got a press release about it this morning that said "embargo lifted" on it. I can only assume that somebody out there jumped the gun, speaking publicly about it before the press embargo had been lifted, and they had to rush to distribute the information.

See: https://www.planetary.org/articles/0015

^^

From Brian Brown's web site:

PlanetXMoon-.jpeg
Artist's concept of Eris and Dysnomia. The sun and other planets appear in the distance. Credit: R. Hurt, IPAC

Why is the discovery of the moon important?

From a series of seven observations using the Keck Telescope and the Hubble Space Telescope, we have now measured the complete orbit of Dysnomia going around Eris. Dysnomia takes almost 16 days to go around Eris. Using equations for used by Isaac Newton to figure out the mass of Jupiter, we can Pluto (with an uncertainty of only 2%). The full orbit can be seen below.Screen Shot 2022-08-02 at 11.35.13 PM.png

While we know that Eris is larger than Pluto, that doesn't neccessarily mean it is more massive than Pluto. For example, a snowball could be bigger than a rock, but still be much less massive. Pluto appears to be a combination of ice and rock. If Eris were purely made out of ice, it could be a good deal less massive than Pluto.

Alternatively, if it is mostly rock, it could be much more massive than Pluto. The one way to find out the mass of an object like Eris is to hope to find a moon around it. Finding a moon, and then determining the distance that the moon is from the planet and how long it takes the moon to circle the planet allows us to precisely measure the mass of the body. A more massive body will pull on the moon tightly and it will circle the body more quickly. A less massive body will allow the moon to have a slow lazy orbit around the planet.

From a series of seven observations using the Keck Telescope and the Hubble Space Telescope, we have now measured the complete orbit of Dysnomia going around Eris. Dysnomia takes almost 16 days to go around Eris. Using equations for used by Isaac Newton to figure out the mass of Jupiter, we can now tell that Eris has a mass 27% higher than that of Pluto (with an uncertainty of only 2%). The full orbit can be seen below.

A second reason that discovering the moon is important is that understanding how moons form provides insight into the history of the solar system. It is quite surprising the 3 of the 4 largest objects in the Kuiper belt (Eris, Pluto, and 2003 EL61) all have moons. Such a large fraction of objects with moons suggests that some very common mechanism must be responsible. In the scientific paper describing the discovery we suggest the hypothesis that the moons of Eris and 2003 EL61 were both formed from a collision between Kuiper belt objects, much like it is thought the the Earth's moon was formed from a collision between the Earth and an object about the size of Mars. Understanding the orbit of the moon around the planet will help to show if this hypothesis is feasible.

Why is the moon of 2003 UB313 called Dysnomia?
Dysnomia is the mythological daughter of Eris. Eris is, of course, the Greek goddess of discord and strife. Dysnomia is the daemon spirit of lawlessness.

Our best images came from the Hubble Space Telescope, and clearly showed (much to our suprise) that Dysnomia is the only moon around. These images, shown below, are so good that you also can see some of the artifacts caused by the Hubble Space Telescope itself. In particualr, the spikes coming out of Eris, the spotty "ring" around Eris, and the slight elongation to the right of Eris are all expected patterns caused by the telescope itself, rather than from anything going around Eris.

eris y moon.jpeg

What is the moon made out of?

Currently we have no direct evidence to tell us what the moon is made out of, but we have some educated guesses. We do know that the moon of 2003 EL61 (the third largest object in the Kuiper belt, after Eris and Pluto) appears to be a ball of almost pure frozen water. We know this because we have been able to look at the sunlight reflected off the moon at infrared wavelengths, and the pattern of the light reflected shows us that there is frozen water and nothing else. From the limited information that we have, Dysnomia appears like it might be similar to the moon of 2003 EL61 (2003 EL61 was code named Santa, by the way, so the satellite is, of course, Rudolph). We are planning to use the Hubble Space Telescope later this year to study Dysnomia in more detail.

How big is the moon?

Right now we are not certain how big the moon is, but we can make some guesses based on how much light it reflects. We know that it is about 500 times fainter than Eris, suggesting that it is perhaps 22 times smaller in diameter than Eris. Eris is about 2400 km in diameter, so Dysnomia is perhaps 100 km in diameter. It is possible, however, that Dysnomia has a darker surface than Eris's very frosty highly reflective surface. In this case, Dysnomia could plausibly be as large as about 250 km.

See: http://web.gps.caltech.edu/~mbrown/planetlila/moon/index.html

Wow, I had no idea that we had such complicated dynamics within the Kuiper Belt and among Kuiper Belt Objects. I'm hoping that this will serve to further enlighten some us as our own solar system's Kuiper Belt becomes more highly defined.
Hartmann352
 
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Great article. Love that 45 degree inclination. There should be many more of these out there, with these inclinations.

A great project for amateur IR astronomers.
 

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