solar system

2003 UB313

2003 UB313 was announced on 29 July 2005 after two years of observations showed it to be the largest known member of the Kuiper Belt.


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2003 UB313

2003 UB313

2003 UB313 Statistics

Basic Data

Object Type: Kuiper Object
Parent Star: Sol (G2 V)
Discovery Status: Imaged

Physical Data

Mass : 0.0069 Earths
Radius: 1430 Jupiters
Periastron Distance: 37.8 AU
Mean Distance: 67.709 AU
Apastron Distance: 97.61 AU
Orbital Period: 557 Years
Eccentricity: 0.4416129
Argument of Perihelion (omega):
151.3115 0
System Age: 4600 Myr


2003 UB313 was announced on 29 July 2005 after two years of observations showed it to be the largest known member of the Kuiper Belt, larger even than Pluto, causing NASA to designate the object as the tenth planet of our solar system in its discovery press release. Best estimates are that the new object has a radius 1.25 times that of Pluto. Radius estimates of KBOs are determined based on brightness and distance, but can be skewed by the object's albedo. If 2003 UB313 has a higher albedo than other KBOs, then it would be smaller than current estimates suggest. Still, the discoverers state that even if the albedo of 2003 UB313 is 100%, which is very unlikely, the new object would still be as large as Pluto. So if Pluto is a planet, so is 2003 UB313. Currend radius estimates are based on the assumption that the new planet's albedo is around 60%, like Pluto, due to the fact the its spectrum is very similar to Pluto's.

The discoverers attempted to find the object with the Spitzer Space Telescope but were unable to detect it due to a technical glitch. Another detection attempt with Spitzer is underway. At the time of the announcement, 2003 UB313 was at 19th magnitude in the constellation of Cetus. It is observable with large backyard telescopes and amateur astronomers were photographing it only hours after the discovery was announced. In fact, if the new planet wasn't orbiting at such a high inclination, 44: to the plane of the ecliptic, it is very probable that it would have been discovered years ago.

2003 UB313 appears to be a scattered-disk object and was probably tossed into its highly inclined orbit by the planet Neptune sometime in the distant past. Near infrared observations of the new planet indicate a surface dominated by methane ice, much like Pluto. In fact, of all the KBOs only Pluto and Neptune's moon Triton share this strong methane signature.

System Components

(Mass in Multiples of Jupiter's Mass)

2003 UB313 sys com

A name for the new planet has been submitted to the IAU but will only be announced when it is approved. The directory of the discoverer's website was /planetlila, suggesting to many that the new name would be "Lila" after the discoverer's newborn daughter, but this is apparently not the case. Apparently the planet has been nicknamed "Xena" by the discoverers after the main character of the TV show of the same name. It is not known if this is the name they have submitted to the IAU.

2003 UB313l

Even before the announcement of 2003 UB313, controversy existed over the definition of a planet. Observations of the Kuiper Belt have turned up many objects, like Varuna, Quaoar, and Sedna, that are two-thirds to one half the size of Pluto and some have suggested that these objects should be called planets. Others site Pluto's own membership in the Kuiper Belt as an indication that the ninth planet is not a planet at all. If 2003 UB313 is indeed larger than Pluto as current observations suggest, it brings home the need for a better definition of what a planet is. Certainly, if Pluto is a planet, then 2003 UB313 is one as well. But if Pluto is not a true planet, then perhaps 2003 UB313 is not one either. Some would even claim that 2003 UB313 is the ninth planet and demote Pluto. Recognizing the need for a better definition of "planet" in the wake of 2003 UB313's discovery, the IAU has recently announced that it will be releasing an official scientific definition of the term soon.

2003 UB313 Orbital Statistics

2003 UB313 orbit

Orbital Period: 557 Years

2003 UB313 size

2003 UB313, the 10th planet, has a moon


On September 10th 2005, astronomers at the Keck Observatory on Mauna Kea took a look at the 10th planet with a new instrument that allows them to see details as precise as those seen from the Hubble Space Telescope. The images of the 10th planet quickly revealed that it has a faint moon in orbit around it! The discovery is so recent that little is yet known about the moon,

How was the moon found?


The 10th planet is so far away that the moon appears quite close to it. So close, in fact, that all of the telescopes that have looked at the planet up until now have completely missed it. The telescopes miss it because the earth's atmosphere blurs the view of the planet as the light makes its way to the ground. In the past few years a technique known as Adaptive Optics has allowed astronomers to measure and then partially correct for the blurring caused by the atmosphere. One limitation of Adaptive Optics has been that you must be looking at a very bright object to be able to measure the blurring, otherwise you can make no corrections. The Keck Observatory has recently commissioned a new cutting-edge addition to its Adaptive Optics system. The new addition uses a laser beam launched from the side of the telescope to make an artificial star in the sky right next to the target. This laser star then allows Adaptive Optics to be used even when the target is not particularly bright. Just one year ago, observations such as these would have required the Hubble Space Telescope. Today they can be made from observatories on earth. The new system is so powerful that many many astronomers want to use it right away, which is why, even though we know that the 10th planet has a moon, we have not been able to obtain any new observations of it since the initial discovery.

How do you know it is a moon?

In the image above you see the small point of light next to the planet. The point of light moved with the planet as the planet moved across the sky relative to the fixed stars. If you look very carefully at the bottom of the image above you can see a slight streak of light. This streak is a faint background star that was blurred as we tracked the planet across the sky. Thus, since the point next to the planet is not blurred, we know that it is not a background star. Another potential thing to worry about is that the point is an artifact of the Adaptive Optics system. Any such artifact, however, would rotate in an arc relative to the planet throughout the night, rather than stay in a fixed position. Thus, since the point does not rotate, we know that it is not an artifact. The only possibility left is that it is a moon circling the tenth planet.

Why is the discovery of the moon important?

While we know that the 10th planet is larger than Pluto, we don't actually know if 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 the 10th planet 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 a planet like 2003 UB313 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 planet. A more massive planet will pull on the moon tightly and it will circle the planet more quickly. A less massive planet will allow the moon to have a slow lazy orbit around the planet. We don't yet know the speed of the moon, but when we do we will suddenly have new insight into the size and even composition of the 10th planet.

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 (2003 UB313, 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 2003 UB313 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

Does this mean that 2003 UB313 is indeed a real planet?

Interestingly, many objects in our solar system have moons besides planets. Many quite small asteroids have moons around them, as do about 10% of the objects in the Kuiper belt (where 2003 UB313 and Pluto reside). So the fact that 2003 UB313 has a moon does not really change the case one way or another for 2003 UB313's claim to planethood. Nonetheless, we know that for some people the existence of the moon makes 2003 UB313 seem a more proper planet. We are happy that they are now happier.

What is the moon called?

Sadly, the committees deciding the fate of 2003 UB313 and its name have still come to no decisions. Since we get tired of saying a name with 9 syllables, we still call the tenth planet by our original code name, Xena. The moon officially goes by the name of S/2005 (2003 UB313) 1. Depending on how you count, that is something like 15 syllables.. It should come as no surprise to some, then, that we very quickly decided to call the moon by the code name Gabrielle. We look forward to the day that the committees finally decides and both of these bodies can get permanent names. In the meantime we are glad that Xena and Gabrielle are together again.

What is the moon made out of?

It was long thought that all moons were made out of green cheese, but that theory has been generally discredited. 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 2003 UB313 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, the moon of 2003 UB313 appears like it might be similar to that of 2003 EL61 (2003 EL61 was code named Santa, by the way, so the satellite is, of course, Rudolph), so we would guess that Gabrielle might be similar, also. We will not be able to study the composition of the moon until next year when we can get dedicated time on the Keck telescope. (Astronomers apply for time on telescopes months in advance, so, again, even though we know the moon is there and we'd like to study it, we have to wait until we will be able to be back to the telescope).

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 60 times fainter than the planet, suggesting that it is perhaps 8 times smaller in diameter than the planet. Interestingly, the planet-moon system appears similar to the Earth-Moon system, except reduced in scale by a factor of about 5-10. Xena is about 5 times smaller than the Earth. Gabrielle is about 8 times smaller than the Moon. And the two are separated by a distance that is about 10 times smaller than the Earth-Moon separation. Not a perfect match but awfully close. The image below shows a view of the relative scales of the two systems.

2005 (2003 UB313) 1'S Orbit Around 2003 UB313

2003-UB313-1-orbit

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