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Finally - the tenth planet is bigger than Pluto

The diameter of the object that will reside next to Eris is about 3,000 km - about 700 km larger than Pluto. It is therefore the largest object since the discovery of Neptune in 1846

Size comparison between the tenth planet and the Earth
Size comparison between the tenth planet and the Earth
The demand to increase the number of planets in the solar system to ten is supported by the findings of a group of astronauts from Germany who observed the planet. According to the team, the new planet 2003 UB313 is larger than Pluto.
By measuring its heat emission, the scientists were able to determine that its diameter is about 3,000 km - about 700 km larger than Pluto. It is therefore the largest object since the discovery of Neptune in 1846. This was published in the journal Nature on February 2, 2006.
Like Pluto, 2003 UB313 is one of the icy bodies in the Kuiper belt that extends beyond the Neptune. It is the most distant object found in the solar system. Its orbit is elongated and at its peak it is 97 times farther from the Sun than the Earth, almost twice as far as the farthest point of Pluto's orbit and therefore it takes twice as long to orbit the Sun.
When the object was first seen it appeared to be at least as large as Pluto but accurate estimates of its size were not possible without knowledge of its rate of return. A team led by Prof. Frank Bertlodi from the University of Bonn and radio astronomer Wilhelm Altenhoff from the Max Planck Institute for Radio Astronomy solved the problem by measuring the heat emitted by 2003 UB313 as a means of determining the size in combination with the optical observations.
"Because 2003 UB313 is bigger than Pluto," Bertoldi said, "it's hard to call Pluto a planet without giving 2003 UB313 the same status."

2003 UB313 was discovered by the team of Michael Brown, Chad Trujillo and David Rabinowitz on January 5, 2005 from images taken on October 21, 2003 and the discovery was announced on July 29, 2005, the same day as two other Kuiper Belt objects: 2003 EL61 and 2005 FY9.

The research team has been systematically scanning for large extrasolar bodies for several years, and has previously been involved in the discoveries of other large trans-Neptunian objects including 50000 Kva-Var and 90377 Sadna. Routine observations were made by the group on October 31, 2003 using the Samuel Osshin Telescope at Mount Plumer Observatory in California, but the object captured in the images was not discovered until January 2005, when later images of the same area revealed its slow motion against the background stars. Observations that followed then enabled a preliminary decision on its orbit, but failed to pinpoint its size. Due to its optical clarity, it was already believed that it was bigger than Pluto.
Astronomers have been finding small planetary objects beyond the orbits of Neptune and Pluto since 1992, confirming a forty-year-old prediction by astronomers Kenneth Edgeworth and Gerard Kuiper that a belt of small planetary objects exists beyond Neptune.
The Kuiper Belt contains objects left over from the process of creating the Solar System 4.5 billion years ago. In their distant orbits they could survive the gravitational sweep of all similar objects swallowed up or rejected by the planets existing in the inner solar system. Some of these objects occasionally enter the inner solar system and we see them as short-lived comets.
In the field of visible light, these objects can be distinguished thanks to the sunlight they reflect. However, the apparent size depends on their size as well as the reflection rate of the surface. This reflection rate ranges from 4% for most comets to over 50% in the case of Pluto, which makes the exact measurement of the size of the object only with the help of light reflection impossible.
The team from Bonn used at least the 30-meter diameter IRAM telescope in Spain equipped with a sensitive X-Planck millimeter bolometer detector developed at the Max Planck Institute. to measure the heat emitted from
UB313 at the wavelengths of 1.2 millimeters where the solar radiation is negligible and the brightness depends solely on the temperature and the size of the object. The temperature of the celestial body can be estimated by knowing its distance from the sun, and therefore the brightness in the frequency range of 1.2 millimeters allows a good estimate of the size of the object. Therefore, the researchers concluded that the surface of UB313 reflects about 60% of the sunlight, similar to the reflection of Pluto.
"The discovery of an object in the solar system larger than Pluto is an exciting discovery," explains Dr. Altenhoff, who has been studying asteroids and asteroids for many decades. "He tells us that Pluto, which is also an object belonging to the Kuiper belt, is not a unique object. There are other such small planets and they can help us understand the birth and development of the solar system. The Kuiper Research Self are remnants of the formation of the solar system, a kind of archeological site that includes primordial objects from the nebula that surrounded the sun and from which the sun and the planets were formed." It was Altenhoff who did the pioneering work to measure the radiation from Pluto in 1988 with an instrument that preceded the detector in the IRAM telescope.

The tenth planet Xena is slightly larger than Pluto

This is evident from images taken by the Hubble Space Telescope
By: Avi Blizovsky

The Hubble Space Telescope was able to clearly observe the "tenth planet" which was nicknamed Xena and found that it is slightly larger than the ninth planet in the solar system - Pluto.
Relying on observations from ground telescopes, the scientists determined that it is about 30% larger than Pluto. However, photographs taken by Hubble on December 9 and 10, 2005 and published this week reveal that the diameter of Xena is about 2,400 km (plus or minus 100 km). The diameter of Pluto is 2,288 km.
"Only a telescope that can take clear photographs and has a clear view can measure Xena's exact diameter," said Mike Brown of the California Institute of Technology in Pasadena, California. The research team led by him is the one that discovered Xena, officially cataloged as 2003 UB313, a discovery approved for publication in the Astrophysical Journal.
Quite a number of photographs are needed to determine the diameter of Xena. The planet is 16 billion km away. This diameter, from this distance takes up a total of 1.5 pixels in the Hubble image but according to the scientists this is enough to make accurate size measurements.
Since Xena is smaller than previously thought, but relatively bright, it must be one of the best reflective objects in the solar system. The more reflective object than it is known is Enceladus, a geologically active moon of Saturn whose surface is constantly covered with ice reflecting light from active geysers.
It is possible that Xena's brightness is due to fresh methane freezing on its surface. This material should have served as its atmosphere, if it were closer to the Sun, but it froze. Another possibility is that the methane is constantly leaking from the planet's hot interior. When the methane reaches the cold surface it immediately freezes and becomes solid, covering the craters and other features and making it uniformly bright to the mourner's eyes.
Zina takes 560 years to orbit the Sun and is now very close to its ephelion – the farthest point from the Sun in its orbit) but the plan is to use Hubble and other telescopes to study the other recently discovered objects in the Kuiper belt that are nearly the same size as Pluto and Zina. The Kuiper Belt is a giant ring of primordial icy comets and larger bodies orbiting the Sun beyond the orbit of Neptune.
The discovery that the largest object in the Kuiper Belt is a Pluto twin could further complicate the debate over whether to categorize these icy worlds that inhabit the Kuiper Belt as planets. If Pluto is considered the minimum size for a planet, Xena should also meet this criterion by the time the World Astronomical Union decides on the official name.
Ydan Pluto

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