An asteroid orbiting the Sun surprised astronomers by turning out to be roughly spherical – possibly the smallest dwarf planet in the solar system, according to a study by an astronomical team.
The asteroid, called Hygeia, is the fourth largest object orbiting in the asteroid belt between Mars and Jupiter, after the dwarf planet Ceres and the asteroids Vesta and Pallas.
Because it is much smaller than Vesta and Pallas, it was believed that Hygeia gives it an irregular shape like it and not a spherical shape that forms under the force of its own gravity, like the Earth and the other rocky planets. However, astronomers have discovered that Hygeia is round and that its relatively low gravity is the reason.
This could qualify him as a dwarf planet within the definition of the International Astronomical Union, which in 2006 plutoically downgraded Pluto's status as the Ninth Planet of the Solar System.
The IAU agreed that a planet is an object orbiting the sun but not a moon of another object; that it has freed its neighborhood in space from smaller objects; and that it has enough mass for its own gravity to pull it into a spherical shape. Since Pluto shares part of its orbit with other objects, including the distant dwarf planet Eris, the IAU downgraded it to a dwarf planet.
The decision means that the IAU sees the solar system as eight rather than nine planets ̵
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Hygeia was first seen in 1849 as a moving point in a telescope. It is believed that it took its current form 2 to 3 billion years ago when it collided with another asteroid.
This collision created one of the largest asteroid families in the solar system, consisting of nearly 7,000 asteroids that can detect this at the wavelengths of reflected sunlight.
It was also assumed that they left a scar.
Astronomer Miroslav Brož from the Charles University in Prague in the Czech Republic, co-author of the study. But instead of finding evidence of an ancient impact, observations with the European Very Large Telescope in the Atacama Desert in Chile showed that Hygeia was unexpectedly round – a result of the massive collision in which the rock fragments behaved like a liquid for a while ,  "The mother body was completely destroyed and then transformed into a roughly spherical body," said lead author, astronomer Pierre Vernazza of the Laboratoire d 'Astrophysique de Marseille in France. "The more spherical shape is thus a consequence of the force of the impact." That's about as tall as Alabama and much smaller than the dwarf planets Ceres (nearly 600 miles) and Pluto (nearly 1,500 miles).
Both authors thought Hygeia should now be considered a dwarf planet.  "I do not think it affects larger objects, but rather smaller ones," Vernazza said. "We can not rule out that even smaller dwarf planet candidates will appear in the future."
"It meets the official definition," Brož said. "Alternatively, should we improve the definition, right?"
But some other astronomers are not sure if Hygeia should be considered a dwarf planet.
"The term dwarf planet always seemed artificial to me," says astrophysicist Steven Soter
. He said the IAU designation was a compromise to preserve Pluto as a kind of planet after astronomers found out that it was just one of thousands of objects in the Kuiper belt in the outer area is the solar system.
"So they introduced roundness as a new criterion," Soter said.