A Chinese scientist considers it too early to draw a conclusion on the tenth planet of the solar system, after American astronomers announced their finding.

On July 29, Mike Brown, an astronomer in planetary science at the California Institute of Technology, made the announcement that his group discovered the tenth planet of the solar system, which immediately drew wide attention.

Reporter of People's Daily interviewed Li Jing, researcher at China's National Astronomical Observatories. Li held that according to the features of the planet, it is yet to be made sure whether it is the tenth planet of the solar system.

The reason for identifying the planet is doubtful

The solar system has many universal rules, one of which is that eight of the nine planets, except for Pluto, almost all orbit the sun on one surface, and their orbits are close to round. Li said, the basic conditions are not seen in the newly discovered object: first its orbital inclination is 44 degree; second, its orbit is oval not close to round.

Besides, Li said, information on the object is rather inaccurate so it cannot be defined as the tenth planet just based on it. Brown's group, according to the brightness of the object, estimated its size and then its mass. Of course, size can be estimated based on brightness, but as for non-luminous celestial bodies, albedo is a must. Now, Li said, as we don't know its albedo, there'll be errors in the estimations. Mass is a very important factor in identifying a celestial body. Orbit can be accurately measured only with accurate mass.

Li added, the American scientists said its period of revolution is 560 years, which is also an estimation. Astronomers calculate the length of an orbit based on the length of the arc after a celestial body moves for a period of time. The longer the observation, the longer the arc and the easier the calculation. However, this object has just been discovered so its known arc is very short, and there are not many observation points. So the calculated orbit and cycle are not accurate. Its identity can be finally determined only through further observation and sufficient information.

It is an accidental discovery in observation

Li said, in fact, astronomers had lost their enthusiasm in looking for the tenth planet of the solar system since the 1990s. As Li considered, Brown and his fellows painstakingly pursuit it but hit upon it.

After the discovery of Uranus, astronomers thought there was another celestial body affecting Uranus, then they discovered Neptune, then they thought there was another celestial body affecting Neptune, so they found Pluto. At that time, astronomers found that Pluto is too small in mass to affect Neptune and Uranus. Then they speculated whether there was a bigger planet that is affecting them. They got down to exploration and called what they were looking for as "X" star. The "X" has two meanings, first, it is the unknown; second, it is just the "ten" in Roman numbers therefore it was just the tenth planet. They did not find it but found a large group of asteroids and galaxies.

In 1989, American Voyager-II sent back information of Uranus and Neptune after exploring Jupiter, Saturn, Uranus and Neptune, which shows many inaccuracies in the basic parameters reckoned through ground observation. Calculation of the orbits of the two planets based on new data showed slight impact on them by other celestial bodies. Astronomers finally realized their problematic reasoning. If this is the case, it should not have be necessary to look for a so-called major planet that affects Uranus and Neptune.

Maybe it's an asteroid in circular belt

Brown's group said, the object is located in the Kuiper belt, a suburb region in solar system. Then what is the Kuiper belt?

Li said, in 1951, Dutch-American astronomer Gerard Kuiper (1905-1973) put forward a hypothesis that there is a circular belt outside Neptune, which contains many small objects. Later scientists did find such a belt thus called it Kuiper belt.

After Kuiper belt was identified, astronomers immediately raised a question �C what is Pluto? In fact, Pluto is not in accordance the rules of major planets: it has a 17-degree orbital inclination, not on the same surface. Its orbit surface is oval not close to round. Meanwhile, Pluto is small as its diameter is only one third that of the moon, one sixth of the earth and its mass is 1/500 of the earth. Now it is almost a consensus in the astronomical circle that Pluto is in the Kuiper belt and it is a relatively big asteroid in the belt.

What planet can be defined as major planet? Usually, there are two kinds of major planets, earth-like and Jupiter-like. In terms of mass, earth-like planet is similar to earth, and Jupiter-like planet is dozens to hundreds of times of earth. In contrast, Pluto is just a secondary planet. Li said, to be accurate, the newly discovered object is merely an asteroid bigger than Pluto in the Kuiper belt.

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The solar system has nine major planets. The eight planets other than the earth, in the chronological order of their discoveries, are Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. The former four, bright and close to the earth, were seen by people in ancient times. Saturn was also discovered in ancient times, but its "rings" were identified by astronomers through telescope in the 17th century. Uranus was discovered in 1781 by German William Herschel (1738-1822) who moved to Britain. Neptune was discovered by in 1846 by German astronomer Johann Galle (1812-1910). Pluto was discovered by American Clyde Tombaugh (1906-1997) in 1930.

By People's Daily Online