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Is Pluto a planet, as have learned generations of schoolchildren? Or is it really a dwarf planet, as determined by the official governing body of astronomy?
NASA administrator Jim Bridenstine says it's the first. "Just for you to know, in my opinion, Pluto is a planet," he told reporters Aug. 23, while he was visiting an aerospace engineering lab in Boulder, in the United States. Colorado. "That's what I learned and I'm attached to it."
Bridenstine may be engaged, but the official call of the planet / dwarf planet does not belong to him. This responsibility lies with the International Union of Astronomy, which transferred Pluto from a planet to a dwarf planet 13 years ago.
Since then, scientists and observers have discussed, sometimes fiercely, the status to be attributed to the 1,500 km wide rocky ball that surrounds the sun once every 248 years at the edge of the solar system.
Long-standing quarrel
Bridenstine's comments will probably not solve the debate that began in the 1990s with the discovery of the Kuiper Belt, a ring of rocky or icy objects surrounding the sun beyond Neptune's orbit.
In the light of this discovery, some astronomers and planetary scientists began to claim that Pluto – whose orbit is located in the Kuiper belt – had more in common with these worlds, mostly small and distant, with the eight other planets. Others felt that Pluto still deserved to be a full-fledged planet.
To resolve the dispute, the AIU based in Paris published in 2006 a new definition of the term "planet". According to this definition, a planet is a celestial body that revolves around the sun, has enough gravitation to assume an approximately spherical shape, and has been able to clear smaller objects from its orbital trajectory, as would bulky objects because of their gravitational tug.
Pluto fulfills the first two conditions, but not the third – and the members of the organization voted in favor of its reclassification.
As a result of Pluto's change of status, hundreds of world scientists have called for a change in the definition, saying it was ill-conceived. For example, one could argue that the Earth, Mars and Jupiter have failed to clean the debris from their own orbit, given the many asteroids that still populate the region. Critics also point out that only 424 of the approximately 9,000 IAU members took part in the vote.
What makes a planet?
Alan Stern, principal investigator of NASA's New Horizons mission, is among the most ferocious defenders of Pluto. He sent a robotic spacecraft to Pluto in 2015. According to Stern, the decision of the AIU is "wrong," claiming that planetary status should be attributed on the basis of the geophysical characteristics of a celestial object. . By this measure, he says, Pluto is clearly a planet.
"Pluto has an atmosphere, mountain ranges, a core, an inner ocean and many other properties, just like the Earth," he says. Pluto also has its own moon, Charon, 200 km long. "Planetary scientists ignore the definition of IAU and consider Pluto to be a full-fledged planet in their research. It is more important than any vote. "
Astronomer Mike Brown of the California Institute of Technology, author of the 2010 book titled "How did I kill Pluto and why did he arrive?" Is on the other side of the Pluto division. In his eyes, Pluto is simply one of the countless objects that surround the sun beyond Neptune. It's not even the biggest: Eris, a dwarf planet in the Kuiper belt discovered by Brown in 2005, is about 25% more massive than Pluto.
New perspectives on an old discovery
Pluto was discovered in 1930 by Clyde Tombaugh, a young astronomer working at the Lowell Observatory in Flagstaff, Arizona. Astronomers have designated it as a planet even though it's not like the farthest planets, the gaseous giants Jupiter, Saturn, Uranus and Neptune. Pluto was a "little silly thing at the edge of the solar system," in the words of Brown, who discovered dozens of small objects beyond Neptune's orbit.
Stern is not influenced. Pluto is not only a planet, he says, but so are the large spherical bodies of the Kuiper Belt and, closer to home, some of the larger objects in the asteroid belt between Mars and Jupiter. If this increases the number of planets in tens or even hundreds, so be it.
Jessie Christiansen, astrophysicist at Caltech, shares this view. "The argument I do not agree with is:" There would be too many planets if we let them all be planets, "she says." There are 100 billion stars in galaxy.We have not stopped calling them stars because their numbers have become too big. "
Will Grundy, astronomer at the Lowell Observatory, said that the IAU astronomers had reclassified Pluto because they were "panicked by the prospect of announcing new planets every year – this which is happening because technology has progressed to the point of making us discover all those little things far away in the outer solar system. But as a global scientist, I think it's fabulous. The more there is, the better!
The debate is likely to heat up again – and not only because Bridenstine has become aware of it. Astronomers have found tantalizing evidence of the existence of a massive "super earth" beyond Pluto. According to them, its existence could be confirmed in the coming years. Perhaps ironically, some refer to the object with the same term once reserved for Pluto: Planet Nine.
As passionate as they are, scientists on both sides of the debate are aware that the argument is largely a semantic argument. As Christiansen says, "Pluto does not care what we call this."
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