"What?! Pluto no longer a planet?"

THE SOLAR SYSTEM IS MORE BIZARRE THAN WE THOUGHT: "WHAT?! PLUTO NO LONGER A PLANET?"

Recently the International Astronomical Union decided that what we were taught in school, that the Solar System has nine planets, was wrong: it has eight. The problem is not so much a mistake made 76 years before, when Pluto was discovered, but rather the fact that we had never understood exactly what a planet is.

A century ago many people believed that the orbit of Neptune had imperfections, which would mean that in its motion around the Sun there was a gravitational interference, possibly by another yet unknown planet (Neptune itself had been discovered this way, in 1846). One of these people was Percival Lowell, famous for his ideas about canals and civilized life on Mars. From his private observatory in Flagstaff, Arizona, USA, he sought the mythical planet "X" to the end of his life, in 1916, without success.

But in 1929 the observatory founded by Lowell hired a 23 year old farmer with much skill for astronomy. One of his duties was to take, with a telescope, many photographs of selected parts of the sky and then compare them using a special device. In two pictures taken on different days of January 1930, the employee, Clyde Tombaugh, saw a difference: a star switched places.

In 1801 Giuseppe Piazzi had also discovered, with a telescope of the Palermo Observatory, a "star" that shift places. It has an orbit between Mars and Jupiter, but despite that Piazzi named it planet Ceres, he failed to see more than just a point. Later it was agreed that it was not a planet but a small body in the Solar System, like comets. From then on bodies like (1) Ceres were simply known as planetoids or asteroids, and labeled with numbers in a catalog.

But what Tombaugh discovered was much brighter. So even though, due to the distance, it was also seen as just a point, it was thought that it might be the size of the Earth itself. A girl who liked mythology much suggested the name Pluto. But it was not the expected planet "X", because it can not disturb Neptune, whose diameter is four times that of Earth's. At present we know that the alleged alterations of the orbit of Neptune came from exaggerated calculations. Planet "X" does not exist.

STUDYING PLUTO

Over time the estimated size of Pluto halved, i.e., to slightly larger than Mercury. Its 248-year orbit resulted very strange: it's not like those of the previous planets, which are almost circular and horizontal, but is very elliptical and highly inclined, such as those of asteroids.

With observations between 1953 and 1955 periodic variations in its brightness were noted, which were interpreted as spots on a rotating planet. Period: 6 days and 9 hours.

In 1976, Dale Cruikshank, Carl Pilcher and David Morrison discovered, by means of sophisticated analysis of the colors, that Pluto is covered by bright ices.

In 1978 James Christy studied several photographs of distant Pluto (to calculate its trajectory, for the U.S. Navy) and noticed that that bright speck had a deformation in one corner. Or Pluto was not round, or had some "accessory". He called in another technician to confirm what he was seeing, and they found that in a sequence of photos the imperfection was spinning around the center: they discovered that Pluto has a natural satellite, later baptized Charon (alternatively, Caron, or even Caronte).

This discovery was extremely important. Charon movement depends on the planet's gravity, which depends on the mass. Thus Robert Harrington found that Pluto's mass is much less than that of the Moon. It was noted that Charon revolves around Pluto in 6 days, 9 hours, the same as Pluto's rotation. This rotation of Pluto must be synchronized by the gravity of Charon, indicating that the satellite is very large relative to the planet, almost like two planets. Or better: a double planet, where the movements of one depends on the other and vice versa.

It was found that the orbit of Charon is vertical, so it was inferred that Pluto is lying horizontally, with the poles at left and right. Around 1983, with high-speed photography to minimize atmospheric turbulence in front of the telescope, Gerd Weigelt and others helped to calculate that the location of the system in 1987 and 1988 would make Charon pass in front of and behind Pluto, in relation to us. Despite not seeing any sphere, these occultations allowed David Tholen to calculate that Charon has between 1150 and 1230 km in diameter, and Pluto between 2260 and 2340 km in diameter.

By having the masses and sizes the density of the system could be calculated: about 2000 kg/m³, heavier than water, but lighter than rock. Probably mixed ice and rock.

In 1988 Pluto passed right in front of a star, so NASA had sent a cargo plane equipped with a telescope to the South Pacific, where the 2-minutes-long event was visible. The star's brightness decreased slowly before disappearing behind the edge of the planet, rather than abruptly, indicating that Pluto has a layer of gases around: a tenuous atmosphere. At that time Pluto was about 30 times farther from the Sun than the Earth (in fact closer than Neptune, and below it) but at the beginning of the 22nd century will be about twice this distance: so far away that this atmosphere, perhaps of nitrogen, carbon monoxide and methane will be frozen by the cold.

In January 2006 NASA finally launched an interplanetary probe to study the Pluto-Charon system. In support of the mission, called New Horizons, the team led by Alan Stern gained access to the Hubble Space Telescope, and with this in 2005 discovered two tiny objects, estimated size of 50 to 180 km, rotating with the system. These small bodies were christened Nix and Hydra. In mid-2011, with the same instrument, the team of Mark Showalter found a fifth object in the system, this one ten times less bright with which a size of only a few tens of kilometers is estimated for it. It was named provisionally S/2011 (134340) 1, or Pluto IV, now formally Kerberos (rather than Cerberus). In the next opposition (of Pluto and the Sun relative to Earth) a year later, in 2012, Showalter's team again, and using the same space telescope, discovered another moon, even smaller, estimated size between 6 and 10 km. Its orbit is inside the others' and its provisional name was S/2012 (134340) 1, or Pluto V, now formally Styx.

PLUTO AND CHARON HAVE COMPETITORS

In 1951 the great astronomer Gerard P. Kuiper proposed that in the outer Solar System must be a second asteroid belt, similar to the one existing this side of Jupiter, but populated by frozen objects. In 1987, David Jewitt and Jane Luu began using the 2,2-m telescope of the University of Hawaii equipped with ultrasensitive CCD electronic cameras to test the idea. In 1992 they published the discovery of an object of some 200 km (provisional name 1992 QB₁) in the region where Pluto moves. Since then hundreds more have been discovered, some like 1996 TL₆₆, of about 600 km, large enough for its interior gravity forces make them round.

But in 2000 the story became complicated: Robert McMillan discovered an object of about 900 km, as large as the largest asteroid, (1) Ceres. It was christened (20000) Varuna. In 2002 Chad Trujillo and Mike Brown announced the discovery of an object larger than (1) Ceres, with a size between 1070 and 1450 km. It was christened (50000) Quaoar. In 2003 the team of Mike Brown, Chad Trujillo and David Rabinowitz discovered, at 90 times the distance separating the Earth from the Sun, an object even larger than Charon, having between 1180-1800 km, and later dubbed (90377) Sedna. And finally on 29 July 2005 they announced the following ones: 2003 EL₆₁, irregular of about 1500 km, 2005 FY₉, with a size of 1600-2000 km, and ... yes, the much anticipated discovery: an object (provisionally called 2003 UB₃₁₃, and later "Eris") with a size of between 2300 and 2500 km, which made many people advertise it as the "tenth planet". But that story did not have a happy ending.

THE PANORAMA IS FULFILLED

Gerard Kuiper in a sense was right: there is a second asteroid belt, frozen, in the outskirts of the Solar System. The truth is that Pluto, (136199) Eris and others yet to be discovered are only the larger individuals of a swarm of objects, which similarly to the rock-metal asteroids between Mars and Jupiter, never came to be aggregated to form up a single large planet. In the main asteroid belt, where (1) Ceres is the largest specimen, Jupiter's gravity keeps these objects in chaotic orbits, and prevents them from coming together to form a single mass. The forces of gravity of Uranus and Neptune should be doing the same with these icy objects beyond the eighth planet. They never got to form a single dominant mass ... which refer us way back, to the times of the formation of the Solar System.

Stars form from clouds of gas and dust in space. The gravitational attraction between the molecules causes them to come together in a whirlwind. In the center, where the cloud is denser, the pressure rises so much that the temperature reaches millions of degrees, producing very powerful thermonuclear reactions in their atoms: a star is born. Around it the remains of the vortex continue spinning. Eventually the particles join together forming larger particles, which then come to form pebbles, which later join together forming large stones, which then get together forming asteroids and comets, and finally are piled by their mutual gravity into larger objects we call planets. Each planet has the composition of the region where it arose within the cloud: Mercury, Venus, Earth and Mars, very close to the Sun, are predominantly rock and metal, heat resistant. In Jupiter, Saturn, Uranus and Neptune, which are cooler, gases and ices abound. And beyond Neptune objects are almost entirely icy, but due to gravitational perturbations these large blocks remain dispersed, unable to join together to form a body dominant in its region: there never was a ninth planet. Even being round and very bright, Pluto never got to be completely outstanding among its neighboring siblings.

What Clyde Tombaugh discovered in 1930 was just the first piece of a cosmic jigsaw puzzle that only in 2006 the members of the International Astronomical Union, meeting in Prague, were finally able to fit together. This is a classic example that the word "discovery" has two meanings: one is "to find", the other is "to understand". Three quarters of a century ago Clyde Tombaugh found Pluto. Now we understood it a little better.

Aldo Loup.

Originally published in ABC Color, on 3 September 2006. Photograph: The Pluto system as seen by the Hubble Space Telescope. From left to right: Pluto (now demoted to minor body number 134340), Charon, Nix and Hydra. In this overview, S/2011 (134340) 1 Kerberos is somewhere almost directly above Pluto, and S/2012 (134340) 1 Styx orbits much closer to Pluto and Charon, almost lost in the brightness of these two. A joke in English said that it was not known whether Pluto was a planet, an asteroid or a dog, created by Walt Disney also in 1930. Credit: NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team. Reproduced under conditions of the Space Telescope Science Institute.