The New Solar System: It's Not Just Planets Anymore
by Louise Good
Montage of solar system objects by Karen Teramura
"Most of the solar system, I think, remains to be discovered, and we're still discovering it with telescopes on Mauna Kea," said IfA astronomer David Jewitt, the speaker for a Frontiers of Astronomy Community Lecture entitled "The New Solar System" on March 18 at the UH Manoa Art Auditorium.
Jewitt explained that the solar system has three domains: the terrestrial planets, the giant planets, and the comets. The terrestrial planets include Mercury, Venus, Earth, Mars, and the asteroids. These rocky bodies grew by collisions between dust particles that stuck together and "became pebbles, then boulders, and so on" until they became planets. "We know a lot" about these planets "because we live on one." The others are "nearby and they are relatively easy to study," and many spacecraft have been sent to them.
Photo by Dan Birchall
The giant planets--Jupiter, Saturn, Uranus, and Neptune--are farther away and more difficult to know, he said. They are similar to the Sun in that they are made up primarily of hydrogen and helium, but they are too small for nuclear fusion to take place. We see the cloud tops in their atmospheres and make inferences from that. Jewitt's preferred model of solar system formation hypothesizes that the giant planets started as rocky planets. When these planets reached a critical size (probably 5 to 10 Earth masses), they had enough gravity to pull in gas from the disk surrounding the Sun. There is evidence that this is what happened: By studying the motions of natural satellites and spacecraft as they go around the giant planets, scientists have determined that the density deep inside these planets varies. According to Jewitt, there is now "pretty good evidence" for the existence of a dense core inside Jupiter, and "undeniable" evidence for such a core in Saturn. Uranus and Neptune are "mostly core," five to 10 Earth masses out of a total of about 16 Earth masses for each of these planets.
The comet domain consists of huge numbers of very small bodies. We know even less about the comets than we do about the giant planets, Jewitt said. In 1950 Dutch astronomer Jan Oort inferred the first source of comets, the Oort Cloud, because comets enter the solar system from all directions. The Oort Cloud is a huge area on the far edge of the solar system that contains about 1 trillion comets. It is where long-period comets originate. These comets probably formed between the giant planets and then were ejected by the gravity of the giant planets.
A second source of comets is the Kuiper Belt, a doughnut-shaped region that lies just beyond the orbit of Neptune. The most famous Kuiper Belt object is Pluto, which was until recently thought to be a planet. But in 1992, Jewitt and Jane Luu discovered the "first" Kuiper Belt object (KBO, also called trans-Neptunian objects), setting off a chain of events that led to the reclassification of Pluto as a KBO and dwarf planet. More than 1,000 KBOs have been discovered, and there may be 70,000 large ones and millions or billions of small ones, but the total mass is quite small, about a tenth of an Earth mass. Short-period comets, which enter the inner solar system more frequently than long-period comets from the Oort Cloud, come from this region.
Jewitt and Henry Hsieh, then a graduate student at IfA, discovered the third reservoir of comets among the asteroids in the main asteroid belt between Mars and Jupiter about three years ago. This new category of objects has been named main-belt comets. They may have contributed water to Earth after our planet cooled sufficiently to have liquid water.
"There has been a qualitative change in the way people think about the solar system," Jewitt said. Twenty years ago, a scientist might have specialized in studying one type of comet. Now scientists know that it is extremely important to understand the connections between the various kinds of comets and everything else in our system.
In summing up, Jewitt said, "Don't think of the solar system the way it was presented to me when I was a kid: 'Here are these planets, there's nothing else and nothing's ever going to change.' It's totally different from that. It's much more interesting. A lot of the information in the solar system is carried by bodies that have almost no mass, but they (the comets) contain a huge amount of information, in part because there are so many of them, and we can get that information by finding them and studying their orbits."
Video of this lecture