We preempt tonight’s science to bring to you breaking news about last week’s science. Astronomers just announced Eris may be smaller than Pluto.
Eris is a dwarf planet discovered only within the last ten years. Along with Pluto, it orbits the Sun in the Kuiper Belt, at over 30 times the Earth’s distance to the Sun. But Eris’s discovery was a day of reckoning. Eris was larger than Pluto. How embarrassing. How could Pluto be a planet, so the public argument went, if an even larger body was not? It was a simple and persuasive argument. Certainly easier to explain than the full reason for demoting Pluto: that some planet-like objects like Pluto are different than the eight solar planets since they don’t clear their orbits. Eris was larger than Pluto–what could be more direct.
Up until now, the size of Eris was estimated by a number of techniques. One way was from its mass. Its mass is measured from the time it takes for its moon, Dysnomia, to complete its orbit. The larger the mass of the central body, the faster its moons will orbit it. (Mathematically, the duration of the orbit of a small body around a large, central one goes inversely with the square root of the mass of the central body.) For example, if Earth were four times as massive, our own moon, Luna, would orbit us in just 14 days. That is, the gravitational force on the Moon from the Earth would be four times as strong and the only way the Moon could travel in a circle around us at its current distance would be to increase its speed by a factor of two. We’d have 24 months to remember. Then, assuming we counted months properly, it would be Christmas in Vigintiquattuorber.
So anyone can compare the mass of Pluto to the mass of Eris and determine which is physically larger right? Wrong. That’s only if you know the two objects have the same density.
To measure the physical size of an object you need to do something geometrical. A few techniques: extracting the cross section from its brightness and even direct imaging gave some results, but up until now with significant experimental uncertainty. The experimental uncertainties were always such that Eris could have been smaller than Pluto, but it just looked like that would be unlikely.
A special astronomical event changed all this. One of the most useful moments in astronomy is when an object passes in front of a star. This event is known as a occultation, and can be thought of as a kind of eclipse. Occultation and eclipse are not exactly the same thing. In an “occultation” the nearer body completely covers the farther one–in this case Eris passing in front of a star in the constellation Cetus. Distant stars appear point-like to us. An “eclipse” can be an occultation but also when a body passes into the shadow of another is a completely different kind of event also called “eclipse”. Sometimes the nearer body does not completely obscure the body behind it, in what is called a “transit”. Too many things to remember? Just tell your friends it is a “syzygy”.
Watch the occultation of a star by an asteroid (0:56)
Stars occupy such a small angle on the sky that predicting these events are hard, but astronomers are up to the task. Eris is so small that its shadow is much smaller than the size of the Earth. Astronomers had to predict which spots on Earth had the best chance and a few found it. When Eris passed in front of a dim star in the constellation Cetus, astronomers measured exactly how long the star was blotted out. The larger Eris is, the longer the star disappears from the sky. A telescope in Chile found it dim for 76 seconds. A few other measurements at other telescopes yielded Eris’s diameter. And to everyone’s surprise, Eris was smaller than its mass suggested. (Actually this was within the range of experimental uncertainties of the previous measurements.) It is a physically smaller, more compact object than Pluto.
For a wonderful account of this observation made by one of the discoverers of Eris, see Mike Brown’s blog about the event, which is where I found most of my information.
Dr. Tyson should come back. And he should bring Dr. Brown with him. I suspect Sheldon needs to have a word with them.