Just about the time that one NASA spacecraft is closing in on Ceres (formerly known as an asteroid, now semi-officially designated as a dwarf planet) and another nears its encounter with Pluto, a group of astronomers has suggested that there may be two or more planets at the outer reaches of our solar system. Didn’t we settle this when we kicked Pluto out of the family of planets?
To keep it short: planet is a word with no universally agreed-upon scientific definition. A very good summary of the subject is here for those who want to dig deeper. But let’s talk about Ceres and the possible “extra” planets.
Ceres and Dawn
The Dawn spacecraft is amazing for a number of reasons:
• It uses an ion propulsion engine to navigate its way around the inner solar system.
• It was the first spacecraft to orbit Vesta, the second-largest asteroid and the fourth discovered.
• It will be (in March 2015) the first spacecraft to orbit Ceres, and therefore the first spacecraft to orbit two different asteroids. Here is the latest (January 13th) series of images from Dawn as it approaches Ceres. These will rapidly improve as Ceres draws nearer!
I hesitate to predict what we will find once we are there. Such predictions nearly always turn out to be wrong! Stay tuned.
More Planets?
Among the many confusing terms used to describe the variety of objects found in our solar system, here we encounter ETNO: Extreme Trans-Neptunian Object. Let’s take it step by step.
The four classical planets in order of their distance from the sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto was the first TNO (trans-Neptunian Object) discovered in 1930, although it was designated as a planet, not a TNO. When the second such object was discovered in 1992 and many more quickly followed, the new term was born. It refers to any object that orbits the sun at a greater average distance than does Neptune. Neptune’s average distance from the sun (it has an elliptical orbit that carries it both closer to and farther away from the sun) is 30 AU, where an astronomical unit (AU) is the average distance of the Earth from the sun: 93 million miles or 150 million kilometers.
Some of these objects orbit at essentially the same distance from the sun as Pluto, although Pluto is by far the largest among them. Some orbits are almost circular, maintaining pretty much the same distance from the sun; others have highly elliptical orbits, swinging in relatively close (although still farther away than Neptune) and then retreating to as far away as 900 AU. For example, here is a portion of the orbit of Sedna, shown in blue. The otherwise most distant orbit shown in white is that of Pluto.
Extreme TNOs are those whose average distance from the sun is greater than 150 AU. Sedna is one such object, discovered only because it is near its closest approach to the sun on its more than 11,000 year long orbit.
The claim being made is that a study of 13 different ETNOs finds suspicious similarities in their orbits. For example, here are two such objects seen from the same perspective, if not from the same distance.
The similarity, which I will be the first to admit isn’t immediately obvious, has to do with where the two orbits dip below the plane of the Earth’s orbit around the sun. The light blue portion of the ETNO orbit is above this plane and the dark blue portion is below it. Here is Sedna’s orbit from a different perspective that emphasizes this feature.
The analysis by these scientists is that there may be two or more large (Earth-sized) planets waaaayy out there—250 AU or more—that are perturbing these orbits. The catch is that an object that far away cannot be seen with any current telescope. The good news is that the upcoming James Webb Telescope, or perhaps one of the extremely large ground-based telescopes that should come on line in five or so years—these should be able to spot any such objects.
Pluto Back in the Spotlight
Pluto is about to be back in the news, and for a good reason this time. The New Horizons spacecraft is currently 1.39 AU away from the little ex-planet, and closing on it at almost 33,000 miles per hour. In May its camera will be close enough to exceed the Hubble Space Telescope in resolution, and in July it will fly by, furiously snapping pictures and taking data. Here is our current best view of Pluto: a video assembled from several Hubble images. If this is unsatisfying, don’t worry. It’s about to get a lot better!
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