Sunday, November 21, 2010

Faraway Sedna

It is amazing to see how much our view of the solar system has changed over the past few years. Once upon a time, the solar system was known to be just a system of several planets in neat orbits around the Sun, together with a population of asteroids and comets. Back then, not much was known to exist beyond Pluto and the solar system seemed to be a simple place to be in.

Today however, the solar system is far from being a simple place as new objects are frequently being discovered as far out as Pluto and beyond. In fact, Pluto is far from being at the edge of the solar system as a huge number of newly discovered worlds are known to exist far beyond Pluto. Many of these newfound worlds rival Pluto in size and in 2005, a newly discovered object named Eris is found to be more massive and probably larger in size than Pluto.

This population of objects that orbit the Sun beyond Neptune are knows as Trans-Neptunian objects (TNOs) and they include objects such as Pluto and Eris. Several TNOs are known to be over 1000 kilometers in diameter and many more of such large TNOs are yet to be discovered. Since the discovery of Pluto in the 1930s, one of the most intriguing discoveries of a TNO was of an object named Sedna in 2003.

What makes the discovery of Sedna so interesting is its extremely elongated and far-flung orbit that is unlike any other TNOs. Sedna’s orbit brings it as close as 76 AU from the Sun out to as far as 960 AU from the Sun and it takes Sedna around 12 thousand years to complete one orbit around the Sun. An AU is a unit of measurement and one AU is basically the mean distance of the Earth from the Sun. When Sedna was discovered in 2003, it was located at a distance of 90 AU from the Sun and approaching perihelion. At its furthest distance of 960 AU, the Sun will appear as a point of light with less than half the brightness of the full moon.

Sedna is so distant that it never comes close enough to Neptune for it to be gravitationally scattered by Neptune into its current highly elongated orbit. In fact, the Earth comes closer to Neptune than Sedna ever does! Since its discovery in 2003, the answer as to how Sedna got kicked into its crazily elongated orbit is still not yet known, making it probably the only known object in the solar system whose orbit cannot be explained. Is something lurking in the outer parts of the solar system that could account for Sedna’s orbit?

Sedna could not have formed in it current orbit since the large relative velocities between planetesimals would have been disruptive rather than constructive. Hence, Sedna’s initial orbit must have been circular otherwise its formation by the accretion of planetesimals would not have been possible. A number of possibilities have been thrown in that might explain Sedna’s intriguing orbit.

The first possibility is that there is a large Earth-sized planet orbiting the Sun beyond Neptune that could have gravitationally scattered Sedna into its current orbit. This hypothesis might be a long shot because any Earth-sized planet located within 100 AU would have been easily detected, especially from its gravitational interactions with other TNOs. However, such a planet might once exist but may have been ejected from the solar system after the formation of the Inner Oort Cloud. The ejection of this planet would not substantially modify the orbits of the objects that have been scattered into Sedna-like orbits.

The second possibility that might explain Sedna’s odd orbit is a chance close encounter with a passing star. Such a star would have to come as close as 200 AU to 1000 AU from the Sun in order to excite TNOs into Sedna-like orbits. An encounter like this would have been “extremely close” give that the closest stars are already a few hundred thousand AU distant. In fact, the probability for such a close encounter in the past 4.5 billion years of the solar system’s history is around 1 percent. This is probably not good odds to base a theory on. For the second possibility, it can also be that Sedna once orbited a brown dwarf or a low mass star, and it was stripped from its parent star when it came too close to the Sun.

The third possibility, which is also the most likely one, assumes that the Sun was formed on a dense cluster of stars and perturbations from numerous neighboring stars gradually excited Sedna into its current elongated orbit. The view from inside one of these clusters would have been an incredibly awesome sight. After 4.5 billion years, the stars that once formed this cluster would have been long lost amongst the hundreds of billions of stars in the vast Milky Way galaxy. If this third possibility is true, then Sedna could serve as a “fossil record” of what happened during the Sun’s birth 4.5 billion years ago!

With two-thirds the diameter of Pluto, far-flung Sedna is already an interesting world in its own right. However remote the possibility may be, the thought that Sedna once orbited another star is rather fascinating because that will make Sedna the first known extra-solar dwarf planet in the solar system. What is Sedna trying to tell us? With just a single object, there will be no way of finding out and the next practical step will be to continue to search the skies for more objects like Sedna. All these explain why I personally think that Sedna is the most interesting TNO discovered so far since the discovery of Pluto.