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.

Saturday, November 13, 2010

Weighing Up Extrasolar Moons

An extrasolar planet is a planet which orbits a star other than the Sun and from the Paris Observatory’s online Extrasolar Planets Encyclopedia, there are about 500 known extrasolar planets as of November 2010. This number is expected to increase dramatically in the next several months with follow-up observations of the hundreds of candidate transiting extrasolar planet released in the first data set by NASA’s Kepler space observatory - a ‘planet hunting’ space telescope. A transiting extrasolar planet is one which periodically blocks a small fraction of the light from its parent star as its orbit happens to bring it in front of the star.

As the number of known extrasolar planets continues to increase rapidly, it undoubtedly brings up the possibility of detecting moons orbiting around these extrasolar planets. Detecting moons around extrasolar planets will be very challenging since such objects are expected to be smaller and less massive than the Earth. However, NASA’s Kepler space observatory might have the sensitivity necessary to detect the largest of such moons around extrasolar planets. Moons around extrasolar giant planets that are close to the size of the Earth can be particularly interesting because a large number of extrasolar giant planets are know to orbit their parent stars at ‘comfortable’ distances where Earth-like surface conditions are possible on such moons!

Recently, I did some research on transit timing variations (TTV) and transit duration variations (TDV) caused by the presence of a planet’s moon perturbing the periodic transit of the planet in front of its parent star. I used the methods outlined in two papers published by David M. Kipping in 2008 and in 2009 respectively, and wrote a program which allows me to play around with the parameters. I used stars, planets and moons of different masses in various combinations and orbital configurations. Additionally, I also used various TTV and TDV inputs to determine the corresponding mass of the moon and the corresponding planet-moon orbital configuration that is responsible the various signals.

In one of my analysis, I have a star with the mass of our Sun and a planet with the mass of the Earth which orbits the star at a mean distance of 100 million kilometers. This planet has a moon that is one-twelfth its mass and the moon orbits the planet at a mean distance of 130000 kilometers. It is also assumed that the planet takes 40000 seconds to transit in front of its parent star. As the periodic transits of the planet in front of its parent star is measured, the moon will induce an observed TTV of around 20 seconds and a TDV of around 35 seconds.

In light of a paper by David M. Kipping (2010) entitled “How to Weigh a Star Using a Moon”, I wrote a separate program to study the methods outlined in this paper. Basically, if a star has a planet, and if that planet has a moon, and if both of them transit in front of their parent star, then the sizes and masses of the star, planet and moon can be precisely measured. Furthermore, knowing the size and mass of an object allows its bulk composition to be constrained. This particular method employs the TTV and TDV signals, and it requires a star to have both a planet and moon that transit it. Although no star is yet know to have both a planet and moon that transit it, NASA’s Kepler space observatory is expected to discover several of such systems.

This method of measuring the mass of a moon of an extrasolar planet is rather interesting because such a moon is likely to be less massive than the Earth and the mass of such an object will not be measurable with radial velocity measurements. Therefore, a method like this offers a means to accurately pin down the masses and sizes of the star, planet and moon respectively. The masses of moons measured in this way could well be the smallest masses that can be directly measured outside of our Solar System.