Solar System has more than one “Planet X”?

12 06 2010

Recent observations of the nearby (44 light-years away) multi-planet star system Upsilon Andromedae have kindled in my mind an intriguing question:  Namely, can our own solar system have one or perhaps many “Planet Xs” hiding in oblique orbits?  Allow me to explain.

New findings show Upsilon Andromedae's planets have oblique orbits. Credit: NASA, ESA, A. Field

It has been known for quite some time that the Upsilon Andromedae star system is composed of at least three Jupiter-sized planets (we can’t yet see Earth-sized planets or smaller, yet).  However, research recently presented at the American Astronomical Society suggests that unlike our solar system where the major planets orbit in the same plane, two of Upsilon Andromedae’s three known planets orbit askew with respect to each other to the tune of nearly 30 degrees tilt.  This defies what we have come to know as a “normal” star system configuration of planets.

While there have been many “Planet X” hypotheses in our own star system over the years, including recent research suggesting the possibility of a large, distant icy planet in our own solar system, (see Tyche post here,) astronomers have not yet been able to locate any of these proposed culprits of periodic extinctions or comet peculiarities.

However, planets are notoriously difficult to find, especially the farther away from the Sun they are.  Planets do not intrinsically emit their own light (except infrared), and their reflections get exponentially dimmer with distance.  So, with the recent Upsilon Andromedae findings in mind, perhaps the reason we’ve yet to find any Planet Xs isn’t because there’s no merit to the ideas, but rather that astronomers have been looking in the wrong orbital planes.

Let’s investigate a step further. With “ordinary” planet formation in a young star system, the conservation of angular momentum causes material around a new sun to flatten into a disk, (called a “proplyd” or protoplanetary disk,) and planets form from the material in this disk.  Hence, planets will be found in an orbital plane around a star, just like ours are.  However, when we look closely, we find that there are even notable oddities in our solar system.  Namely, Uranus is tilted almost completely 90-degrees onto its side, and Pluto is not only tilted sideways, but it also orbits obliquely, much like its Jupiter-sized kin in Upsilon Andromedae.  What does this mean?  At the very least, it means that the evolution of any star system is a dynamic process.  At most, this is an indicator that we’ve yet to fully describe our own system.

On this note, Upsilon Andromedae is actually a “quiet” binary star system.  The main star, Upsilon Andromedae A, is a yellow-white star not unfamiliar to human eyes.  However, it does have a dim, red dwarf brother (unsurprisingly called Upsilon Andromedae B) in a wide orbit, far enough away to leave the planets orbiting Upsilon Andromedae A alone, so far as we are able to tell.  However – it does beg the question: Might subtler interactions of Andromedae’s red dwarf or perhaps outer, dimmer planets we have yet to find be responsible for the oblique orbits we see?  And if so, have we found a distant mirror suggesting there might be more places to look for Planet X in the far reaches of our own system?

Food for thought.

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