XO-3b: 'Quite bizarre' planet rekindles debate

By Thomas R. Webber

While the discovery of any planetary body orbiting another star is always exciting, XO-3b is a particularly fascinating world.

A transiting planet — that is, one that passes in front of its host star as seen from Earth — XO-3b has a period of less than four days. Even more peculiar is that, despite such a close proximity to its star, it has a very elliptical, or elongated, orbit.

But the piece de resistance for this particular extrasolar planet is that it is roughly 12 to 13 times the mass of Jupiter!

Christopher Johns-Krull, an astronomer at Rice University, put it very succinctly: “This planet is really quite bizarre.”

But there is even more to this tale.

A ball of gas becomes a star when it reaches a mass approximately 80 times that of Jupiter. The pressures and temperatures found at the core of such a massive object ignite nuclear fusion and hydrogen atoms fuse into helium and other heavier elements. This is sometimes called “hydrogen burning,” although the process must not be confused with the act of chemical combustion.

Brown dwarfs, sometimes called Large Gas Giant Planets (LGGPs), are objects that fall just short of being a full-fledged star. However, those more massive than about 13 Jupiters theoretically have the potential to also initiate nuclear reactions in their cores, in this case the fusion of deuterium.

Deuterium, or “heavy hydrogen,” is a stable isotope of hydrogen, possessing one proton and one neutron in its nucleus (as compared to hydrogen, which just has the one proton). Deuterium represents 0.015 percent of the hydrogen in the universe.

XO-3b seems to be right on the boundary between these two scenarios. Both the Hubble Space Telescope and the Spitzer Space Telescope are scheduled to study XO-3b in more detail in an effort to help fill the gaps of our understanding about the nature of such massive bodies.

If XO-3b does turn out to be a brown dwarf, it will be the first one found via the transiting method.

However, the discovery of XO-3b has also brought an old definition debate back to the forefront: When is an object a star? A brown dwarf? An LGGP Large Gas Giant Planet?

On one side of the table is a group that contends that if an object can fuse deuterium, regardless of any lower-mass cutoff, then it is a brown dwarf. Should an object have a mass near 13 Jupiter masses but not be undergoing deuterium burning, then it would be an LGGP.

The other side argues that the origin of the body is the real issue: Did the object form independently or as part of a planetary system?

Does this sound like deja-vu all over again? It was less than a year ago that the issue over the definition of planethood was being deliberated — a debate that ultimately ended with Pluto being booted off the list of planets in our solar system.

So once again, scientists are faced with discoveries and realizations that force us to reconsider our most basic definitions and assumptions. As frustrating as it may be, we must always remember that the universe does not exist in neat little packets that we can easily identify and sort. Rather, we try to impose, and sometimes force, our egocentric ideas on a vast and strange universe.

And as we learn more, it is humanity, not the universe, that has to change. We must continually question, challenge and grow.

Indeed, this is the very heart of what makes science work.