Planet Orbits Its Star example essay topic

In the past 8 years, astronomers have discovered over 100 probable planets around other stars. According to the standard model of planet formation (pre-1996), many of these planets are too close to their stars to have formed in their present locations. This has lead to a few astronomers suggesting that these are not, in fact, planets at all. Rather, the scientists argue, these objects are really small stars. Because our most successful detection method only gives a lower limit on the object's mass, distinguishing the two classes of objects is difficult. However, I feel that there is enough evidence to conclude that many, or even most, of these objects are really planets.

Before explaining why I conclude that these are planets, allow me to remind the reader of how we have detected extra-solar planets. As a planet orbits its star, being pulled by the star's gravity, it exerts and equal and opposite force on its star. (This is in accord with Newton's 3rd law, which says that for every force of A on B, B exerts and equal force opposite in direction on A.) The parent star is much more massive than the orbiting planet, so the star accelerates much less than the planet does. (According to Newton's 2nd law, Force = Mass x Acceleration.) So that star orbits about the center of mass of the system in a manner similar to the planet, but with a much smaller semi-major axis. This motion is too small to see by looking for movement on the sky, but measuring the Doppler shift can tell us if the star is moving towards or away from us.

Unfortunately, if we do not know the plane of the planet's orbit, we cannot tell what the mass of the planet is. (For a given amount of Doppler shift, a planet orbiting in a plane that lines up with our line of sight has the smallest mass. A planet orbiting in a plane nearly perpendicular to our line of sight has the highest mass, since most of the induced motion is not along our line of sight.) Most objects detected so far have lower limits on their masses that are roughly Jupiter's mass. Stellar formation theory says objects that are this small probably did not form as stars do, but rather they formed like planets. At about thirteen times the mass of Jupiter, stellar formation models say that the objects could be "brown dwarfs", as these small, not-quite-stars are called. For masses less than this, it is considered unlikely that the objects are brown dwarfs, however.

(Brown dwarfs are thought to range in mass from 13 times the mass of Jupiter to about 80 times the mass of Jupiter. When they become more massive than this, they start to have fusion in their cores and become true stars.) One reason to think that these objects are planets is statistics. Specifically, with over a hundred objects detected, it is unlikely that all of them are orbiting nearly perpendicular to our line of sight. This means that at least some of the objects ought to be near the lower limit of the mass range. As I have stated above, at the lower limit of the mass range most objects should not be brown dwarfs. The only ways to argue that most of these objects are much more massive than their lower limits (that is, orbiting nearly perpendicular to our lines of sight) are to either argue that we have just been very (un) lucky in our searches so far or that there are just far, far more brown dwarfs out there than planets.

The former argument is highly unlikely and so safely dismissed for now. The latter argument requires far, far more brown dwarfs than planets. Searches specifically aimed at understanding brown dwarfs do not appear to show that brown dwarfs are sufficiently numerous to be this common. So the latter argument is also unlikely. And so, statistically speaking at least some of these objects are really planets.

The next argument is stronger, but only applies directly to a few cases. Astronomers have now measured the brightness of a few stars orbited by the planet-sized objects. They have seen the brightness drop and then rise back to its original levels as planets pass in front of their stars. (Much as the Moon can pass in front of the Sun during a solar eclipse, or as Mercury passes in front of the Sun in a transit event.) Any such system requires that orbiting body have an orbital plane aligned with our line of sight. (Otherwise, the object cannot pass between the star and us.) This being the case, the mass is around the lower limit of the range. Again, this requires that the object be a planet and not a brown dwarf.

My final argument that these are really planets is a theoretical one. Astronomers were initially concerned about how close many of these objects were to their stars. The pre-existing planet formation models suggested that planets could not form so close to their stars. However, even before the detection of these objects, it was suggest that planets might migrate towards or away from their stars as they form or early in their lives.

Since the detection of the first of these objects, more work has been done which shows that planet migration is a very plausible behavior. In fact, there is evidence in the outer solar solar system indicating that Neptune probably has migrated outward from the Sun. (The evidence is the large number of Kuiper-belt objects which are in orbital resonances with Neptune. Astronomers believe that the most plausible way to "capture" some many objects into these orbits is by slowly moving Neptune outward.) So the original concerns about the orbits of the objects have been relaxed significantly since their initial detection. Although, to be fair, much about the migration theory is still not understood.

Notice that I admitted a weakness in my argument above. It is good practice in science to do this when you can see such weaknesses. This is both the intellectually honest thing to do and it tells your audience that you have thought about potential problems in your reasoning. Given that the statistically at least some of the objects we have tentatively label as planets should really be planets (and not brown dwarfs) and that in at least a few cases they really are too small to be brown dwarfs, it seems safe to conclude that we have, in fact, detected planets around other stars.

This is a very important step for astronomers. It means that we can start to analyze the trends in these planets and hopefully start to understand planets in general. (Until now, we have only had our own system to base our theories upon.) This, in turn, helps us understand the likelihood of finding habitable or even inhabited planets elsewhere in our galaxy.