Baseballs, not umbrellas…

As NASA’s search for extrasolar planets really begins to amp up thanks to its new exoplanet-finding mission Kepler, I thought I’d dedicate today’s post to exoplanets and the search for life elsewhere in the universe.

First off, for those of you who don’t know what an exoplanet is, the definition is tricky. Basically an exoplanet is a planet that orbits a star other than the Sun (reminder: the Sun is a star, you have no idea how many times this is misunderstood); however, according to the International Astronomical Union’s (the governing body of Astronomy) ruling on the definition of a planet that demoted Pluto back in 2006 (see previous “Alas poor Pluto…” post), these extrasolar planets are not technically planets by definition. So, I guess the correct way to define an exoplanet would be “a non-stellar celestial body that orbits a star other than the Sun”.

In any case, according to NASA’s Planetquest, we have found 490 exoplanets orbiting 412 stars other than the Sun to date, but none that are “Earth-like” or similar in mass to Earth. Most of the planets we have found are what we call “hot Jupiters” or gas giants that are several times more massive than Jupiter with orbits at distances about that of Mercury’s. Not that there’s anything wrong with Jupiter or Jupiter-like planets, but if we want to find life (which is really what this search is all about) most scientists agree that we will need an Earth-like planet. This isn’t necessarily true, but it’s what we know and scientists generally have a very anthropocentric (human-centered) view of the universe. The truth though is that our methods of detection for exoplanets make it extremely difficult for us to find Earth-like planets.

The first method and by far the most successful/common (the one Kepler uses) is to look for a dip in a star’s brightness as a planet “transits” or passes in front of the star. But if you think about this, Jupiter is about a 1/10 the diameter of the Sun and the Earth about 1/100, that means that the disk of the planets are roughly 1/100 and 1/10,000 of the Sun’s disk respectively. That’s basically as if you held an open umbrella and a baseball in front of a lighthouse and tried to see the drop in brightness in the beacon light. The point is, you’re going to notice a lot more umbrellas and very few baseballs.

The second method is trying to find the “wobble” of a star from a planet’s gravity. Every star-planet system orbits around the system’s center-of-mass, which is not the same as the star’s center-of-mass, but is usually somewhere still within the interior of the star (as is the case with the Sun). So the star moves, even if very slightly, and we can use the star’s light (namely its “spectra”- or light broken up into its component wavelengths using a spectrograph) to detect that and the existence of the planet that’s causing it.

The third method, “gravitational microlensing”, is really technical, very difficult, not very successful, and has to do with Einstein’s theory of general relativity so I won’t go into much detail about it. The basic concept is that the mass of the planet-star system that exists between us and another star that’s further away can bend the light from that far star as it travels pass the system on its way to Earth.

So, basically, to be able to improve our most successful method of exoplanet discovery, the transit method, we’ll need to develop super sensitive (read very large and expensive) space telescopes that will be able to resolve the minute brightness drop from small (Earth-like) planets. We’ve got to pay the star athletes lots of money if we want to catch those baseballs.


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