Amazing Space Thingy Pt I: Exoplanets

I’ve been interested in astronomy since I was very young – my interest kick-started by this great book – and much of what I learned over twenty years ago is still true now. There are two areas, though, where a lot has changed in the last decade or so. Here’s the first.

I understood that there were nine planets around the Sun- this was before the days of Eris and Pluto’s demotion. There were also asteroids, comets and moons. Everything else that we saw in the sky was a star. The experts reckoned that there probably were planets around other stars, but we couldn’t really know. For anyone who wonders if we’re alone in the universe, the question of exoplanets is a key one, as it feeds into the Drake Equation.

The difficulty in detecting planets around other stars comes from the fact that the planets are small and dim, and are (in astronomical terms) right next to huge bright stars. I remember a simple demonstration – it may have been on Tomorrow’s World – where a presenter lit a match, then brought it closer and closer to a bright light bulb. The match eventually became invisible. The difference in brightness between a star and its planet is much greater than the difference between a bulb and a match, so the idea of seeing exoplanets directly seems to be a non-starter.

Fortunately, some clever astronomer types thought that there may be better ways of finding planets than direct observation. The first phenomena that might be explained by planets were noted in the late eighties and early nineties. Detections used tiny accelerations in the stars’ movements caused by their planets, or changes in the timing of pulsars’ radio bursts. The first confirmation of an exoplanet’s existence was in 1992, using the pulsar timing method.

Since then, more detection methods have been used, and now there are 208* known exoplanets. Many of these are of a type called hot Jupiters: very close to their parent star (typically within 0.05 AU) and with a mass similar to or greater than that of Jupiter. These are common amongst the known exoplanets because they are comparatively easy to detect by their affect on their parent star. Personally I suspect (without any evidence whatsoever) that’ll we find that there’s nothing special about hot Jupiters, and we’ll find as many types of planet, in as many different orbits, as we are capable of detecting.

Recently even direct observation has become practical for some kinds of planets. Last year the planet 2M1207b was directly imaged. There are a number of other candidate planets that have been directly observed, but it can take a while to determine if these are planets or brown dwarves.

Future telescopes, both ground- and space-based, will doubtless discover far more exoplanets, of a wider variety and at greater distances than currently possible. The Terrestrial Planet Finder and Darwin missions are both still in the planning stages, but could potentially be used to detect and image planets of a size similar to Earth’s. If the presence of oxygen in their atmospheres can be inferred, then there will suddenly be very strong evidence for the existence of life elsewhere in the universe.

The universe is full of interesting things, and we’re going to discover a lot more of them over the next couple of decades.


* According to the Wikipedia entry on extrasolar planets (http://en.wikipedia.org/wiki/Extrasolar_planet, accessed 01/10/06) there are 202. I’m taking the figure from the The Extrasolar Planets Encyclopaedia (http://exoplanet.eu/, accessed 01/10/06) instead.

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