by I. E. Lester


Everyone knows what a planet is. Or at least so we thought until last year. For the last 76 years we have had nine planets in our solar system; every schoolchild has learnt this as an absolute fact. But the truth is that there has never been a precise definition of exactly what a planet is.

Times change though, and in a very well publicised meeting of the International Astronomical Union (IAU), in August 2006, a formal definition of a planet was created. To qualify as a planet the celestial body must:

  1. be in orbit around a star.
  2. be of sufficient mass that the body's self-gravity has overcome the rigid body forces of the body's matter and formed a hydrostatic equilibrium (that is, the body is nearly spherical).
  3. have cleared its orbital neighbourhood of other celestial bodies (or captured them as moons).

The new category created by the IAU is Dwarf Planet. Like Planets, these must also meet the first two criteria above but not the third. Pluto falls into this new category, as do Ceres (in the asteroid belt) and Eris (the Trans-Neptunian body initially known as 2003 UB313, and informally as "Xena").

So now everyone knows we have just eight planets. But this is not exactly true, or at least not if you are willing to look outside the Earth's own Solar System, and consider Exoplanets (or Extrasolar Planets).

In fact just over a month before the IAU removed Pluto's planetary status, the count of planets orbiting other stars had reached 200 with an announcement in The Astrophysical Journal (July 20, 2006) of five new Extrasolar Planets.

The History of Exoplanet Detection

Astronomers have been looking for planets orbiting other stars since the mid-nineteenth century, but all claims of planets orbiting stars including 70 Ophiuchi and Barnard's Star, all having been later shown to be due to observational errors.

All this changed in the late 1980s, or the early 1990s depending upon your point of view. 1988 saw a group of Canadian Astronomers (Campbell, Walker and Yang) make a rather cautious announcement of an Extrasolar Planet orbiting around Gamma Cephei (Alrai). Their announcement met with a sceptical response from the astronomical world, mainly due to the fact that the observations had been made at the very limit of the equipment being used. In fact, it wasn't until 2003 when improved technology and techniques confirmed the team's original results.

So although the planet orbiting Gamma Cephei was the first to be announced, the long delay until its confirmation effectively handed the honour of the first detection over to two other astronomers - Canadian Dale Frail and Pole Aleksander Wolszczan. In 1992 they discovered two planets around a pulsar, PSR B1257+12. This was confirmed in short order, and so mankind had conclusively discovered a planet orbiting another star.

This discovery was not entirely expected. PSR 1257+12 is a pulsar - a type of star previously not considered a candidate for having planets. Pulsars are rotating neutron stars, the remnants of supernovas. It had previously been believed that such stellar explosions would destroy any planets in orbit around the star.

Three years after this, in October 1995, the first discovery of a planet around a main sequence star was made. Two astronomers (Michel Mayor and Didier Queloz) of the University of Geneva detected a planet around 51 Pegasi, a very Sun-like star fifty light years from Earth.

This planet, 51 Pegasi b (these planets are designated in order of detection, not distance from the parent star - with the letter "a" reserved for the parent star) is not a likely candidate for life, however. Like the other early Extrasolar Planets detected it is a large world (in our solar system it would be a close second to Jupiter in terms of size) and it orbits very close to its star causing it to have a high surface temperature around 1200 degrees Centigrade. It has been given the informal nickname "Bellerophon".

Methods of Detection

Campbell, Walker and Yang's 1988 detection was made using a technique known as Radial Velocity (or the Doppler Effect). By analysing the Doppler Shifts in the spectral lines emitted by the star, slight variations in the speed (relative to Earth) can be detected. Variations like this can only be explained due to the gravitational effects of other celestial bodies and the technique has, so far, proven the most productive of all the Extrasolar Planet detection methods.

Mayor and Queloz used a technique known as Pulsar Timing Variations to discover their planets. Pulsars emit extremely regular radio waves as they rotate. Variations in the frequency of these emissions can be proven to be the effect of orbiting planets.

Over the years, as technology has improved, new techniques have been developed and there are now many methods of detecting Extrasolar Planets.

When one star is directly in front of a more distant star from the perspective of an observer on Earth, the gravitational field of the front star can act like a lens, causing the far star to be magnified. Any planets orbiting the nearer star can be detected due to their effect on this magnification.

Astrometry is the technique of measuring the exact position and path of movement of a star, and examining the movement for perturbations caused by gravity fields from nearby planets.

Periodic variations in the brightness of stars can be proven to be due to planets passing in front of the star (from our point of view), obscuring some of the light and causing the star to appear dimmer. If this dimming happens regularly, the only cause is an orbiting body.

A further technique involves analysing disks of dust surrounding stars. These disks absorb some of the star's light, and then re-emit this energy as infrared radiation. Features in these disks can suggest the presence of planetary sized bodies.

The Chances of Life

The majority of Exoplanets detected thus far have been orbiting Sun-like stars. A main reason for this is that one goal of the work is to discover an Earth-like world, so programmes have been concentrating on similar stars.

However, there is also good reason to concentrate on these stars. Large stars (tens of times larger than our Sun) emit so much radiation that they inhibit planetary formation. Smaller dwarf stars are believed less likely to have planets, and it is generally believed that these planets will be smaller than current detection methods can discover. Given that planets have now been detected around multiple star binary systems (another environment thought not to be likely homes to planets) this narrowness of search may well need to be expanded.

Although the majority of the search is concentrated on Sun-like stars, our currently available technology and detection methods are not capable of finding other Earths out there. Up to February 2007 the vast majority of the 200+ discovered Exoplanets are Gas Giants, often greater in size and mass than Jupiter.

This is to be expected, for these will produce much greater anomalies than rocky planets could. The smallest Exoplanet discovered orbiting a main sequence star is OGLE-2005-BLG-390Lb. At a mere 5.5 times the mass of the Earth, it orbits its parent star at 2.6 times the Earth-Sun distance. As the majority of Extrasolar Planets orbit very close to their star, this makes this body the most Earth-like world we know about.

So what do these discoveries mean for the chances of life out there? They do prove that planetary formation is not uncommon. There are a number of other criteria that are believed to be necessary for life to develop. But given the fact that planets are being detected that are comparable to the size of Earth, orbiting similar stars to our own Sun at Earth-like distances, the chances are looking good.

Article Copyright © 2007 by I. E. Lester. All rights reserved.
Illustration Copyright © by Heidi Kristensen. All rights reserved.

About the author

I.E. Lester is a lifelong fan of science fiction, having acquired the bug whilst on a washed-out family holiday as a child when, sheltering from the rain in a seafront kiosk store, the cover on a collection of Isaac Asimov short stories attracted a nine year old eye.

Having worked through all the fiction of Asimov (as well as Heinlein, Clarke, Moorcock, and many others) he moved onto Asimov's non-fiction, encouraging a love of science.

He studied Mathematics and Astrophysics whilst at University and works as a software designer. When not reading sf or factual science, he can often be found watching cricket or rugby, or wandering medieval streets in France or Italy.

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