No Place Like Home

planet_Art
Scene from a moon orbiting the extra-solar planet in orbit around the star HD70642.
Credit:David A. Hardy, astroart.org (c) pparc.ac.uk

On the evidence to date, our solar system could be fundamentally different from the majority of planetary systems around stars because it formed in a different way. If that is the case, Earth-like planets will be very rare. After examining the properties of the 100 or so known extrasolar planetary systems and assessing two ways in which planets could form, Dr Martin Beer and Professor Andrew King of the University of Leicester, Dr Mario Livio of the Space Telescope Science Institute and Dr Jim Pringle of the University of Cambridge flag up the distinct possibility that our solar system is special in a paper to be published in the Monthly Notices of the Royal Astronomical Society.

In our solar system, the orbits of all the major planets are quite close to being circular (apart from Pluto’s, which is a special case), and the four giant planets are a considerable distance from the Sun. The extrasolar planets detected so far – all giants similar in nature to Jupiter – are by comparison much closer to their parent stars, and their orbits are almost all highly elliptical and so very elongated.

‘There are two main explanations for these observations,’ says Martin Beer. ‘The most intriguing is that planets can be formed by more than one mechanism and the assumption astronomers have made until now – that all planets formed in basically the same way – is a mistake.’

SIM reference design
SIM, scheduled for launch in 2009, will determine the positions and distances of stars several hundred times more accurately than any previous program.
Credit: NASA / JPL

In the picture of planet formation developed to explain the solar system, giant planets like Jupiter form around rocky cores (like the Earth), which use their gravity to pull in large quantities of gas from their surroundings in the cool outer reaches of a vast disc of material. The rocky cores closer to the parent star cannot acquire gas because it is too hot there and so remain Earth-like.

The most popular alternative theory is that giant planets can form directly through gravitational collapse. In this scenario, rocky cores – potential Earth-like planets – do not form at all. If this theory applies to all the extrasolar planet systems detected so far, then none of them can be expected to contain an Earth-like planet that is habitable by life of the kind we are familiar with.

However, the team are cautious about jumping to a definite conclusion too soon and warn about the second possible explanation for the apparent disparity between the solar system and the known extrasolar systems. Techniques currently in use are not yet capable of detecting a solar-system look-alike around a distant star, so a selection effect might be distorting the statistics – like a fisherman deciding that all fish are larger than 5 inches because that is the size of the holes in his net.

It will be another 5 years or so before astronomers have the observing power to resolve the question of which explanation is correct. Meanwhile, the current data leave open the possibility that the solar system is indeed different from other planetary systems.

Timelines

1990
- Hubble Space Telescope launches aboard Space Shuttle Discovery, as Earth Orbiting Observatory

1994
- Hubble Space Telescope finds evidence of black hole in the center of M87
- Hubble Key Project begins studying Cepheid variable stars to better define Hubble Constant, and the size of the universe

The TPF
The Terestrial Planet Finder will search for Earth-like planets orbiting 250 of the closest stars.
Credit: NASA

1996
- Sidney van den Bergh and Gustav Tammann debate Hubble Constant and the scale of the universe

1998
- Jim Peebles and Michael Turner debate nature of universe and whether cosmology is solved

1999
- John Cowan confirms age estimates of globular clusters and universe by dating metal-poor stars
- Wendy Freedman and Allan Sandage debate Hubble Constant and the scale of universe

2001
- Hubble Space Telescope detects an atmosphere around an extrasolar planet

2002
- Chandra X-ray Observatory finds evidence for new matter in "quark stars", matter so dense it exceeds terrestrial nuclear material with 1.2 million degree temperatures

2003
- Final mission in NASA Great Observatory series, the infrared observatory, or Spitzer Space Telescope, finds evidence for organic molecules in intergalactic regions
- Microwave measurements precisely date the Big Bang at 13.7 billion years ago, with a remarkable 1% error prediction

2006
- French COROT mission will look at 50,000 to 60,000 stars and should find a few dozen terrestrial planets and several hundred close-in gas-giant planets during a two- to three-year mission

2007
- Kepler, Extrasolar Terrestrial Planet Detection Mission, designed to look for transiting or earth-size planets that eclipse their parent stars [survey 100,000 stars]. Scientists expect to find thousands of planets, and perhaps 50 Earth-like candidates.
- Likely de-orbit for Hubble Space Telescope [date announced is highly fluid but assumes no planned shuttle visits from NASA]

2009
- Planned launch for NASA-ESA Next Generation Space Telescope, or NGST [James Webb Space Telescope], a near-infrared telescope that will succeed the Hubble Space Telescope.
- Planned launch for Space Interferometery Mission (SIM)

2012-2015
- Planned launch for TPF and Darwin missions


Related Web Pages

Night Lights
The Earth as Seen by Galileo
Blue Dot, Red Planet
Life in the Clouds
Commitment to Life on This Earth