Our Planetary Neighbourhood


ESO/L. Calçada

This article is taken from the Winter 2011 issue of I, Science.

Nicola Guttridge explores the hunt for planets around other stars, and how
this is helping to answer questions about the possible existence of alien life in our universe.

There is no place more inhospitable and mysterious than space, although if you have read Exploring The Deep and Sub-surface Life?, you might disagree! This is one of the reasons that it inspires such passion and dedication amongst astronomers, as in spite of our many years of stargazing and spaceflight, we still know very little about the universe in which we live.

Great progress has been made in the hunt for planets that lie outside of our solar system. Known as extrasolar planets or exoplanets, analysis of their atmospheres and possible surface conditions allows scientists to postulate on one of the most poignant and scientifically frustrating questions known: Are we alone?

This question has occupied the scientific community since the early nineties, when researchers Wolszczan and Frail stumbled across the existence of the imaginativelynamed planetary system PSR B1257+12. Since this discovery, there have been many other forays in search of new planets.

The launch of MOST, the Microvariability and Oscillations of Stars project, in 2003 was followed by three subsequent missions in 2005, 2006 and 2009, all supported by the invaluable Hubble Space Telescope. Planet-searching equipment known as HARPS has played a key role from the ground. This blur of acronyms has led to an impressively high success rate; currently, the tally stands at over 680 planets discovered, orbiting over 470 stars – a remarkable achievement in just under 20 years.

The discovery of new exoplanets inevitably leads to questions concerning how many of these worlds could be home to life – intelligent or otherwise. One recent study tackled exactly this, directly comparing both exoplanets and planets within our own solar system to planet Earth.

By attributing values from 0 to 1 to the planets based on various characteristics (1 being the Earth itself), the ‘Habitability’ index considered factors such as atmosphere, magnetic field, available sunlight, surface organic compounds and tides. Another index, ‘Earth Similarity’, dealt with density, distance from parent star and size.

Three exoplanets from the Gliese 581 system topped the ‘Similarity’ list, with Mars, Mercury and the Moon also scoring above 0.5. However, although the Moon may be physically similar to us, we know that this does not mean it is able to support life. To remedy this, the ‘Habitability’ index was applied to the same planets with vastly different results. Saturn’s moon Titan, Mars, Jupiter’s moon Europa and again the Gliese system were the top scorers, with Jupiter, Saturn, Venus and Enceladus also appearing.

There are a vast range of bodies considered to be habitable, and this raises new questions. For example, who says life should be carbon-based? Is oxygen necessary? Does the planet need a Moon for life to evolve? Does the planet need a solid core? These culminate in a single question: does a planet need to be similar to Earth to sustain life?

This question remains unanswered, but as shown by the study above, the answer may lie within our own solar system. Most people have heard of the past history of water on Mars, and recent NASA research has uncovered evidence for liquid water on Jupiter’s icy moon Europa. Titan, as previously mentioned, is very Earth-like in many respects, and has not yet been fully explored. Another rocky body that has piqued interest is Saturn’s moon Enceladus, a moon postulated to house an underground ocean of liquid water.

The progress achieved by missions such as NASA’s PlanetQuest is promising. However, as the search progresses, it becomes apparent just how much there is left to explore in our solar system, galaxy and universe – and just how alien our stellar neighbourhood is to us.

More > For all things space, check out Nicky’s blog Space For Thought.

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