Astronomers find planets in unexpected places

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Are we alone in the Universe? This is a question which divided philosophers as long ago as the age of the ancient Greeks. Epicurus thought that the number of worlds was infinite, while Aristotle believed that the Earth was unique and at the centre of the Universe. Twenty-four centuries of debate and discussion went by before astronomers were able to prove Aristotle wrong! The Earth is not unique, and since 1995 we have known that there also exist planets orbiting stars like the Sun: the extrasolar planets (exoplanets). More than 200 of these worlds are now known, and the number increases monthly. None of these planets really resembles the Earth, but astronomers are sure that exoEarths will be found as soon as appropriate instruments become available.

So, there are other solar systems. Their exploration by astronauts cannot be accomplished for centuries to come, but by 2010, due to space missions still at the planning stage, we should know if there are rocky, Earth-like planets out there. They may orbit their parent stars at distances not too near, and not too far, for life to have gained a foothold.

Twenty years from now we could have the necessary instrumentation to be able to search these exoEarths for signs of life: an atmosphere containing oxygen, or the signature of chlorophyll.

So many discoveries - and surprises - have come in quick succession since 1992, when the only solar system we knew of was our own! This was the year in

Aristotle (348-322 BC) believed the Earth, at the Epicurus (341-270 BC) envisaged the centre of the Universe, to be unique. existence of multiple worlds.

1.1 Twelve years of discoveries and surprises 3

1.1 Twelve years of discoveries and surprises 3

The Earth - a life-bearing planet with oceans, land masses, clouds and polar caps (seen here from Apollo 12) - bears all the hallmarks of a planet where life has developed. We are a long way from being able to say the same of exoplanets.

Another Earth, 10 light-years away? A

synthetic image of an exoEarth 10 light-years away, as it might be seen by an armada of 150 3-metre space telescopes travelling 150 km apart - a project still in the files of the space agencies!

The Earth - a life-bearing planet with oceans, land masses, clouds and polar caps (seen here from Apollo 12) - bears all the hallmarks of a planet where life has developed. We are a long way from being able to say the same of exoplanets.

Another Earth, 10 light-years away? A

synthetic image of an exoEarth 10 light-years away, as it might be seen by an armada of 150 3-metre space telescopes travelling 150 km apart - a project still in the files of the space agencies!

which radio astronomers sprang the first surprise: Earth-sized planets orbiting a pulsar - a star at the end of its life cycle. This was absolutely the last place where one might have expected planets to be, since surely no planet could have survived the supernova explosion which results in a pulsar.

In 1995 there was another surprise when a Swiss team announced the discovery of an exoplanet comparable in mass to Jupiter. This planet orbits a fairly ordinary Sun-like star called 51 Pegasi (in the constellation of Pegasus - the number referring to John Flamsteed's eighteenth-century catalogue of naked-eye stars in order of right ascension, from west to east).

A third surprise awaited astronomers: 51 Pegasi b (as this planet is sometimes designated) orbits its star in little more than 4 days, compared with the 12 years it takes Jupiter to complete one orbit of the Sun. This means that 51 Pegasi b lies at a distance twenty times less than that of the Earth from the Sun, or an eighth of the distance of Mercury from the Sun. It is easy to imagine that it must be quite a hot world!

Since 1995 the rate at which discoveries have been made has been remarkable, and the number of known exoplanets increases month by month. By July 2006 more than 200 such planets, orbiting more than 170 stars, were listed. With the exception of the three planets associated with the pulsar PSR 1257+12, they are nearly all heavyweights, with masses between 0.02 and 14 times that of Jupiter. Thus we talk of exojupiters, exoSaturns and exoNeptunes; but exoEarths still elude us, as current techniques cannot detect them near ordinary stars.

The quest for the first Earth-like planet capable of harbouring some kind of life is driving some very ambitious and difficult programmes of research. Early observations depended on instruments such as the 2-metre telescope at the Haute-Provence Observatory, used to discover 51 Pegasi b; but in future programmes, much larger instruments will have to be utilised. There is talk of Earth-based telescopes with apertures of 30-50 metres; and in space, flotillas comprising perhaps dozens of telescopes. There are many ideas for programmes of which the feasibility remains to be proved... but the difficulty of the task reflects the value of the prize.

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