Pale blue dots

Four centuries ago, in 1609, Galileo Galilei pointed his homemade telescope at the heavens and discovered the four large satellites of Jupiter. It was a great year for astronomy, but an annus horribilis for religious doctrine, as it proved that the Earth was not the centre of the Universe.

Given that the planets of our solar system orbit the Sun, and that many of them are orbited by moons, it was a fairly safe bet that other stars would be host to families of planets. The trouble was that planets beyond our solar system, otherwise known as extrasolar planets or exoplanets, were impossible to see, partly because of their great distance from us, but also because of their extremely low luminosity when compared to their parent suns.

This changed in 1995, when astronomers reported the detection of an object, believed to be a planet in the Pegasus constellation (see box on p24 'Exoplanet Discovery'). It was a discovery that, like Galileo's before it, promises to disrupt the status quo not only within the astronomy community but in society at large.

Remote imaging

To the naked eye, the planets of our solar system look very much like stars, and even the largest terrestrial telescopes can provide only indistinct images of the closer bodies, which is why space telescopes and interplanetary probes have been developed.

Unfortunately, using currently available propulsion technologies, it takes a long time to reach the outer reaches of our own astronomical backyard. NASA's New Horizons probe to Pluto, for example, is currently three years into its nine-and-a-half-year trip to the edge of the solar system, which itself is only a fraction of the way to the nearest star. The spacecraft may be travelling at a record-breaking 17 km/s (40,000mph), but it would still take more than 70,000 years to get there.

This means that if we wish to detect planets around other stars, we must rely on the technologies of remote imaging, much like Galilei. The difference, of course, is that imaging technology has moved on somewhat since 1609.

It was Cornell astronomer Carl Sagan who famously lobbied for NASA's Voyager probe to look back towards the Sun and take a 'family portrait' of the solar system. Thus, on 14 February 1990, some six billion kilometres from Earth, Voyager's Imaging Science Subsystem produced the so-called 'Pale Blue Dot' image of the Earth. Sagan's vision was justified… and he got a book out of it.

Voyager's imaging system used a 1,500mm telephoto lens and a 25mm-diameter magnetic deflection vidicon, a type of television camera tube incorporating a photoconductive surface on which electric charge patterns form in response to incident light.

The image was converted into a TV signal by scanning the pattern with an electron beam. Each frame comprised 800 lines, each with 800 picture elements (pixels), which made it the equivalent of a 640 kilopixel digital camera.

According to NASA, the crescent Earth occupied just 0.12 of a pixel, and as Sagan wrote in his book, 'Pale Blue Dot: A Vision of the Human Future in Space': "The Earth is a very small stage in a vast cosmic arena." 

Apart from anything it might have said about our place in the Universe, the image showed how difficult it is to detect a planet orbiting a star, even from within its own solar system and when its position is accurately known.

Space telescopes

It is no surprise that the majority of the exoplanets discovered so far have been gas giants (see box on p25 'Detection Techniques'), but as detection methods have advanced, rocky terrestrial-type worlds have become the new target.

A specialist meeting held in January 2009 at the Royal Astronomical Society in London considered potential developments in the detection of 'Super-Earths' of only a few Earth-masses. Attendees discussed not only methods of detection, but also the possibility of modelling the planets' atmospheres, surfaces and internal structures. One even considered the subject of 'Exomoon Detection'.

Although terrestrial methods are by no means exhausted, it is likely that many of the most important and detailed discoveries will be made by instruments carried aboard spacecraft. Indeed, a search is currently underway by Europe's COROT satellite. COROT stands for convection, rotation and planetary transits. 'Convection and rotation' refer to a capability to study acoustic waves that ripple across the surfaces of stars (a technique known as astroseismology), and 'transit' refers to the detection of exoplanets by the dimming of starlight.

Launched in December 2006, the plan was that COROT would monitor about 120,000 stars during the course of its two-and-a-half-year mission. Although it has already discovered a number of exoplanets, none of them has been Earth-like. The closest was Corot-7b, announced in February 2009, which is rocky and less than twice the diameter of Earth, but has a surface temperature of between 1,000°C and 1,500°C.

COROT's payload is a telescope with a 30cm-diameter primary mirror and a two-part camera, half of which is designed to detect the subtle variations in a star's light caused by those acoustic ripples, the other half - to look for planets. According to the French space agency CNES, which commissioned the spacecraft, both functions require the camera to be sensitive to changes in a star's light of "just one part in one hundred thousand".

The camera itself has an imager comprising four frame-transfer CCD detectors with a 2048 x 4096 pixel array. In a frame transfer CCD, half of the detector area is covered by an opaque mask. This allows the image collected by the exposed half to be transferred quickly to the covered 'storage region' and read out slowly, thus ensuring the best possible image quality. Meanwhile, a new image is forming (typically with a
long integration time) in the active area. The alternatives - flying two similar detectors (complete with cost and mass impacts) or halving the effective telescope time - were less attractive.

The next step up and the first mission designed specifically as a 'terrestrial planet finder' is NASA's Kepler observatory, which was launched in March 2009. For about three-and-a-half years, the telescope will observe a particular area of space continuously and simultaneously monitoring the brightness of more than 100,000 stars in the hope of detecting planets as small as Mars, which is about half the diameter of Earth.

As might be expected some three decades after Voyager's 640 kilopixel vidicon tube, the technology and the pixel count are much improved. The photo-meter, or 'light meter', that forms the core of the Kepler instrument comprises an array of 42 CCDs of 50 x 25mm, each of which has 2200 x 1024 pixels, giving a total of 95 megapixels. The CCDs are not used to make images, however. Only the information from the pixels that contain stars brighter than about magnitude 16 is recorded, stored onboard for about a month and then transmitted to the ground for analysis.

The instrument, based on a 0.95m-diameter telescope, has a very large field of view for an astronomical telescope - 105 square degrees - comparable to the area of a hand held at arm's length and more than 100 times that of a typical telescope. It needs that wide a field to observe the large number of stars.

Kepler uses the transit photometry method of exoplanet detection which involves monitoring the small changes in a star's brightness (typically one in 10,000) when a planet moves in front of it. Of course, the observation must be repeatable and the effect identical to prove the existence of a planet, a fact that argues for a space-based photo-meter because it avoids the day-night cycles of ground-based observing.

Moreover, to avoid the similar interruption experienced by observatories in low Earth orbits, Kepler is stationed in an 'Earth-trailing heliocentric orbit' (an orbit like the Earth's about the Sun).

According to NASA, if most of the theoretically observable planets are about the same size as Earth, Kepler would be expected to detect "about 50", but if most have a radius only a third greater than Earth the number expected increases to "about 185". According to Lawrance Doyle of the SETI Institute in California: "At the photometric precision achievable by the NASA Kepler mission, [it] should be able to [detect] over 1,000 additional giant planets".

Signatures of life

Despite the impossible distances involved, the confirmed detection of a doppelganger-Earth would have profound societal implications, especially if it appeared habitable. 

A mission now under development by the European Space Agency (ESA), called aptly enough Darwin, is designed to detect the chemical signatures of life in the atmospheres of any "nearby" exoplanets (in other words, those less than 75 light-years away - 17 times the distance of the nearest star).

The mission is unusual in using a flotilla of telescope-carrying spacecraft flying in formation. According to ESA, one spacecraft will be a central communications hub, while the others act as 'light collectors', redirecting light beams to the hub spacecraft.

The system relies on a technique known as nulling interferometry, involving the destructive interference of light rays from parent stars and the constructive interference of light from planets. The result is that the feeble glow of the planet is revealed despite the overwhelming glare of the star.

One engineering challenge of the Darwin mission, among many, is the requirement to keep the telescopes and the hub in formation with millimetre precision, but ESA is "confident of achieving this" using a variation of GPS satnav techniques.

As NASA is considering a similar mission, it is possible that the two agencies will engineer a joint undertaking which would also save money.

Less than 15 years ago, such missions would never have got beyond the proposal stage, but as the exoplanet catalogue grows, it becomes increasingly likely that a habitable planet in another solar system will be found.

Galileo Galilei's discoveries shook the foundations of 17th century society, and confirmation of another 'pale blue dot' will do the same for the 21st.

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