IS the Earth alone in the Universe or are there vast numbers of planets orbiting other stars, occupied perhaps by alien civilisations? Earth-bound astronomers have never succeeded in detecting signals broadcast from outer space, and the 鈥渇lying saucer鈥� image of these searches makes them unpopular with budget-conscious politicians, who recently cancelled NASA鈥檚 search for extra-terrestrial intelligence (SETI) programme. But another approach may prove more fruitful 鈥� looking for Earthlike planets rather than aliens. Next week, astronomers and both European and American space agency officials are gathering in Boulder, Colorado, to thrash out their ideas for how to put this into practice.
NASA has been interested in the search for Earthlike planets for some time, and recently initiated a programme called ASEPS (Astronomical Studies of Extrasolar Planetary Systems). Last month, ASEPS hosted a workshop at the Jet Propulsion Laboratory in California to work out the best strategy for detecting planets around other stars. Meanwhile in Europe, ESA鈥檚 Horizon 2000+ consultation process has identified planet detection as a priority and supporters of two rival European missions will be at Boulder to present their ideas.
Beyond the obvious political problems of who works with whom on what, and who picks up the tab, there are significant technical challenges ahead. One of the biggest is that even large planets are very faint compared with the star they orbit. From about 13 light years away 鈥� next door by astronomical standards 鈥� the Earth would appear as a point of light only 0.25 seconds of arc from the Sun and almost 10 billion times fainter than Earth. Even detecting a planet the size of Jupiter, the largest planet in our Solar System with a mass more than three hundred times that of Earth, would be difficult because tiny imperfections in telescope mirrors scatter starlight into a halo that can swamp the feeble light from nearby planets.
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Even a perfect telescope cannot concentrate light from a distant star into a single point. Instead, diffraction spreads the light into a series of bright and dark rings and the amount of light in these rings, called the diffraction pattern, can easily drown out the image of a nearby planet. Worse still, distortions caused by the Earth鈥檚 atmosphere usually limit the resolution of star images to about 0.5 arc seconds. Until recently this has precluded attempts to image extrasolar planets from Earth, but according to Roger Angel, an astronomer from the University of Arizona, adaptive optics (鈥淣ew eyes for an ageing star鈥�, New 杏吧原创, 14 January) could soon change this.
Angel believes that fitting adaptive optics to large telescopes, with mirrors at least 6 metres across, will make it possible to detect Jupiter-sized planets orbiting around nearby stars. If the planet was at its widest separation from the star, as seen from the Earth, he says it might take only a few hours of observing time to pick it out.
In the meantime, several different groups have been trying more indirect approaches, looking at the behaviour of the central star for hints of a lurking planet. One such approach, called astrometry, relies on the fact that very large planets 鈥� even bigger than Jupiter 鈥� should exert a detectable gravitational pull on their star. Jupiter is a cold gaseous world unable to support life, and other Jupiter-sized planets would presumably have formed by similar processes and have similar characteristics. But the presence of a Jupiterlike planet around another star may also indicate the existence of smaller, more hospitable planets in the same system.
Wobbly stars
The gravitational effect of a Jupiter-sized planet would be detectable because planets do not actually orbit around a star. Instead, the whole system revolves around the combined centre of mass, or barycentre. If the planets are small, the barycentre will more or less coincide with the centre of the star. But with one (or more) massive planets, the barycentre will be some distance from the star鈥檚 own centre. As the star circles around the barycentre, there is a telltale 鈥渨obble鈥� in its position, with a period equal to the 鈥測ear鈥� of the massive planet 鈥� 12 years in the case of our own Solar System which is dominated by the Sun and Jupiter.
The only astrometric search today is being carried out by a small team led by George Gatewood of the Allegheny Observatory of the University of Pittsburgh. Almost every clear night, Gatewood and his colleagues use an instrument called a Multichannel Astrometric Photometer (MAP) to measure the position of about twenty nearby stars. This instrument moves a grating of clear and opaque lines along the focal plane of the telescope. The stars in the viewing field blink on and off as the lines from the grating pass by, and the time sequence of the blinking depends on the relative positions of the stars in the field. If one of the stars has moved since the last observation, the blinking pattern will have changed slightly. Since the blinking, and hence the star positions, can be measured very accurately, it is possible to detect small movements of the star and to search for the wobbles caused by large planets.
Even though Gatewood has been observing for seven years he has seen no evidence for any planets. The problem is not the quality of the data 鈥� by now, he should have detected any planets with half the mass of Jupiter around nearby Barnard鈥檚 star or objects twice Jupiter鈥檚 size around a number of other stars. 鈥淚t seems that very large planets are rare,鈥� he says. 鈥淚t was once thought that Jupiter was a run-of-the-mill giant planet, but perhaps Jupiter is a giant giant planet and not typical at all.鈥� Gatewood is now trying to develop a more advanced version of the MAP with ten times better resolution; this should be able to detect planets the size of Uranus and Neptune, both around fifteen times the mass of the Earth, which he hopes are more common than 鈥淛upiters鈥�.
Meanwhile, other astronomers are trying a different approach 鈥� looking for the motion of a star along rather than across the line of sight. If a giant planet is orbiting in a plane that is more or less edge on as seen from the Earth, the movement of the central star around the barycentre will cause the star to move towards and away from the observer over a period of years. This produces small but measurable changes in the frequency of the star鈥檚 light.
The effect is tiny: Jupiter causes our own Sun to move backwards and forwards with a radial velocity of only 12 metres per second, introducing a shift in the frequency of light of less than 3 parts in 10 million. But although most ways of studying the radial velocities of stars are only accurate up to a few hundred metres per second, various researchers have produced more accurate systems to search for planets.
Bruce Campbell of the University of Victoria in Canada and his colleagues recently completed a 12-year search using the 3.6 metre Canada-France-Hawaii Telescope on Mauna Kea, Hawaii. They placed a cell of hydrogen fluoride gas in the telescope beam to calibrate the spectrum from the star. The idea was that instrumental variations, which would otherwise limit the accuracy, affected both the starlight and the reference beam and cancelled out, producing an impressive accuracy of 13 metres per second.
Similar techniques have been used by other teams, including Bill Cochran at the University of Texas McDonald Observatory, who uses an iodine cell as a calibrator. These other searches have been in progress for about seven years but so far have not detected any planets, even though they should by now have been able to find any really massive objects, the 鈥淪uper-Jupiters鈥�, around the stars observed.
Spectroscopic techniques are improving all the time. Geoff Marcy of San Francisco State University has equipment that has recently been upgraded, and is beginning to deliver accuracies of between 2 and 4 metres per second. This would certainly be good enough to detect a planet the size of Jupiter, and perhaps even slightly smaller.
The lack of success of any of these programmes to date makes it clear that even finding even a Jupiter sized planet is no easy task. Finding an Earth-sized one will be even harder. According to Angel, even with adaptive optics, detecting Earth-sized planets is unlikely to be possible from the ground and so a move into space is probably necessary. Not that it would be exactly easy there: even above the distorting effects of the Earth鈥檚 atmosphere, taking a direct image of any Earthlike planet would need a space telescope with a mirror about twice the size and ten times smoother than the Hubble Space Telescope. This would be very expensive, and is unlikely to be built for many years, if ever.
Planets in transit
A much cheaper alternative is a mission called FRESIP (Frequency of Earth-Sized Planets), put forward late last year by Bill Borucki of NASA鈥檚 Ames Research Center in California as part of NASA鈥檚 鈥淒iscovery鈥� programme. FRESIP would monitor the brightness of Sunlike stars to search for changes caused by planets as they pass across them. The effect is small 鈥� the Sun loses only 0.01 per cent of its brightness as the Earth passes in front of it 鈥� but the diminution stays the same for several hours, lasts for a fixed time and happens once every year, which should make it possible to pick it out against a background of random variations in the star鈥檚 brightness caused by flares or starspots. Borucki says that three such repeatable events will be enough to establish the existence of an Earthlike planet and should allow researchers to predict the next transit, which would prove the issue.
Transits can be detected only if the observer is roughly level with the orbital plane of the star鈥檚 planets and from geometric considerations the likelihood of this is only about 1 per cent. Since there is no way to know in advance which stars to start with, the best plan is to observe a large number of stars in the hope of picking some at least that have planets in the right plane, and catching the planets in the act of passing in front of their star. These sorts of observations require round-the-clock monitoring and are best done from space, where there is no interference from daylight, or bad weather.
Borucki鈥檚 FRESIP would use a 1-metre wide-angle telescope with a field of view of about 10 掳 and a massive array of very sensitive detectors. It would be placed in orbit around one of the Earth-Sun Lagrangian points, regions in which the gravitational influences of the Sun and Earth virtually cancel each other out and a satellite can stay in position using very little fuel. From here, FRESIP could stare at the same area of sky all the year round, pointing in a direction chosen to include about 5000 stars like our own Sun. If planetary systems are very common, FRESIP should detect about 50 transits per year.
While FRESIP was described by the 100-strong peer-review committee assessing it earlier this year as the only feasible technique to detect Earthlike planets, there were some doubts about whether it could be built within the Discovery programme budgetary guidelines of less than $150 million per mission. Two independent panels are now reviewing the estimates and if FRESIP is approved as part of the Discovery programme next year, it would be launched around the turn of the century.
Because of their much smaller mass and hence smaller influence on their parent star, detecting Earthlike planets by astrometry would require instruments that are precise to about a tenth of a microarc second. This could be achieved using interferometry, where the beams of light from two telescopes some distance apart are combined to mimic the resolving power of a single telescope with a mirror as wide as the distance between the two telescopes.
At NASA鈥檚 Jet Propulsion Laboratory in Pasadena, Michael Shao and his colleagues are building an infrared interferometer that can be used for very high precision astrometry. They want to detect extrasolar planets about the size of Uranus and Neptune around stars up to 40 light years away. Shao鈥檚 interferometer 鈥� two 40 centimetre telescopes 100 metres apart 鈥� is being funded by NASA to demonstrate the technology.
NASA is also making arrangements to use the twin 10-metre diameter Keck telescopes on Mauna Kea, Hawaii, for planet searches. One approach is to build a number of small 鈥渙utrigger鈥� telescopes near to the main Keck telescopes and run in parallel with one of the main Kecks as a multi-element interferometer. Shao believes that with such a set-up accuracies of a few microarc seconds and detection of planets somewhere between the sizes of Earth and Neptune should be possible.
The final step, detection of an Earth-sized planet by astrometry, will be very challenging. Subtle complicating factors, like the effects of starspots on the light from the target star, may introduce errors that are difficult or impossible to remove. Even if an astrometric programme eventually succeeds, it will still leave open the question of whether the planets are habitable, or whether they are inhabited. DARWIN, a major mission proposed to ESA late in 1993 by a team led by Alain L茅ger of the University of Paris, would look for signs of life using a space-based interferometer made up of two or more infrared telescopes between 1 and 2 metres in diameter and about 10鈥�30 metres apart. A mission of this general type is also being considered as part of the ASEPS deliberations.
Sharper contrast
DARWIN鈥檚 equipment would observe in the infrared region of the spectrum for a number of reasons. First, the contrast between a star and a planet is greater for infrared than for visible light. This is because Sunlike stars, with temperatures of about 5000 K, emit most of their light in the visible region of the spectrum whereas planets, with temperatures of only a few 100 K or less, have their peak emission in the infrared. Although the greater size and higher temperature of a star makes it much brighter than a planet at all wavelengths, in the infrared end of the spectrum, the difference in brightness is less, so the planet is easier to spot. At a wavelength of about 10 micrometres the Earth is the brightest planet in the solar system, although it is still about 10 million times fainter than the Sun. The DARWIN interferometer would be arranged so that starlight in the various beams would interfere destructively and largely cancel each other out, making it easier to detect the faint signal from an Earthlike planet.
Since oxygen has no easily detectable spectral lines in the infrared, the planet鈥檚 signal would be examined for a strong absorption band from ozone at a wavelength of 9.6 microns, right in the wavelength range in which an Earthlike planet should be relatively bright compared with its central star. As with the Earth, the presence of an ozone layer would indicate considerable amounts of oxygen in the atmosphere below. Since oxygen is so reactive that it is normally removed quickly from the atmosphere, its presence would suggest that the oxygen was being replaced by biological activity akin to photosynthesis, and that the planet supported life.
DARWIN is one of two planet-detecting projects being considered by ESA. Between now and the year 2000 DARWIN, and its astrometric rival GAIA, which would cost about the same as DARWIN, will be studied in more detail and eventually one will be adopted as a cornerstone of the ESA鈥檚 Horizon 2000+ long-range space programme. It will be a difficult choice. GAIA should be able to detect planets, but the technically more challenging DARWIN could detect life as well.
Actually seeing what a distant planet looked like would be the most difficult task of all 鈥� even the projects that detect the planet directly would see it only as a point of light. It would require space-based interferometers with telescopes separated by vast distances to mimic huge telescope mirrors. This could not be done with a single spacecraft, but Shao believes that it will soon be possible to use different spacecraft as components of a giant interferometer.
He envisages three spacecraft in solar orbit, placed at the corners of an equilateral triangle with sides 1000 kilometres long. Two would be telescopes, the third a platform at which the beams would be combined and analysed. Lasers would be used to make extremely precise measurements of the distance between the three elements so that the beams could be combined correctly. Such a project would form the final stage of ASEPS and would deliver the first detailed image of a world beyond the Sun.
It will be a picture well worth waiting for.
Pulsar planets
THE first extrasolar planets have already been found, but not around a Sunlike star. In 1992, Alexander Wolszczan of Pennsylvania State University reported in Nature that at least two planets were orbiting the millisecond pulsar PSR B1257+12 (New 杏吧原创, 鈥淧uzzle of the pulsar planets鈥�, 18 July 1992). Wolszczan suspected the existence of the planets when very accurate timing of the pulses from this collapsed star revealed systematic inconsistencies, which could not be explained by changes in the pulsar itself. Instead, he reasoned that they were caused by minute gravitational tugs from small planets orbiting the pulsar.
These planets are most unlikely to be habitable and were probably formed from the wreckage of a companion to PSR B1257+12, destroyed many millions of years ago.