Ordinary Meeting, 2007 January 31
The Wide-Angle Search for Planets
The WASP project, Dr Wheatley explained, was searching for planetary systems around nearby stars. Before discussing that work, however, he reviewed our own Solar System as a point of reference. Excluding Pluto, there were eight known planets, of two distinct varieties. The inner four were terrestrial planets, with rocky surfaces and physical densities similar to the Earth's 5.5 g cm-3. The outer four were gas giants, having gaseous surfaces and much lower physical densities – in Jupiter's case, 1.3 g cm-3.
It was interesting to calculate how easy it would be to detect the planets of our Solar System from an observatory several light-years distant. One might imagine that their intrinsic faintness would be problematic, but this was in fact not so. The absolute magnitude of Jupiter – the magnitude with which it would be seen at a distance of 10 pc – was 27. Though this was faint, it would be quite within the reach of modern telescopes; in other words, a hypothetical civilisation living 10 pc away, with an exact replica of our state of technological development, would be able to detect it. The absolute magnitude of the Earth was 30 – though more challenging, even this would be a plausible photometric target.
The principal difficulty in imaging extra-solar planets (a.k.a. exoplanets) was not their faintness, but contrast. The absolute magnitude of the Sun was 4.8 – brighter than Jupiter by 22 magnitudes. Distinguishing planets from the glare of their host stars was a great challenge. Using large telescopes such as the 8.2-m Very Large Telescope (VLT) in Chile, fitted with adaptive optic systems to correct for atmospheric seeing and achieve sub-arcsecond angular resolution, this was now at the verge of feasibility. One exoplanet, orbiting a star called 2M1207, had now been imaged by the VLT. This, however, had been a five-Jupiter-mass (MJ) planet orbiting a brown dwarf, which itself weighed a mere 25 MJ – as the first ever image of an exoplanet, it was a profound achievement, but it was a far-cry from imaging anything akin to the Earth.
Looking ahead, the 40-m Extremely Large Telescope (ELT), proposed by the European Southern Observatory (ESO), would be able to image planetary systems similar to our own, but it was not scheduled to be built until 2020. The Darwin Observatory – a space-based constellation of four telescopes planned by ESA – would not only image exoplanets, but also take spectra of them. This would allow the composition of their atmospheres to be studied. Atmospheric scientists had identified certain features, termed biomarkers, which, if detected in an exoplanet spectrum, were thought to be sure indications of the presence of life. One example was ozone – O3 – which indicated the presence of free oxygen atoms in a planet's atmosphere. Over time, one would expect free oxygen to react with rocks to become bound up in minerals; a continuing supply indicated replenishment and in turn respiring organisms. As with the ELT, however, Darwin's work was some years distant: it was scheduled for launch around 2015. The remainder of this talk, therefore, would look at what was possible with current instrumentation. There were indirect techniques for detecting exoplanets, which did not obtain images of the planets themselves, but which inferred their presence by detecting their influence on their host stars.
The most prolific such technique worked by detecting the small motions of stars induced by the gravitational pull of planets orbiting them. Whilst simplified pictures always showed planets orbiting around stationary stars, more precisely, both revolved around their common centre of gravity. The motion of a star due to the gravitational pull of its planets was normally neglected because it was so small – as an example, whilst Jupiter orbited at more than 30,000 mph, the corresponding motion of the Sun was at a mere 30 mph – but if this motion could be detected, it allowed the presence of planets to be inferred from observations of their host stars alone.
This method – termed the radial velocity method – had yielded the first ever exoplanet discovery in 1995: a 0.46 MJ object orbiting 51 Peg. To date it had yielded more than 150 discoveries. Most of these planetary systems were rather unlike the Solar System – in them, large Jupiter-like planets were in close orbits around stars, often with orbital periods of only a few days. Part of the explanation of this was that massive planets in close orbits produced the most conspicuous radial motions in their host stars, and so were the easiest to detect. But it was still surprising that such planetary systems existed at all. Currently it was impossible to ask whether they were 'typical'; they were simply those which radial velocity searches were most apt to find.
Though radial velocity searches were the most prolific source of exoplanet discoveries to date, they revealed little about the physical nature of the objects found: only their masses and the eccentricities of their orbits. A complementary search strategy, and the only other to have been employed to date, worked by detecting the fractional dimming of stars when planets transited their faces. This method could only detect edge-on systems with planets of comparatively large radii – which produced detectable transits – but was more enlightening of the physics of the planets discovered.
Initially, this approach had been trialed on stars already known to have planetary systems from radial velocity measurements. One such star was HD209458, which a radial velocity survey in 1999 had discovered to have a 0.69 MJ planet orbiting it at a distance of 0.045 AU. In 2001, the brightness of HD209458 was observed to decrease by 1.7% for around 3 hours – the first detected transit of an exoplanet. A humble off-the-shelf 10" telescope had been used to make this detection; HD209458 was bright (mag 7.7), and so the photometry did not require a large aperture.
This detection had brought a much greater understanding of HD209458's companion. From the degree of dimming during the transit could be estimated the companion's radius; the larger the planet, the more light it would have obscured. The resulting radius estimate was 1.43 times that of Jupiter, suggesting the companion to be a gas giant, most probably inflated by the intense heat of HD209458, a mere 0.045 AU distant. Later in 2001, the HST had taken a spectrum of HD209458 during a subsequent transit, finding that the dimming in sodium spectral features was appreciably greater than that seen elsewhere in the spectrum. This was interpreted as evidence for a sodium-rich atmosphere around the companion planet. For the first time, some indication of the chemistry of an exoplanet was available.
Following up on this success, the speaker amongst others had set up two observatories, termed collectively SuperWASP, dedicated to the continuous monitoring of the brightnesses of around seven million stars. To give coverage of both hemispheres, sites in La Palma and South Africa had been chosen. Each observatory housed eight wide-angle telescopes, having between them a combined field-of-view of around 1% of the whole sky. To keep the cost down, off-the-shelf components had been used: 11-cm Canon telephoto lenses and standard 2048×2048 pixel CCD arrays.
Soon after these observatories had become operational, large numbers of stars had been identified as having suspect dips in their light-curves. Most of these had turned out to be variable or eclipsing binary stars. Before any of these dips could be declared exoplanet discoveries, an independent radial velocity detection was required as confirmation. After many disappointments, SuperWASP had recently made two confirmed discoveries, named WASP-1b and WASP-2b.
A significant outstanding question in planetary science was what determined the radius of gas-giant planets. Their masses and temperatures were undoubtedly important, but the details of their internal structures were not well understood. Exoplanets detected by both radial velocity and transit methods were valuable probes of this, because their masses, radii, and distances from their host stars, could all be estimated. SuperWASP would increase the number of such planets over coming years, and perhaps shed some light on this.
Following the applause, Mr Nick James asked how the automation of SuperWASP's search procedure was progressing. Dr Wheatley replied that the construction of stellar light-curves and identification of transit candidates was now fully automated. However, it was still necessary for humans to manually inspect the light-curves of these candidates to confirm them as genuine.
The President then adjourned the meeting until March 28 at 5.30pm at the present venue.
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Dominic Ford