A new satellite is expected to find 3,000 of them
BEFORE 1992 astronomers could only presume that alien planets existed. That was the year the discovery of the first such worlds, orbiting a pulsar called PSR 1257+12, about 2,300 light-years from Earth, was announced. These days, astronomers have more exoplanets than they know what to do with. The Extrasolar Planets Encyclopaedia lists 3,767 confirmed worlds as of April 18th, with thousands more detections awaiting confirmation.
This torrent of discovery has made exoplanetology one of the most exciting fields in astronomy. But it is also frustrating, for the majority of those planets are so far away that, besides the fact of their existence, little can be learned about them. Data on most are limited to the orbits they trace around their parent stars, and estimates of their sizes and masses.
That is about to change. On April 18th a space telescope called the Transiting Exoplanet Survey Satellite (TESS), blasted into orbit from Cape Canaveral in Florida. TESS is designed to examine almost the entire sky, looking for evidence of planets around the nearest (and thus brightest) stars—those, in other words, that are easiest to study in more detail.
Over the course of its two-year mission, TESS will stare at around 200,000 of the stars closest to Earth, watching for telltale dips in brightness caused by a planet crossing in front of its star. The transit method, as this is known, relies on the orbital plane of a planet being aligned with the instrument doing the observing, so only a small fraction of any planets orbiting the stars in question will be detectable. Nevertheless, assuming Earth’s cosmic neighbourhood is similar to the galactic average, TESS should turn up around 3,000 planets.
The idea is to provide a target list of the most interesting worlds for follow-up observations by other instruments using other methods. One such method (the one by which PSR 1257+12’s planets were detected) is to measure the wobble that the planet’s gravity causes in its parent star as it orbits. Such measurements reveal a planet’s mass. Since TESS’s transit method reveals a planet’s size, the two together give enough information to calculate its density, and therefore to deduce whether it is made of rock or gas. Several telescopes around the world are being upgraded with just such TESS-assisted observations in mind.
Based on existing data, around 500 of the 3,000 planets that TESS is expected to find are likely to be rocky worlds with radii up to twice that of Earth. Of those, a couple of dozen might orbit in the habitable zone of their parent stars, where temperatures are right for liquid water to exist on their surfaces.
Since liquid water is required by all forms of terrestrial life, looking for it in the cosmos makes a good starting-point in the quest for life of the non-terrestrial sort. And because TESS’s planets will be so close to Earth, it should be possible to check whether they do in fact possess water, not merely whether they might.
A new generation of instruments, including ultra-large ground-based telescopes and the delayed, over-budget James Webb Space Telescope, scheduled to launch in 2020, will be able to observe starlight that has passed through an alien planet’s atmosphere on its way to Earth, and from that infer the chemical composition of the exoplanet’s air. The presence of certain gases would be tantalising hints that the planets in question might play host to life. The simultaneous presence of methane and oxygen, for example, would excite interest because these two gases react together quickly, and would thus need constant renewal to coexist.
It may eventually be possible, for the closest worlds, to take photographs of the planets. That could show general details of geography, such as how much of the surface of a rocky body is covered by liquid and how much by land. Going, in the space of less than 30 years, from knowing nothing at all about alien planets to making rough maps of them would be a feat of extraordinary proportions.