NASA’s Kepler satellite has continued to set the pace in discovering planets around other worlds, even though its pointing mechanism failed in 2013. Its latest catalog, released earlier this month, contains more than 4000 exoplanet candidates.
But ground-based astronomers are not taking this lying down. A group of European institutions today announce first light on a new planet-hunting instrument in Chile, the Next-Generation Transit Survey (NGTS). They hope it will open up the world of super-Earths, relatively unfamiliar objects between the size of small rocky planets and gas giants, of which we have no examples in our solar system. “It’s one of the most interesting classes of planets,” says astronomer Peter Wheatley of the University of Warwick in the United Kingdom, one of the project leaders.
Exoplanets can only very rarely be seen directly, because they are so faint and their host stars so bright. So astronomers have to detect them by other methods. Kepler and NGTS both do this by transits: If the orbit of the planet is oriented favorably, it will periodically pass in front of its star, causing the star to dip in brightness. Using an instrument called a photometer, which measures brightness very accurately, researchers can detect those dips and—from their duration and frequency—can deduce the size of the exoplanet and its orbit.
Previous transit surveys by ground-based telescopes have been able to detect only large Jupiter-sized or bigger planets because of the blurring effect of Earth’s atmosphere. Kepler’s mission was to stare at the same patch of sky for years on end so as to identify planets with long orbits and small size, down to Earth-sized or smaller. NGTS aims to fill the gap in between, focusing on planets between two and eight times the diameter of Earth—super-Earths to exo-Neptunes.
Based at the European Southern Observatory’s (ESO’s) Paranal observatory in Chile, NGTS is an array of 12 telescopes, each with a mirror 20 centimeters across, built by a consortium of U.K. and Swiss universities and the German space agency DLR. Although NGTS is not part of ESO, any planets of interest can be followed up in more detail using ESO’s large telescopes in Chile, including the Very Large Telescope and the upcoming Extremely Large Telescope. “We’re a planet finder for their instruments,” Wheatley says.
“Super-Earths are very diverse, but we don’t understand them very well,” Wheatley says. Some of those discovered so far appear to be rocky, whereas others seem to be fluffy and gaseous, or even to be “waterworlds” with deep oceans covering their entire surfaces. Unlike Kepler, NTGS will not always focus on the same area but will move its gaze so it can detect larger samples of different planet types. “You need a decent sized sample to study” the diversity of super-Earths, Wheatley says.
“It’s very interesting, because NGTS is just filling the gap we have at the moment” between rocky planets and gas giants, says astronomer Isabelle Baraffe of the University of Exeter in the United Kingdom, who is not involved in the project.
One key part of follow-up observations is measuring a planet’s mass, which must be found by a different method, such as detecting the back-and-forth wobble of a parent star caused by the planet’s mass as it orbits. With a planet’s mass and radius in hand, astronomers can calculate its average density, which gives them a handle on what it is made of. Here NGTS has a key advantage: Kepler’s focus on Earth-like planets makes many of its candidates too small or their stars too dim to get a mass measurement, but NGTS’s wider field of view allows it to detect a larger number of more massive planets around brighter stars. So it will be easier to get mass measurements using the wobble method. “That’s the strength of NGTS,” Baraffe says.