Astronomers have used many different methods to discover planets beyond the solar system, but the most successful by far is transit photometry, which measures changes in a star's brightness caused by a mini-eclipse. When a planet crosses in front of its star along our line of sight, it blocks some of the star's light. If the dimming lasts for a set amount of time and occurs at regular intervals, it likely means an exoplanet is passing in front of, or transiting, the star once every orbital period.
NASA's Kepler Space Telescope has used this technique to become the most successful planet-hunting spacecraft to date, with more than a thousand established discoveries and many more awaiting confirmation. Missions carrying improved technology are now planned, but how much more can they tell us about alien planetary systems similar to our own?
A great deal, according to recently published studies by Michael Hippke at the Institute for Data Analysis in Neukirchen-Vluyn, Germany, and Daniel Angerhausen, a postdoctoral researcher at NASA's Goddard Space Flight Center in Greenbelt, Maryland. They show that in the best-case scenarios upcoming missions could uncover planetary moons, ringed worlds similar to Saturn, and even large collections of asteroids.
"We expect a flood of discoveries from these new missions, so we want to get a feel for the possibilities so scientists can make the most of the data," Angerhausen said.
Both NASA and the European Space Agency (ESA) are building on Kepler's success. NASA's Transiting Exoplanet Survey Satellite (TESS), scheduled to launch no later than 2018, will be the first-ever spaceborne all-sky transit survey. Over the course of two years, TESS will monitor some 200,000 nearby stars for telltale transits. ESA's Planetary Transits and Oscillations of stars (PLATO) satellite, which is expected to begin a six-year mission in 2024, will search for planets around roughly a million stars spread over half the sky.
The amount of stellar dimming caused by a transiting planet tells astronomers how big the planet is in proportion to its star, while recurring events can tell us how long it takes for the object to orbit its star. Additional transits increase confidence the dimming isn't caused by another cosmic object (such as a faint star), dark sunspot-like regions on the host star, or noise in the detector. Over the operational lifetime of a satellite, the strongest signals always come from larger planets orbiting close to their stars because they produce both a deeper dimming and more frequent transits.
"Planets with sizes and orbits similar to Mars or Mercury will remain out of reach, even when six years of PLATO data are combined," said Hippke. "But worlds similar to Venus and Earth will show up readily." Kepler has demonstrated the presence of planets smaller than Earth in very close orbits around stars smaller than the sun, but these sweltering worlds are unlikely to support life. TESS and PLATO will reveal Earth-sized worlds in Earth-like orbits around stars similar to the sun. Orbiting within the star's habitable zone, these planets may possess pools of liquid water, thought to be a prerequisite for the development of life as we know it.
Jupiter and Saturn each take more than a decade to orbit the sun. Similar worlds may only transit once during the TESS and PLATO missions but will produce a strong event. If, like Jupiter, the planet has a few large moons, their transits could show up in the data too. "We wouldn't have a clear detection, and we wouldn't be able to say whether the planet had a single large moon or a set of small ones, but the observation would provide a strong moon candidate for follow-up by other future facilities," explained Angerhausen.
Today, rings have been detected around only one exoplanet, called J1407b. The ring system is more than 200 times larger than Saturn's. Considering how a more Saturn-like planet would appear to PLATO, the researchers show that the transiting ring system produces a clear signal that precedes and follows the planet's passage across its star. These findings were published in the Sept. 1 issue of The Astrophysical Journal.
In a second study, published in the Sept. 20 issue of the same journal, the researchers explored the possibility of detecting asteroids trapped in stable orbital zones called Lagrange points, locations where a planet's gravitational pull balances its sun's. These areas lead and follow the planet in its orbit by about 60 degrees. In our solar system, the most prominent example occurs near Jupiter, where at least 6,000 known objects have gathered in two groups collectively called the Trojan asteroids. Less well known is that Earth, Mars, Uranus and Neptune similarly have captured one or more asteroids along their orbits, and astronomers now refer to all objects trapped in this way as Trojan bodies.
The same phenomenon will occur in other planetary systems, so Hippke and Angerhausen combined Kepler observations of more than 1,000 planet-hosting stars to hunt for an average dip in starlight that would indicate transits by Trojan bodies. They turned up a subtle signal corresponding to the expected locations of objects trapped in two Lagrange points.
"As good as the Kepler data are, we're really pushing them to the limit, so this is a very preliminary result," Hippke said. "We've shown somewhat cautiously that it's possible to detect Trojan asteroids, but we'll have to wait for better data from TESS, PLATO and other missions to really nail that down."