The first exoplanet orbiting a main sequence star was discovered in 1995. Today, the field of exoplanet research is booming, with new planets being announced regularly and exciting new discoveries being made about these alien worlds.
There are several techniques for detecting planets outside our Solar System, and one of these is the transit method. Just as Venus and Mercury can sometimes be seen passing in front of the Sun’s disc, exoplanets also pass in front of their parent stars. However, we cannot resolve these stars, and so only a slight periodic reduction in the star’s light can be measured.
Earth-based observations are limited because the atmosphere interferes with measurements, meaning that only large planets in tight orbits can be found via the transit method. While these hot Jupiters are certainly interesting objects, we need to find Earth-like planets if we are ever to determine if we really are alone in the Universe.
This is where NASA’s Kepler mission comes in. Launched on 6 March 2009, Kepler’s vantage point above the Earth’s atmosphere gives it the unique capability of detecting small rocky planets around other stars.
While Kepler’s main objective is to find potentially habitable terrestrial planets, it can also discover larger planets as these are easier to detect. So far, dozens of gas giants and super-Earths have been found. The planet that is most similar to Earth so far is Kepler-22b, which is 2.4 times the radius of the Earth and orbits a Sun-like star.
Artist illustration of Kepler-22b, the first planet confirmed by NASA's Kepler to exist in the habitable zone of a sun-like star. Image credit: NASA/Ames/JPL-Caltech
Kepler was launched in 2009, and yet this potentially habitable planet was only announced in late 2011. Why did it take so long for this discovery to be announced, and why aren’t there more of them?
The answer lies in the size of the planet. Earth lies within what is known as the habitable zone (HZ) of the Sun. This is the area where the temperature is just right for liquid water to exist and thus for life to flourish. A potentially habitable Earth-like planet orbiting a Sun-like star will also have to be in this HZ, and thus have a similar orbital period around the star, i.e. one year.
Kepler will thus only observe one dip in a Sun-like star’s light in one year if it has an Earth-like planet, and a single transit is not enough to indicate that a planet might be present. Three or four transits are needed, which means that Kepler will only be able to detect these planets towards the end of its primary mission. The primary mission is due to end in November 2012, but this could be extended for a further three years.
Kepler also monitors stars that are cooler and hotter than Sun. In fact, it is monitoring more than 150,000 stars to search for dips in the light. The 105 square degree star field is located within the constellations of Cygnus and Lyra, and this field was chosen due to both the amount of stars present and because it is above the ecliptic plane.
If Kepler were to observe a star field within the same plane as the Earth, Moon and Sun, then these objects would be constantly blocking the telescope’s view of its target stars. This is also why Kepler was placed in a helio-centric Earth-trailing orbit, as a low Earth orbit would cause the Earth to periodically block the telescopes view.
Staring continuously at the one star field for years made the design of the Kepler spacecraft quite simplistic, although it does turn towards Earth once a month to transmit data via its antenna.
Finding the dip in a light curve that indicates that a planet might be orbiting a star is only the first step in the process. These dips could also be caused by binary stars, and so follow up Doppler spectroscopy must be performed to confirm the mass of the transiting object, and thus if it is actually a planet. This is why the Kepler team have announced so many candidates compared to confirmed planets.
This Doppler spectroscopy, otherwise known as the radial velocity (RV) method, is performed using a 10 metre telescope at the Keck observatory. The majority of exoplanets discovered so far have been found via the RV method. A planet will cause a star to “wobble,” as the star’s centre of gravity becomes offset. This can be detected as the stars spectral lines move back and forth, and combining this technique with the transiting method can yield the mass of the planet.
However, many of the planetary candidates orbit stars that are very faint, which makes it difficult for the RV followup. “Kepler was optimised to study as many stars as possible within its 100 square deg. field of view,” explains Gibor Basri from the University of California, Berkeley, and Co-Investigator on the Kepler team. “There are not a sufficient number of bright main sequence stars, so it has to reach down into stars that are more difficult for RV followup. Followup of terrestrial planets is very difficult even for bright stars (only the closest planets can be seen), so Kepler could not be constrained by the availability of RV measurements.”
Kepler has additional objectives, such as collecting data on the orbital parameters of planets and determining how many planets orbit in multiple star systems. Systems like this have already been found, such as Kepler-34b and Kepler-35b.
Kepler has also discovered several systems that have multiple planets, and aims to determine how common multiple planetary systems are. ”Our multiple systems are only those which show multiple transits (or transit time variations),” Basri tells Sen. “Thus it is only a lower limit to the frequency of multiple systems. My guess is that almost all planetary systems contain more than one planet; it is a different question whether they can all be detected by current techniques.”
Kepler also determines the parameters of stars, and it does so via a technique known as asteroseismology. Stars can pulsate, growing or shrinking in a way that makes it seem like they’re “breathing.” These pulsations cause the star to become periodically brighter or dimmer, which is thus measurable via Kepler.
Kepler is powered by a solar panel array, as well as an onboard battery. The solar array performs a dual function, as it also shields the photometer from the direct influence of the Sun. The Kepler telescope has a 1.4 metre mirror, and a photometer that has a 0.95 metre aperture and is based on the Schmidt telescope design. It is the largest photometer that NASA have ever launched into space, and it is this instrument that is responsible for collecting the light from the stars.
The general public can also participate in planet hunting by sifting through the Kepler data to find what the computers have missed. This project is known as Planet Hunters, and it is part of the Zooniverse network. The human brain is adept at picking out patterns and signals, and so far several planetary candidates have been identified by citizen scientists.