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NASA Kepler: the search for exoplanets

Dr Amanda Doyle, Feature writer
Mar 26, 2012, 7:00 UTC, Updated Sep 19, 2014, 1:35 UTC

Sen—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 are often made about these alien worlds.

Many of these discoveries are thanks to NASA's Kepler mission, and the main objective of this mission is to find planets like our own.

Kepler was launched on 7 March 2009 with a primary mission scheduled for three and a half years and this mission was then extended until 2016. Unfortunately, in July 2012 one of the four reaction wheels failed, followed by a second in May 2013. 

The Kepler mission was designed to stare at 100,000 stars in a 105 square degree star field continuously for several years. The loss of functionality in the reaction wheels meant that this was no longer possible.

All was not lost, however, as scientists designed a new mission for Kepler. The K2 mission has new objectives and can work well with only two reaction wheels. 

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 original mission - searching for habitable planets
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 transiting 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. 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. One of the planets 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.

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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 or needed, which means that Kepler was only be able to detect these planets towards the end of its primary mission.

Kepler also monitors stars that are cooler and hotter than Sun. The Kepler 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 or transit timing variations, 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."

The general public can 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.

Asteroseismology
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 has revolutionised asteroseismology, and pulsations have been detected in thousands of stars. This had led to impressive precision in stellar parameters, such as mass, radius, and age. There have also been many exciting new discoveries in stellar astronomy, such as strange periods in RR Lyrae variables and speedy core rotation in red giants. 

The K2 mission
The K2 mission involves the spacecraft pointing at a number of different fields towards the ecliptic. The spacecraft can remain stable for 75 days while pointing at a star field, before moving on to the next one. There will be nine different fields observed, which will take observations up to 2016.

Unlike the original Kepler mission where the goals were decided by NASA, the K2 mission is accepting proposals from the scientific community as to what targets it should look at.

While the K2 mission will also study exoplanets and perform asteroseismology, it is also capable of much more. K2 will observe open star clusters, star-forming regions and even capture supernovae as they occur. It can now look at bright stars, which the original mission couldn't observe at all, and the fields that point above the Galactic plane will open up a window to perform extragalactic science.

The original Kepler data still hasn't been completely analysed, so between Kepler and K2 there are still many more exciting discoveries to be made.