Hardware utilizado

Ground based transiting exoplanet surveys have been able to push the limit on lightcurve precision down to 0.1% using different observing and advanced reduction techniques. The

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earth’s atmosphere further limits the attainable precision to 0.01% through variable extinc- tion and scintillation. This places a limit on the size of the exoplanets that are able to be detected using these telescopes. To find exoplanets smaller than Neptune in size or planets the size of earth, the only option is to place a telescope in space beyond the atmosphere.

CoRoT

The Convection, Rotation and planetary Transits (CoRoT) (Baglin 2003, Auvergne et al. 2009) satellite was launched on 2006 December 27 with first light reported on 2007 January 7. It was a joint venture by the French Space Agency and European Space Agency along with other international partners. The satellite was designed with two objectives in mind. The first was to perform astroseismology by studying oscillations and pulsations in stars. The second objective was to find and characterise small radii exoplanets (ones with radii approaching earth radius). Consisting of a 27 cm diameter off-axis afocal telescope attached to four 2K × 2K CCDs manufactured by E2V Technologies, the field of view was roughly 2.7◦ × 3.05◦ _{with a pixel scale of 2.3}00 _pixel−1_{. Photometric precision of 7.1 × 10}−4 _was

obtained for stars with R = 15 mag.

Of the 26 exoplanet discoveries, none have V < 11. CoRot-1b (Barge et al. 2008) was the first exoplanet to have the secondary eclipse measured in the optical waveband due to the extremely high precision lightcurve produced by the satellite. Other notable discoveries include CoRoT-2b (Alonso et al. 2008), a planet orbiting a star with significant semi-periodic stellar flux variations due to star spots, and CoRoT-7b (Queloz et al. 2009, L´eger et al. 2009), a planet with a radius of only 1.7 R⊕.

The satellite’s on-board instrument stopped communicating with ground stations in 2012 November and it was announced in 2013 June that the satellite had been placed in an orbit that would destroy it in the earth’s atmosphere.

Kepler

The Kepler spacecraft (Borucki et al. 2009) which was launched in 2009 March has been the most successful spacecraft at discovering transiting exoplanets. With the primary goal of determining the frequency of exoplanets with radii as small as the earth or smaller in the habitable zone (the range of orbital radii at which liquid water could exist on the surface of a planet) η⊕, the satellite has found many other interesting objects and completely reshaped

the understanding of exoplanet formation.

Studies, using the Kepler data found the occurrence rate of Earth-sized (0.5 - 1.4 R⊕)

planets, orbiting stars with Tef f < 4000K, to be 0.48+0.12−0.24 per star and for slightly larger

planets (0.5 - 2 R⊕) the occurrence rate increase to 0.51+0.10−0.20(Kopparapu 2013).

The satellite consisted of a 95 cm aperture telescope with 42 2K × 1K CCDs with a field of view of about 115 square degrees. The telescope pointed at the same part of the sky toward the constellation of Cygnus for the entire duration of the mission. Stars in that region were pre-selected for observations, as the entire field of view contained too much data

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1.6 Transiting Exoplanet Surveys 31

for the spacecraft to be able to process and store the data on board the satellite systems. Altogether ∼ 150000 stars were monitored with a measurement of the brightness of the star taken every 6 minutes.

To date the Kepler spacecraft has found over 960 exoplanets and over 4000 possible exoplanet candidates still need to be confirmed. Most of the Kepler candidates are faint (V > 13) making them beyond the capabilities of the current RV surveys and as such cannot be confirmed to be planetary in nature as the true mass cannot be determined at present. Fressin et al. (2012) found that the false positive rate for all size exoplanets in the Kepler dataset for quarters 1 - 6 was ∼10%. This would mean that the number of candidates being actual planets could be as high as 2700. Noteworthy discoveries include Kepler-9 (Holman et al. 2010), the first system to have multiple transiting planets, Kepler-16b (Doyle et al. 2011), the first planet to be discovered orbiting a binary system, Kepler-20e (Fressin et al. 2012), the first planet with a radius smaller than the earth, Kepler-22b (Borucki et al. 2012), the first potentially habitable earth-like planet and Kepler-37b (Barclay et al. 2013), the smallest planet yet discovered, with a radius that makes it slightly smaller than Mercury. In 2012 July the Kepler spacecraft reported the failure of one of the four reaction wheels the satellite uses to point the telescope. Although only 3 reaction wheels are required to point the telescope, a failure of another reaction wheel at the end of 2013 May effectively ended the scientific operations of the Kepler Spacecraft. Attempts to recover from the failure of the last reaction wheel finally failed in 2013 August and NASA reported that Kepler would no longer be able to do exoplanet discovery, but the spacecraft was capable of doing other types of science and enlisted suggestions by the scientific community for possible avenues of research for the “crippled” spacecraft.