Exist four different techniques in searching exoplanets:
– Radial velocity
– Direct imaging of the exoplanet
We have to consider that exoplanet investigation is an area relatively young.
2. Radial velocity technique
This technique measures very precisely the radial velocity of the motion of a star due to the existence of a planet. The most successful experiment is the use of the HARPS 1 instrument (High Accuracy Radial velocity Planetary Search project) of the Geneva Observatory, coupled to 3.6 m La Silla telescope. It has a high precision spectrometer, around 10-4 angstroms, thanks to its optic fiber net. It can measure radial velocities below 10 ms-1, allowing the detection of light giant planets (the perturbation of Jupiter in the Sun creates a radial velocity of 12.5 ms-1). Its precision is only exceeded by FLAME instrument in VLT telescope. The objectives of its observations are selected using the very accurate HIPPARCOS catalogue.
Other investigation using this technique is MARVELS 2 (Multi object APO Radial Velocity Exoplanet Large-area-Survey), with a precision of 12 ms-1. It will search giant gaseous planets with orbital periods from hours to 2 years, and masses between 0.5 and 10 Jupiter masses. It will analyze 11,000 stars and it is expected to detect between 150 and 200 new exoplanets.
Also we can emphasize TEDI 3 instrument (Triple Spec-Exoplanet Discovery Instrument), with a near infrared spectrometer , coupled to 5 m Hale telescope in Palomar Observatory. It will search exoplanets of low mass around M, L and T type stars, and in brown dwarfs stars. The prototype has reached a 5 ms-1 precision.
In this technique we can see that the existence of a high precision catalogue and very sensitive spectrometers have allowed the discovery of new exoplanets. Also these instruments are the basis for a new generation of more precise instruments which would discover lower size planets.
3. Direct imaging of exoplanets
This is a complex technique due to the bright of the star blocks us from observing the planet. To avoid this problem are being used the coronagraphs.
GPI 4 (Gemini Planet Imager) uses adaptive optics and a coronagraph coupled in South Gemini Observatory. Its objective is to take exoplanets images with a mass similar to Jupiter, and with a orbit of almost 5 AU. It has a high contrast system, which will be the basis for future investigations of protoplanetary discs detection around the stars.
EPICS 5 (Earth like Planet Imaging Camera Spectrograph) also uses adaptive optics and a coronagraph. Its objective is the detection and characterization of rocky planets in habitable zone, and cold gaseous giant planets. It will use the following bands:
– R Band (600 to 800 nm) to detect O2
– J Band (1,100 to 1,430 nm) to detect CH4 and H2O
– H Band (1,380 to 1,800 nm) to detect CH4, CO2 and H2O
DARWIN 6 project (now Canceled) will consist in 4 satellites with 3 m optics. It objective will be analyze Earth-like planets and their atmospheres, and to develop the space interferometry. It will have between 10 and 100 times better quality than other telescopes, even more than James Webb Space Telescope.
HiCIAO 7 will be coupled in 8.2 m Subaru telescope with the objective of searching planets with masses between 1 and 13 Jupiter masses, and protoplanetary discs. Using adaptive optics and a coronagraph it will study 500 solar type near stars. Its method will consist in divide the image into 2 or more images and use different polarization planes or spectral filters that allow to distinguish faint objects. It will use the following instruments:
– SDI with methane filter, for searching planets with temperatures below 1,400 K.
– PDI with a 0.03” resolution. It will be the basis for the studies of protoplanetary discs with ALMA project.
NACO 8 also will find exoplanets using adaptive optics and a coronagraph coupled to VLT. It will find exoplanets with temperature between 130 and 800 K, and between 1 and 25 Jupiter masses.
In the case of direct imaging, the technique has benefited of the use of adaptive optics and the coronagraph, and these technique is improving the high contrast and resolution systems, and in the case of DARWIN, will be an essential part of the development of space interferometry.
This technique is based on the detection of fluctuations in the bright of the star when an exoplanet is in the same plane of the star and the observer, and transits between the star and the observer.
The French COROT 9 satellite (Convection Rotation Planetary Transit) finds Earth-like planets with a camera that it is used both asteroseismology (stellar seismology) and exoplanet search. The camera is coupled to a 0.3 m telescope. Thanks to be in the space it is not affected by the atmospheric turbulence and thanks to be a mini-satellite, the costs are significantly reduced.
EPOCh 10 is a NASA satellite with two instruments: HRI that it is a high resolution spectrometer, and MRI to observe transits at medium resolution. It has a defect in the camera: it cannot focus correctly, but this problem has become as an advantage due to the object appears more extended and with less noise, reaching high precision in the measurements.
Kepler 11 satellite is a 0.95 m Schmidt space telescope with a 42 CCDs photometer, takes images with a field not reached with other telescopes. Kepler is searching Earth-like planets in habitability zone, measuring 100,000 main sequence stars to increase the probability. It is expected to detect 465 new exoplanets.
TrES 12 (Trans-Atlantic Exoplanet Survey) is a net of 3 small telescopes in Lowell Observatory, Palomar Observatory and Canarian Islands Observatory, whose objective is to search Jupiter-like planets.
Also, using small instruments, we have XO 13 instrument, which is a 0,2 m telephoto that cover an specific RA each night and each star is studied every 10 minutes by each camera.
Finally, the SuperWASP 14 instrument (Wide Angle Search for Planet) have an 8 CCD cameras array with a superb field, taking images all the night. Each image contains 50,000 stars and they are compared with a catalogue to make a photometric analysis.
This kind of technique has benefited from the great development of photometric techniques in high precision measurements (Specially thanks to the CCD cameras) and is allowing the improve of future studies of simultaneous photometry of thousand of stars, and also, thanks to the defect in the focusing system of EPOCh, a new way to improve the precision of the measurements.
Also the investigation is benefiting from the amateur astronomers collaboration. Although they use small telescopes, using CCD cameras it is possible to observe transits. The big advantage of this collaboration is that they have unlimited observing time.
This technique is the latest and more advanced, that consists in observe the microlensing events due to exoplanets.
REX 15 (Robotic Exoplanet discovery network) is a British project of three 2 m robotized telescopes, in three different places (Chile, South Africa and Australia) to observe 24 hours. It will search Earth-like planets in 4 AU orbits, studying 1,000 microlensing events known. Also it will study transits.
GEST 16 (Galactic Exoplanet Survey Telescope) space telescope will search, looking at galactic centre, exoplanets between 2 and 3 AU orbits and with low mass, around F, G and K type stars. It will have a 1 to 1.5 m primary mirror.
The search of exoplanets is a very young investigation area, which has benefited of techniques used in other astronomy areas. In particular:
– Radial velocity: fiber optic spectrometry and HIPPARCOS catalogue.
– Direct imaging: adaptive optics, coronagraphs and interferometry.
– Transits: CCD photometry.
These investigations are discovering hundreds of new exoplanets, which is not only great to characterize other stellar planetary systems, also allow us know better our own planetary system.
The effort in this area will develop a new instruments generation, which they will be space low-cost telescopes using interferometry, ground-based telescopes with high resolution systems or low-cost telescopes nets for the systematic study.
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, viewed 6 February 2010, http://www.darwin.rl.ac.uk
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(This article was presented at University of Central Lancashire in 12/02/2010 by Francisco José Sevilla)