The search for exoplanets using transits the first successes

Velocimetry has been used to find more than 200 giant exoplanets in less than twelve years; yet it still carries the great disadvantage of providing only a lower limit for the mass of a planet, since the angle of inclination of the orbital plane of the planet vis-a-vis the observer is unknown. Neither can the method reveal any information about the physical nature of the planet; for example, its diameter.

There is, however, another method which can circumvent these problems, especially in combination with velocimetry: the transit method. This is used when the plane of the planet's orbit is observed edge-on (see diagram below). This configuration causes the planet to occasionally move across the face of its star, thereby hiding a small part of the star's surface. Similar transits are observed locally when, for example, Venus passes in front of the Sun. During the transit the luminous flux received from the star will be slightly reduced. In the case of a planet like Jupiter, with a diameter of about 0.1 that of its star, the reduction will be of the order of 1%. An Earth-sized planet, with a diameter 0.01 that of its star, will involve a reduction of 0.01%.

The first condition for the use of this method is, of course, that the planet should (from the observer's viewpoint) pass in front of its star; but the

The planetary transit method. When the planet passes in front of its star, the luminous flux received from the star will be reduced. In the case of a planet like Jupiter, the reduction (comparing the situation when the planet is in position 1 and position 3) will be of the order of 1(T2; and for an Earth-sized planet, of the order of 10~4, assuming a solar-type star.

Transit of an exoplanet observed simultaneously by occultation and velocimetric methods.

photometry (transit)

2.8 When planets cross the disks of stars 31

Transit of Venus across the face of the Sun on 8 June 2004, based on successive images taken at Waldenburg, in Germany - the first at 05.41 GMT and the last at 11.04 GMT. This was the first transit of Venus since 1882. Venus is about the same size as the Earth, and its orbit is closer to the Sun. A transit does not occur every time Venus passes this side of the Sun, since the orbits of the Earth and Venus are not in the same plane. These transits occur in pairs separated by 8 years, every 105.5 (105.5+8) and 121.5 (121.5+8) years.

Transit of Venus across the face of the Sun on 8 June 2004, based on successive images taken at Waldenburg, in Germany - the first at 05.41 GMT and the last at 11.04 GMT. This was the first transit of Venus since 1882. Venus is about the same size as the Earth, and its orbit is closer to the Sun. A transit does not occur every time Venus passes this side of the Sun, since the orbits of the Earth and Venus are not in the same plane. These transits occur in pairs separated by 8 years, every 105.5 (105.5+8) and 121.5 (121.5+8) years.

probability of this occurring also depends on the size of the star, and is greater when dealing with a large star. The distance of the planet from the star is another factor, and the nearer it is, the greater is the chance that a transit will occur.

Astronomers systematically carried out photometric studies of all the stars known to have companions discovered by velocimetry, in the hope of detecting transits. In the case of HD 209458 b they struck lucky. Based on velocimetric data, the first transit of an exoplanet was observed by David Charbonneau and Timothy Brown in September 1999. They were able to determine not only the radius of the planet, but also its orbital inclination as seen from Earth, from which data they were able to calculate its exact mass. It is 0.69 times the mass of Jupiter, and its diameter is 1.5 times greater than Jupiter's diameter. This is therefore not a very dense exoplanet (0.3 g/cm3) as confirmed by existing theoretical models. A transit of HD 209458 b was later observed, with even greater accuracy, by the Hubble Space Telescope.

Archive material from the astrometric satellite Hipparcos also confirmed the existence of HD 209458 b. Hipparcos was launched by the European Space Agency in 1989, and over many years it determined the positions of more than 100,000 stars, with a view to establishing a comprehensive and highly accurate catalogue of their proper motions and magnitudes. Its data revealed evidence of a transit of a planet across the star HD 209458 on five separate occasions, when the light from the star was very slightly attenuated. This unusual behaviour had passed unnoticed at the time.

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