Planetary transits

4.2 Planetary transits

As the number of known close orbiting gas giants increases, there becomes a rea- sonable chance that the plane of some of their orbits will include the Earth and so, once each orbit, the planet might occult the star, giving a measurable drop in its brightness (Figure 4.5).

Let us estimate the magnitude drop if a Jupiter sized planet occulted our Sun as seen from a great distance. The Sun has a diameter which is ∼10 times that of Jupiter, so that its cross-sectional area will be ∼100 times that of Jupiter. When Jupiter occulted the Sun, the effective area will drop from 100 to 99 – a ratio of

Extra-solar Planets

Figure 4.5 The eff ect of a planetary transit on the brightness of a star.

0.99 – and give a drop in brightness of 1%. This corresponds to a magnitude drop given by:

∆m ⫽ 2.5 log 10 (0.99) ⫽ ⫺0.011 magnitudes.

With care, such accuracy in measurement is achievable and on November 5, 1999 two teams detected the transit of a planet, previously discovered by the radial veloc- ity method, in orbit around the star HD 209469. During the transit, the brightness of the star dropped by 1.7%.

In 2002, a planet OGLE-TR-56B was discovered by the transit method and later confi rmed using the radial velocity method. Then, in 2006, the Hubble Space Telescope made a survey of 180 000 stars up to 26 000 light-years away towards the central bulge of our Galaxy. The survey discovered 16 candidate extra-solar planets of which three have since been confi rmed. Such confi rmation is required as the technique has a high rate of false detections. If all 16 were confi rmed, it would imply that there would be of order 6000 million Jupiter sized planets in the galaxy. Five of the newly discovered planets were found to orbit their sun with periods of ⬍1 day. The candidate with the shortest period – just 10 h – is only 1.2 million km from its relatively small, red dwarf sun and has an estimated surface temperature of 1400 K! It must be at least 1.6 times the mass of Jupiter in order to prevent the tidal forces from the star splitting the planet apart (Figure 4.6).

Apart from the high rate of false detections the transit method has the problem that transits can only be observed when the planet’s orbit is nearly edge on. About 10% of planets in close orbits would show transits, but the fraction is far smaller for planets with large orbits as the alignment has to be more precise – only ∼0.5%

Introduction to Astronomy and Cosmology

Figure 4.6 An artist’s impression of a transiting exoplanet. Image: ESA – C. Carreau.

of Earth-like planets in orbit around stars similar to our Sun would cause transits. Two space missions called Kepler and COROT will have very large fi eld of views, enabling them to continuously monitor many stars. The 95 Megapixel CCD array used with the 0.95 m Kepler telescope will monitor more than 100 000 stars with very high precision during an initial 3.5 year observing period. It is hoped that Kepler and COROT will, for the fi rst time, enable the detection of a signifi cant number of Earth sized planets.

The transit method does have two signifi cant advantages. The fi rst is that, as a planet will take some time to fully cover its star, the size of the planet can be deter- mined from the light curve. When combined with the planet’s mass, determined by the radial velocity method, the density of the planet can be determined and so we can learn about its physical structure.

The second advantage is that it is possible to study the atmosphere of a planet. When the planet transits the star, light from the star passes through the atmosphere of the planet. By carefully studying the star’s spectrum during the transit, absorp- tion lines will appear that relate to elements in the planetary atmosphere.

The extra-solar planet HD 209458b, provisionally nicknamed Osiris, was the fi rst planet observed transiting its sun (Figure 4.7). Observations by the Hubble Space Telescope fi rst discovered a tail of evaporating hydrogen which may, in time, completely strip the planet of gas leaving a ‘dead’ rocky core. More recent Hubble Space Telescope observations have shown that the planet is surrounded

Extra-solar Planets

Figure 4.7 An artist’s impression of the planet HD 209458b showing an extended envelope of carbon and oxygen and tail of evaporating hydrogen. Image: ESA and Alfred Vidal-Madjar (Institut d’Astrophysique de Paris, CNRS, France).

by an extended envelope of oxygen and carbon believed to be in the shape of a rugby ball. These heavier atoms are caught up in the fl ow of the escaping atmo- spheric atomic hydrogen and rise from the lower atmosphere rather like dust in a whirlwind.