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Late Stages of Planetary System Evolution: the Case of V 391 Peg b
12 Oct, 2007 11:15 am
Exoplanets are of high interest, not only because questions regarding alien life in the universe have accompanied humanity for a long time, but also because studying exoplanetary systems allows to study the past and the future of our own solar system.
Picture: This image represents the system V 391 Pegasi as it was about 100 million years ago, when the star was at its maximum red giant expansion and close to explosive helium ignition, the so-called 'helium flash', that causes the expulsion of the star's outer layers. At that time, the stellar radius of about 100 million km was not much smaller than the orbital distance of the planet, of the order of 150 million km (i.e. the distance between the Earth and the Sun)."
Image courtesy of HELAS, the European Helio- and Asteroseismology Network, funded by the European Union under Framework Programme 6; Mark Garlick, artist.
Detection of the pulsars's planets was performed using the so-called "timing method", which consists in measuring the small differences in the arrival times of the photons due to the changing distance of the pulsar from us, because it is rotating around the barycentre of the system.
Three years later, Mayor & Queloz  detected an exoplanet orbiting the star 51 Pegasi, a main sequence star burning hydrogen in its core (like our sun). This discovery was obtained using the more popular method of the radial velocities (RVs), ie, measuring the Doppler shift of the spectral lines of the star induced, again, by the orbital motion of the star around the barycentre of the system.
The discovery of 51 Pegasi b (this is the name of the planet), opened an exciting season of several new discoveries so that today more than 250 exoplanet candidates are known (see http://vo.obspm.fr/exoplanetes/encyclo/ for an updated list). Most of them were found using the RV method, even though exoplanets are currently detected also through transits (an attractive method for when we observe large numbers of stars as in the case of the COROT and Kepler space missions), microlensing, and direct imaging. As well, most exoplanet candidates orbit main sequence stars.
In the last years, first discoveries of planets around stars in evolutionary phases different from the main sequence were reported. When their core hydrogen runs out, main sequence stars undergo a red giant expansion that modifies the planetary orbits and can easily reach and engulf the inner planets. The same will happen to the planets of our solar system in about 5 Gyr, and the fate of the Earth is matter of debate [3,4]. Recently, a few planets orbiting red giant stars have been found . However, what happens to the inner planets at the maximum red giant expansion or after is largely unknown.
For this reason, to detect a planet around a post-red giant star is of high interest ; in particular, when this planet, V 391 Peg b, is relatively close to its parent star at 1.7 astronomical units (AU; 1 AU being the mean distance between Earth and sun).
The star, V 391 Peg, is an old star, with an age of presumably ~10 Gyr or more, that has already experienced the red giant expansion, at the end of which explosive helium ignition took place in its core (the so-called "helium flash"). After the helium flash, stars contract again and enter a new period of quiet nuclear burning, the so-called "horizontal branch" phase (this name is due to the position of these stars in the Hertzsprung-Russell diagram), which has a typical duration of 100 million years (whereas, for comparison, the main sequence has a typical duration of ~10 billion years for a star similar to our sun).
During the red giant phase, stars loose a significant fraction of their mass. V 391 Peg belongs to a particular class of stars, the hot subdwarf B (or sdBs), for which mass loss was particularly high, leaving a star with a very thin envelope and a high surface temperature near 30,000 K. The reason why these stars have this particularly strong mass loss is not clear and might be related to the presence of a companion . According to few authors, even the presence of planets could play a role in this process .
V 391 Peg is a pulsating star, with a maximum of light every ~6 minutes. In studying the star pulsation for seven years, a small advance or delay of about 5 seconds in the arrival time of the maxima was registered, due to the presence of the planet. Functionally, it is equivalent to the timing method used to find planets around pulsars. The discovery of V 391 Peg b proves that this method can be successfully applied also to pulsating stars, in particular to the short-period compact pulsators, such as the sdB and white dwarf pulsators [9, 10].
The interest for V 391 Peg b is increased when we try to figure out what happened in the past, when its orbital distance was probably smaller than the present value of 1.7 AU. If we consider only the effects of mass loss (and we neglect possible tidal interactions that are more difficult to quantify), it turns out that the orbital distance of V 391 Peg b, during the main sequence, was likely near 1 AU, the same distance of the Earth from the Sun.
But the analogy with the Earth stops here because the two planets are very different: V 391 Peg b is much more massive, at least 3.2 times Jupiter (or ~1000 times the Earth), gaseous, and very hot. From the incoming flux radiation from its parent star, we can estimate, for this planet, a temperature of about 470 K (or 200 degrees Celsius).
Silvotti R, Schuh S, Janulis R, et al. (13 September 2007). "A giant planet orbiting the 'extreme horizontal branch' star V 391 Pegasi". Nature 449, pp. 189-191.
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