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The tragic destiny of the gaseous extrasolar planets

1er février 2009 The tragic destiny of the gaseous extrasolar planets

Thanks to the method of planetary transits, more than forty gaseous extrasolar planets analogs to Jupiter or Neptune have been discovered since ten years, orbiting within less than 0.1 AU of their central star (1). A team made up of astronomers of Paris Observatory and the Astrophysical Research centre of Lyon (ENS Lyon, University of Lyon) showed that all these planets had an unstable orbit because of the effects of intense tides between the star and the planet, leading to an inescapable collision between the two bodies in extremely short times, compared to the lifespan of the system. This discovery restarts the debate on the mode of formation and evolution of the extra-solar planetary systems discovered until now.

The influence of tidal effects on the orbit and rotational velocity of two bodies in gravitational interaction is well-known in the Solar system, in particular in the systems made up of a planet and a satellite. Part of the mechanical energy of the system is dissipated and lost by mechanical friction within the bodies and gradually tends to make their orbit circular and to synchronize their internal rotation period with the period of orbital revolution as can illustrate partly the history of the system Earth-Moon. However, this progressive evolution towards such an equilibrium is possible only because the total angular momentum of the system is larger than a ctitical value, which depends only on the mass of the bodies, their distribution and the universal constant of gravitation (2). In the contrary case, the two bodies irremediably tend to approach and ultimately collide.

Figure 1 : Diagramme indiquant la stabilité des systèmes planétaires extrasolaires accueillant une planète détectée par la méthode des transits vis à vis des effets de marées. Les planètes sont classées par valeur croissante du rapport entre le moment cinétique total du système planétaire et un moment cinétique critique. Quand le rapport est plus grand que un, le système peut évoluer vers un état d’équilibre stable. Quand il est inférieur à un, le système n’a pas assez de moment cinétique, conduisant ainsi à une collision entre l’étoile et la planète. La barre d’erreur tient compte des incertitudes relatives aux mesures des paramètres du système (masses, rayons, vitesses de rotation). Une estimation du temps restant avant la collision est aussi indiquée entre parenthèses (3). Seule la planète HAT-P-2b apparaît à la limite de la stabilité. Cliquer sur l’image pour l’agrandir

In studying the destiny of all extra-solar planets called in transit and detected by a brightness drop of their star at the time of their regular passages in front of it, the French astronomers discovered that the totality of these planets had to come and die tragically in a future collision with their central star (Figure 1). An update of this study shows that the planet in transit discovered recently by the CoRot satellite should also undergo the same fate.

To evaluate the remaining life time of these planetary systems, numerical simulations of the evolution of their orbit were carried out by using traditional models of tidal effects for gaseous planets (Figure 2). In a surprising way, these life-time vary between only a few dizens of million years and a few billion years, therfore being surprisingly short compared to the age of the systems, close to a few billion years. Simulations have shown that, contrary to the generally accepted ideas in the astrophysical community, the eccentricity of the orbit decreased significantly only at the time of the relatively sudden final stage of the two bodies approach before the collision, because of the great sensitivity of tidal effects on the distance between the bodies.

Figure 2 : Exemples d’évolution future du demi-grand axe de l’orbite de la planète HD 209458b (en trait noir plein) et OGLE-TR-132b (en trait vert plein) en UA ainsi que de l’excentricité de HD 209458b (en trait noir pointillé). Les échelles correspondantes sont respectivement sur la gauche et la droite de la figure. On notera la décroissance brutale et rapide du demi-grand axe et de l’excentricité avant la collision. L’orbite de OGLE-TR-132b est considérée comme strictement circulaire. Cliquer sur l’image pour l’agrandir

These results fully restart the debate on the formation and the evolution of these extra-solar systems. Indeed, the majority of these systems have currently quasi-circular orbits, which suggests, according to the preceding conclusions, that we would have the immense privilege to observe all these planets just falling on their star ! The probability of such an event being a priori very low, this means that it is not the tidal effects between the star and the planet the responsible for the circularization of their orbit. This circularization could then result from older processes like interactions between the planet and the primordial proto-planetary disk, for example. The other possibilities to explain the observation of the planets on unstable orbits could be our ignorance of the time scales of the mechanisms of dissipation from tidal effects within gaseous bodies, or the presence of planetary companions still undetected, preserving the orbital stability of the first planet. This last assumption should soon be checked in the next observation campaigns of these systems.

Notes

  • (1) AU = Astronomical Unit = distance Sun-Earth, 149,6 million km.
  • (2) The total angular momentum of the planetary system star-planet is the sum of the orbital angular momentum and the internal angular momentums of the star and the planet, due to their own rotation. It remains constant for a binary system considered as isolated.
  • (3) 1 Gyr = 1 billion years and 1 Myr = 1 million years.

Reference

  • Falling transiting extrasolar giant planets Astrophysical Journal Letters, 692, L9-L13, 10 Février 2009 B. Levrard, C. Winisdoerffer & G. Chabrier

Contact

  • Benjamin Levrard (Observatoire de Paris, IMCCE)
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