Exoplanets in close-in orbits around their host stars can
become tidally distorted. Being in a close-in orbit also makes it likely that
the planet is tidally-locked with the same planetary hemisphere perpetually
facing its host star. For a tidally-locked exoplanet, the effect of tidal
distortion tends to stretch the planet into a triaxial ellipsoid where the
planet’s longest axis is always oriented towards its host star. If the planet
transits its host star, the effect of tidal distortion and the resulting asphericity
in the planet’s shape can cause the planet’s size to be underestimated, and
subsequently, if the mass of the planet is measured, the planet’s density to be
overestimated.
Figure 1: Artist’s impression of a planet transiting a star.
The effect of tidal distortion on close-in, tidally-locked
exoplanets has been explored only for gas giant planets due to the general
assumption that tidal distortion is only relevant for such planets. It is
commonly assumed that the effect of tidal distortion is too small for rocky
exoplanets. A study done by Saxena et al. (2014) show this is not the case.
Rocky-exoplanets in close-in orbits around smaller red dwarf stars can become
tidally distorted to an observable extend. In particular, the study focuses on
rocky exoplanets with 1, 1.5 and 2 Earth radii orbiting M5V and M1V red dwarf
stars. An M5V red dwarf star is smaller and less massive than an M1V red dwarf
star.
Results from the study show that for a 1.5 or 2 Earth radii
rocky exoplanet circling an M5V red dwarf star near the fluid Roche limit, the
effect of tidal distortion can stretch the planet sufficiently such that the
ratio of the planet’s longest to smallest axis can approach 3:2. Basically, the
fluid Roche limit is the minimum distance a planet can be from its host star
before the planet’s own gravity can no longer hold itself together and the
planet starts to disintegrate. For rocky exoplanets around M1V red dwarf stars,
the effect of tidal distortion is less significant such that the longest axis
is at most only ~10 percent larger than the shortest axis.
Figure 2: The ratio of the longest to smallest axis for
tidally distorted rock exoplanets of 1, 1.5 and 2 Earth radii orbiting M5V and
M1V red dwarf stars. Saxena et al. (2014)
Tidal distortion decreases the projected area of a planet
when viewed along the planet’s longest axis. As a result, when a planet
transits its host star, the effect of tidal distortion causes the transit depth
(i.e. fraction of starlight the planet blocks) to be shallower. This can lead
to the planet’s size being underestimated. For rocky exoplanets with 1 to 2.25
Earth radii around M5V red dwarf stars, underestimates can reach ~10 percent at
near the fluid Roche limit. For M1V red dwarf stars, underestimates only reach 2.5
to 5.5 percent. The size underestimates can lead to density overestimates for
these planets.
Figure 3: Radius underestimates for different sized rocky
exoplanets due to tidal and rotational distortions. Saxena et al. (2014)
The planetary models used in this study assume an Earth-like
composition. Nonetheless, a more rigid planet (e.g. a solid, pure-iron planet)
would be less susceptible to tidal distortion than a less rigid planet. Since
tidal distortion is sensitive to a planet’s rigidity, observations of a
planet’s asphericity due to tidal distortion might provide insights to the
planet’s interior structure and bulk composition.
Red dwarf stars tend to have very compact planetary systems
and these stars also tend to have smaller planets. As a result, it is more
likely for red dwarf stars to harbour rocky exoplanets on close-in orbits that
can cause these planets to be subjected to strong tidal distortion. Furthermore,
red dwarf stars are the smallest and least massive stars. This means that a
planet around a red dwarf star can induce a proportionally larger observational
signature compared to a same planet around a Sun-like star.
Reference:
Saxena et al. (2014), “The Observational Effects and
Signatures of Tidally Distorted Solid Exoplanets”, arXiv:1410.2251
[astro-ph.EP]