Unusual Exoplanet Discovery Using Star Pulsations: KIC 7917485b

[|Glitch|]

This particular exoplanet I think deserves a special thread because of the method used to make its discovery. This exoplanet is unusual in that it is one of the few exoplanets discovered orbiting a spectral type A star, and because it was the only one discovered using the pulsations of the star's helium layer.

Astronomers find a planet through a never-before-used method
...

But astronomers came up with a novel idea to use the variability of the star itself as a way to look for exoplanets. The star pulses because of helium changes in its lower layers. It puffs up, cools and dims, shrinks, heats and brightens, and then repeats the process multiple times in a day. In a Kepler light curve, this shows up as a periodic dimming and brightening, like clockwork. But this clock shows a delay. The pulsations appear a little early or late, and by calculating this delay, astronomers can measure that the star is actually moving in a back-and-forth, orbital motion. And this movement is due to the gravitational tug of a nearby planet.

Source: Astronomy Magazine, By Korey Haynes, October 04, 2016

KIC 7917485
M_☉ = 1.63
R_☉ = 2.16
L_☉ = 9.9
T_eff = 7,067°K ± 192°K

KIC 7917485b
M_J = 11.8 (+0.8, -0.6)
Orbit = 840 ± 20 days (2.06 ± 0.04 AU est.)

According to Kopparapu, et al. (2014) the "conservative" Habitable Zone for this star is between 2.779 AU and 4.798 AU. The snow/frost line (160°K) for this star is approximately 13.62 AU. Considering such an exoplanet could not have formed any closer, it would have had to migrate inward quite a distance.

Source:
A planet in an 840-d orbit around a Kepler main-sequence A star found from phase modulation of its pulsations - arXiv 1608.02945

 

[mfb]

So basically pulsar timing, but with stellar oscillations instead of pulsar pulses. Interesting concept, and it requires an amazing long-term stability of the oscillation. The star moves by just 13 light seconds in the 2.5 year orbit of the planet. It corresponds to a radial velocity amplitude of 160 m/s, which should be obvious for all radial velocity searches - if they look at this star.

 

[|Glitch|]

So basically pulsar timing, but with stellar oscillations instead of pulsar pulses. Interesting concept, and it requires an amazing long-term stability of the oscillation. The star moves by just 13 light seconds in the 2.5 year orbit of the planet. It corresponds to a radial velocity amplitude of 160 m/s, which should be obvious for all radial velocity searches - if they look at this star.

Not pulsar timing, since: 1) This star will not end up as a pulsar or neutron star when it dies; and 2) Pulsar timing is caused by a completely different method. This exoplanet was discovered using the helium layer phase modulation within the variable star and then converted the time delays into radial velocities using the formula from Murphy & Shibahashi (2015).

Source:
Deriving the orbital properties of pulsators in binary systems through their light arrival time delays - Oxford Journals, MNRAS, Volume 450, Issue 4, Pp. 4475-4485 (arXiv free reprint)

 

[mfb]

but with stellar oscillations instead of pulsar pulses

The timing is the common feature.