Confirming Predictions of Orbital Decay

Frederic Rasio, the Joseph Cummings Professor of Physics and Astronomy at Northwestern University, was one of the first researchers to make the prediction of orbital decays in some planetary systems. Rasio, who was not involved in the study, remarks: “We’ve all been waiting nearly 25 years for this effect to be detected observationally. The implications of the short timescale measured for orbital decay are also very important.

WASP-12b transits its parent star WASP-12. Image credit: NASA

“In particular, it means that there must be many more hot Jupiters that have already gone all the way. When they get to the Roche limit — the tidal disruption limit for an object on a circular orbit — their envelopes might get stripped, revealing a rocky core that looks just like a super-Earth or maybe a mini-Neptune if they can retain a bit of their envelope.”

WASP-12b was discovered in 2008 by astronomers using the transit or photometry method of exoplanet detection. This is when a planet crossing in front of its parent star — or transiting it. This transit causes a dip in the brightness of the star, from which many of the exoplanet’s characteristics can be inferred.

An illustration of the photometry technique which relies on a planet crossing its parent star, blocking some of the light it emits (NASA)

As this transit occurs each time the exoplanet completes an orbit, astronomers can infer an orbital decay by the intervals between resulting ‘dips’ in the star’s light output becoming shorter. WASP-12b’s shrinking orbit was made measurable by a 29 milliseconds per year shorting in the interval, something first noted by the paper’s co-author Kishore Patra, who was at the time, an undergraduate at the Massachusetts Institute of Technology.

Researchers were unable to conclude that the orbit was shrinking with this 2017 observation, as there could be other factors which cause shortening in the transit interval. For example, If WASP-12b’s orbit is more oval-shaped than circular, the apparent changes in the orbital period could be caused by the changing orientation of the orbit.

To ensure that what they were watching was actually the result of a decaying orbit, was by observing WASP-12b and it passes behind its star — a process known to astronomers as occultation. Should the orbit be changing direction as a result of an elliptical or eccentric orbit, the actual orbit period isn't changing. This means that if transits are occurring more quickly, occultations should be happening at a slower rate. But, if the decay is actually taking place, the researchers should see the timing of both transit and occultation changing to the same extent and in the same way — both becoming more frequent.

Using the Spitzer Space Telescope the team has spent the past two years collecting data to finally confirm orbitally decay, or not, as the case may be.

“These new data strongly support the orbital decay scenario, allowing us to firmly say that the planet is indeed spiralling toward its star,” says Yee. “This confirms the long-standing theoretical predictions and indirect data suggesting that hot Jupiters should eventually be destroyed through this process.”

Winn concludes that this discovery should help astrophysicists better understand the internal composition of stars and tidal interactions between exoplanets and their parent body.

“This tells us about the lifetimes of hot Jupiters, a clue that might help shed light on the formation of these strange and unexpected planets.”