Exoplanet Spirals Towards its Star and Certain Doom

Robert Lea
Jan 9 · 6 min read

New research indicates that exoplanet WASP-12b is spiralling towards its parent star and its own certain destruction.

A team of astrophysicists led by researchers from Princeton University has revealed that the exoplanet WASP-12b — a gas giant orbiting a star 600 light-years from Earth — is spiralling towards its parent star and its own annihilation. A paper documenting the team’s findings is published in the latest edition of the Astrophysical Journal Letters.

WASP-12b is one of a family of planets referred to as ‘hot Jupiters’ — huge gaseous planets much like their namesake in our solar system. The primary difference between WASP-12b and Jupiter, however, is the proximity to their respective host stars. WASP-12b is in close orbit to its star — WASP-12 — making a full orbit in just 26 hours, which is an incredibly short compared to the near 12-year orbit of Jupiter and far shorter than even the closest planet to the Sun, Mercury which takes 88 days to complete its orbit.

An artist created this illustration of the searing-hot gas planet WASP-12b and its star. A Princeton-led team of astrophysicists has shown that this exoplanet is spiralling in toward its host star, heading toward certain destruction in about 3 million years. Image credit: NASA/JPL-Caltech

“Ever since the discovery of the first ‘hot Jupiter’ in 1995 — a discovery that was recognized with this year’s Nobel Prize in Physics — we have wondered how long such planets can survive,” explains Joshua Winn, professor of astrophysical sciences at Princeton and one of the coauthors of the paper. “We were pretty sure they could not last forever. The strong gravitational interactions between the planet and the star should cause the planet to spiral inward and be destroyed.”

Despite being fairly assured of planets such as WASP-12b’s ultimate fate, thus far scientists have been unsure just how long this spiralling-in process would take. Estimates ranged from millions to even trillions of years. The research from Princeton indicates that WASP-12b has approximately 300-million years left before it meets its fate.

“Now that we have measured the rate, for at least one system — it’s millions of years — we have a new clue about the behaviour of stars as fluid bodies,” Winn continues.

The team’s finding of the orbital decay of WASP-12b makes WASP-12 the first planetary system for which astronomers can be certain to have observed orbital degradation.

Time and Tidal Forces Wait for no Exoplanet

WASP-12b’s death spiral is a result of the tidal forces exerted on it by its gravitational interaction with its parent star. As WASP-12b — an exoplanet with incredibly low-density — orbits its host star, the two bodies exert a gravitational pull on each other. This raises a ‘tide’ in the gaseous planet in a similar way that the Moon raises ocean tides on our planet.

WASP 12b’s shape is distorted by its close proximity to its parent star and the tidal forces that arise between the two. Image credit: NASA

This tidal force also causes an effect in the parent star, tidal waves cause the star to distort and oscillate. Friction causes these oscillations to die down but more importantly, also results in gravitational energy from the planet being converted to heat within the star. As this is happening, friction associated with tides is exerting a torque on WASP 12-b. These factors cause the exoplanet to spiral towards the star.

This orbital decay, whilst being of significant interest in itself, also gives astrophysicists a way of investigating the interiors of stars. This is achieved by measuring how quickly the orbit is shrinking. Thus, in turn, this tells them how quickly the star is dissipating gravitational energy.

“If we can find more planets like WASP-12b whose orbits are decaying, we’ll be able to learn about the evolution and eventual fate of exoplanetary systems,” says Samuel Yee, first author of the paper and a graduate student in astrophysical sciences. “Although this phenomenon has been predicted for close-in giant planets like WASP-12b in the past, this is the first time we have caught this process in action.”

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.”

Original research: “The orbit of WASP-12b is decaying,” by Samuel W. Yee, Joshua N. Winn, Heather A. Knutson, Kishore C. Patra, Shreyas Vissapragada, Michael M. Zhang, Matthew J. Holman, Avi Shporer and Jason T. Wright, the Astrophysical Journal, 2019.

Rob is freelance science journalist from the UK, specialising in physics, astronomy, cosmology, quantum mechanics and obscure comic books.

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Robert Lea

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Freelance science journalist. BSc Physics. Space. Astronomy. Astrophysics. Quantum Physics. SciComm. ABSW member. WCSJ Fellow 2019. IOP Fellow.

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