This Massive Planet Is Causing Its Parent Star To Age Prematurely

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There’s a gas giant located about 330 light-years from here that’s not only unusually large, it’s also orbiting its host star at an incredibly close distance. According to a new study, this combination of factors is wreaking havoc on the star’s innards.

The exoplanet is named WASP-18b and it’s about 10 times heavier than Jupiter. So this thing is absolutely huge. Not only that, it’s so close to its parent star, WASP-18, that it completes one single orbit in less than 23 hours. It’s one of the most extreme examples of a hot Jupiter that scientists have ever seen.

Indeed, astronomers are starting to learn about the interesting dynamics that occur in such systems. For example, it was recently discovered that these gas giants can cause their parent stars to wobble like a top. And now it appears that they can also cause its host to display traits of an older star.

A team led by Ignazio Pillitteri of the Istituto Nazionale di Astrofisica (INAF)-Osservatorio Astronomico di Palermo in Italy dated WASP-18 between 500 million and 2 billion years old. That’s young by cosmological standards. By comparison, our sun, which is at its mid-life, is about 5 billion years old.

But here’s the thing: Younger stars tend to be more active, spewing out stronger magnetic fields, larger flares, and more intense X-ray emissions than their older counterparts. That’s why things are weird with WASP-18. The Chandra X-Ray Observatory explains:

Magnetic activity, flaring, and X-ray emission are linked to the star’s rotation, which generally declines with age. However, when astronomers took a long look with Chandra at WASP-18 they didn’t detect any X-rays. Using established relations between the magnetic activity and X-ray emission of stars, as well as its actual age, researchers determined WASP-18 is about 100 times less active than it should be.

"We think the planet is aging the star by wreaking havoc on its innards," said co-author Scott Wolk of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

The researchers argue that tidal forces created by the gravitational pull of the massive planet – similar to those the moon has on Earth’s tides, but on a much larger scale – may have disrupted the magnetic field of the star. [emphasis added]

The strength of the magnetic field depends on the amount of convection in the star, or how intensely hot gas stirs the interior of the star.

"The planet’s gravity may cause motions of gas in the interior of the star that weaken the convection," said co-author Salvatore Sciortino also of INAF-Osservatorio Astronomico di Palermo in Italy. "This has a domino effect that results in the magnetic field becoming weaker and the star to age prematurely."

WASP-18 is particularly vulnerable to the impact of tidal forces owing to its convection zone, which is narrower than most stars.

Read the entire study at the pre-print journal arXiv: "No X-rays from WASP-18. Implications for its age, activity, and the influence of its massive hot Jupiter".

Image: X-ray: NASA/CXC/SAO/I.Pillitteri et al; Optical: DSS; Illustration: NASA/CXC/M.Weiss

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