The Mystery of XO-3b: Is it a Planet, Failed Star, or Active Star?

Thanks to data collected by NASA’s Spitzer Space Telescope, scientists were able to study a mysterious world known as XO-3b. But their analysis has left them with even more questions, such as whether XO-3b is even a planet or something else.

XO-3b is a gas giant that orbits an F-type star. It has a mass that is 7.29 times that of Jupiter, but has a radius that is only 1.41 times larger than our Solar System’s Gas Giant. It orbits its host star at an exceptionally close range of 0.04529 AU (one AU is the distance that Earth is to the sun).

A year on XO-3b lasts just three Earth days and during that time, it goes through two different seasons – a one-day summer and a two-day winter. In a recent news conference that was held by the American Astronomical Society, Lisa Dang, who is a Ph.D. candidate in astrophysics at McGill University in Canada, stated, “We saw seasonal temperature variations hundreds of times stronger than what we experience on Earth.”

Furthermore, the seasons on XO-3b are due to its oval orbit around its host star and the amount of radiation it gets. Since the planet and the star are so close to each other, the furthest part of XO-3b’s orbit continues to be brought closer to the star because of their gravitational interactions which causes the unusual orbit.

With that being said, it is believed that the planet has only been orbiting the star for a relatively short amount of time as Dang noted, “The oval shape that we see here on this very short-period planet suggests that we're catching it in the middle of migrating.”

Another odd fact about the planet is that based on the detection of infrared light, it is much hotter than initially thought, which can’t be explained by the rapid and extreme seasons. Dang explained this further, “This extra heating that we saw with Spitzer isn't seasonal, it's seen throughout the year,” adding, “Our investigation of this hot Jupiter finds that it's heated not only by the nearby planet star, but it's also heated from the interior of the planet.” Additionally, it is “puffier” than expected.

As for why the planet has so much extra heat, it could be caused by tidal heating caused by the star’s gravity pulling at the planet, causing it to become deformed and stretched. Another option is that it might not even be a planet after all. It could be a brown dwarf (also known as a “failed star”) or “...it might be not necessarily a failed star yet, but at the peak of its lifetime as a star.” Whatever it is, it’s definitely unusual. (An artist’s impression of XO-3b can be seen here.)

The study was published in The Astronomical Journal where it can be read in full.