Puzzling Hot Jupiter That Formed Much Too Quickly Offers Clues To Planet Formation

Artist's impression of a hot Jupiter. ESA/ATG medialab

According to most astronomers, the accepted theory is that gas giant planets like Jupiter and Saturn take about 10 million years or more to form. But CI Tau b clearly has not been informed of that rule and somehow ended up fully assembled at just 2 million years old, according to new research presented at the American Astronomical Society meeting in St. Louis this week.

This perplexing planet, located about 450 light-years from Earth in the constellation of Taurus (the bull), was observed directly in infrared for four years by researchers from Rice University and the Lowell Observatory. Now, their study, which will be published in the Astrophysical Journal Letters, details their findings of the properties of CI Tau b, and sets what they call an "empirical benchmark" for understanding newborn hot Jupiters.

CI Tau b is a "hot Jupiter," a gas giant that orbits very close to its star – in this case, it orbits it in just nine days – making it extremely hot. The planet is 11.6 times heavier than Jupiter and is about 134 times fainter than its parent star, an orangish star cooler and smaller than the Sun. The team was able to directly study the light from the planet, the first time this has been achieved of a planet so close to a star so young.  

“Direct observational evidence of the mass and brightness of CI Tau b is particularly useful because we also know it orbits a very young star,” lead author Laura Flagg, a graduate researcher at Rice University, said in a statement. “Most of the hot Jupiters we’ve found are orbiting middle-aged stars. CI Tau’s age gives a tight constraint for putting models to the test: Can they produce a planet this bright and this massive in so little time?”

One possibility for this planet to have formed so quickly is through a different method to Jupiter and Saturn. This hypothesis is called a “hot start”, in which the material in the protoplanetary disk collapses directly into a giant planet, rather than amassing its mass by slowly accreting material, and this is what the researchers think happened. This object is certainly an excellent test bed for current hypotheses.

“The most valuable way to learn how planets form is to study planets, like CI Tau b, that are either still forming or have just formed,” added Christopher Johns-Krull, professor of physics and astronomy and co-author of the study.

The discovery of dozens of hot Jupiters has meant new theoretical models to describe how planets form. Understanding more about the properties of CI Tau b, which the researchers believe their study helps confirm is the youngest confirmed exoplanet discovered yet, offers a unique yardstick by which to measure competing theories. 

“At about 2 million years old, CI Tau b is by far the youngest hot Jupiter directly detected,” Lisa Prato of Lowell Observatory said. “We now have a mass and brightness for it — the only directly measured mass and brightness for a young hot Jupiter — and that provides very strong tests for planet-formation models.”

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