Do Exoplanets Transform Between Classes?

A new analysis suggests that hot super-Earths might be the skeletal remnants of hot Jupiters stripped of their atmospheres.

An artist's depiction of an early stage in the destruction of a hot Jupiter by its star.  NASA/GSFC/Frank Reddy

An artist's depiction of an early stage in the destruction of a hot Jupiter by its star.
NASA / GSFC / Frank Reddy

Most alien planets are unlike any planet in our solar system. Hot Jupiters, for example, are broiling gas giants circling closer to their stars than Mercury orbits the Sun. Astronomers suspect that the star-planet tidal interaction will ultimately drag a hot Jupiter inward toward its doom.

More recently, astronomers have discovered a second class of star-hugging planets in the wealth of data from NASA’s crippled Kepler space telescope. These so-called hot super-Earths are rocky or icy planets that can be up to 10 times Earth’s mass and also orbit extremely close to their host stars.

Astronomers have speculated that the two classes may be related. But Francesca Valsecchi (Northwestern University) and her colleagues now take this a step further, suggesting that these odd planets are stripped hot Jupiters. Instead of forming as super-Earths, they are the skeletal remnants of gas giants peeled of their atmospheres.

The underlying theory is relatively simple. As the exoplanet spirals in toward its host star, the system will eventually reach the point where the two Roche lobes touch.

“The Roche limit or Roche lobe is the region around a planet or star (or moon or lump of bread dough) where that object's gravity dominates — it's a ‘sphere of influence’ so to speak,” explains coauthor Jason Steffen (also at Northwestern University).

When a fluffy star in a binary system overflows its Roche lobe, it can pour material down onto its smaller, denser companion star. In a similar way, when a hot Jupiter reaches the point where its Roche lobe and its star’s Roche lobe meet, the interaction opens a gravitational path along which mass can transfer from the exoplanet to the star. So the hot Jupiter inevitably starts shedding its gaseous envelope.

Valsecchi and colleagues modeled this transformation for several different cases. But they started early in the planet’s history, placing the hot Jupiter not under the glare of its bright star, but in the chilly outer reaches of the planetary system where astronomers think gas giants first form. Due to the star-planet tidal interaction, the planet migrates inward toward the star. But once the planet’s Roche lobe reaches the star’s Roche lobe, something interesting happens. The planet’s orbit reacts to the mass transfer by moving slightly outwards.

But this slight movement doesn’t stop it from shedding its entire atmosphere. Once the rocky or icy core is exposed, tidal forces take over again, causing the orbit to shrink once more and bring the planet close enough for the star to swallow it, explains Valsecchi.

“Broadly the idea makes sense,” says expert David Trilling (Northern Arizona University). “The only evidence will be indirect, so the question is really whether this theory explains the observational evidence better than all other competing theories.”

Trilling and colleagues first mentioned this idea briefly in a paper published in 1998. But at the time, only a few hot Jupiters had been detected and no hot super-Earths. We’re now in a much better position to understand how planets might transition between classes.

The research team did compare their results to observations, finding that most known hot super-Earths have similar orbital periods and masses to those modeled.

If the results hold, hot Jupiters might be about three times as common as what astronomers have inferred directly from observations, because the number of single super-Earths observed is nearly twice the number of hot Jupiters.


Francesca Valsecchi et al. “From Hot Jupiters to Super-Earths via Roche Lobe Overflow” Astrophysical Journal Letters, Accepted

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