A new study suggests that close-in gas giants may heat up electrically like toaster coils plugged into their host stars via the power lines of the stellar wind — explaining why the planets inflate.
In a study presented at the meeting of the American Astronomical Society last week in Indianapolis, astronomer Derek Buzasi (Florida Gulf Coast University) proposes that hot Jupiters are like toasters.
To be precise, his modeling suggests that these planets are heated by an electric current running deep through the planet's interior. It's plugged into an external power source — the host star. The interplanetary power supply would offer a new solution to a mystery that has vexed astronomers for over a decade.
Since 2000, transit observations have allowed astronomers to directly measure the sizes of exoplanets based on how much light they block as they pass in front of their host star. These measurements have revealed that gas giants close to their host stars are "inflated" — larger in size than a planet of the same mass further away. A 2011 study of data from NASA's Kepler telescope by Brice-Olivier Demory and Sara Seager (Massachusetts Institute of Technology) found that hot Jupiters have a baseline average radius of around 87% that of Jupiter. But when the planets orbit closer in and the incoming energy from the host stars reach about 150 times what Earth receives, the planets begin to puff up. Around a star like the Sun, this is at 0.08 AU, or about 1/4 the distance of Mercury's orbit.
But why? Atmospheric-heating models can currently account for only some of the inflation trend, says Adam Burrows (Princeton), who is not involved with Buzasi's study. These models incorporate the planet's mass, its composition, distance from the star, and the star's type and age. Due to the intense radiation of its star, "the [planet] sizes could be up to 1.3 Jupiter radii quite easily," says Burrows. But a handful of hot Jupiters — "WASP-19, WASP-12, TrES-4b," he rattles off — are inflated far beyond any model, some nearly twice the radius of Jupiter.
Toasters Among Us
Astronomers have proposed a variety of extra heat sources for the deep interior that could puff up these planets. For example, observations show that giant-planet host stars are composed of more heavy elements than our Sun; perhaps, proposed a team led by Burrows in 2007, the hot Jupiters also have heavier makeups that absorb more incoming energy than current models predict. And in 2010, a Caltech duo (Konstantin Batygin and David J. Stevenson) suggested that the supersonic winds in a hot Jupiter could generate a significant heating current as ions in the atmosphere streak across the planet's magnetic field at up to 1 km per second (2,200 mph). Most promisingly, tidal heating due to gravitational interaction with the star (and perhaps other planets) could heat the inside of a hot Jupiter directly.
Buzasi’s solution invokes the magnetic field of the star. In our solar system the Sun is like a power station: its winds of charged particles are interplanetary power lines, carrying current through the solar system. That current links up with the Earth's magnetic field at its poles, where it penetrates into the ionosphere, the layer of charged particles ionized by the Sun at altitudes of hundreds of kilometers. Combined with thunderstorm action, this creates a global electric circuit, driving a weak current flowing high above Earth's magnetic poles.
Now apply this model to a gas giant whizzing around its star in a close orbit, and the planet heats up to around 1,400K (2,060°F). It also becomes a much better conductor, more like a metal, as its atoms ionize. Buzasi calculates this current is much stronger and penetrates into the interior of the planet itself, where the pressure is thousands of times that of Earth's atmosphere.
"Instead of having thousands of amps and a hundred thousand volts [as on Earth], we have millions of volts and billions of amps," said Buzasi at a press conference Tuesday. It turns out that the interplanetary electric grid can provide enough energy to heat — and inflate — the planet, like a toaster plugged into its star.
How To Spot a Toaster
So is there any observational evidence? Buzasi says one piece comes from the Kepler database. Of the hot Jupiters that Kepler has found, the inflated ones exist only around highly active stars, hinting that magnetic activity and a strong stellar wind are involved. Another "suggestive" observation, Buzasi writes in his study, is that the 0.08 a.u. cutoff, closer than which hot Jupiters begin to inflate, "roughly corresponds to the Alfvén radius for the Sun," where the solar wind begins to dominate over the Sun's magnetic field.
Burrows doesn't like invoking an arbitrary threshold because he says some of the inflation trend can be explained by models without an extra heat source. He says he isn't yet "wedded to any particular model" — even his own. But he finds Buzasi's model "interesting" and says it "deserves further consideration" because it provides a way for the current to close deep inside the planet. "The depth at which you deposit the heat is rather crucial," says Burrows. "The deeper you put this heat, the more effect it has."
Burrows says more statistics would help convince him. "For example, for stars that are the same but have a higher level of [magnetic] activity, do you get a larger radius for the planets?" he wonders.
Buzasi noted one way that the effect could be directly confirmed: the interplanetary magnetic field should induce aurora, which could be observed in the radio or UV. "If the planet has any kind of magnetic field, those have to be very powerful."