Hot Jupiters: Astronomers Blew It

The experts were (and are) wrong about how planets form, new data from a weird exoplanet show.

The case of HD 80806b may undo decades of theory about planet formation.  According to Science Daily, this planet, orbiting a star in Ursa Major with a wildly eccentric orbit of 111 days, cannot have formed the way astronomers thought. It’s an oddball in the class of “hot Jupiters” (large planets that orbit close to a star), which themselves were theory-busters when they were first detected.

For centuries, the solar system was viewed as a standard blueprint for planetary systems in the universe, with a star (our sun) at the center of a circular track, and a planet orbiting within each lane. Smaller, rockier planets fill the interior lanes, and larger gas giants orbit further out.

But over the last 20 years, more powerful telescopes have revealed, far from our solar system, a host of exotic systems with completely unexpected configurations. “Hot Jupiters,” for example, are massive “roaster” planets that circle scorchingly close to their stars. Scientists have puzzled over how these gas giants, which supposedly form far from their stars, end up on such blistering orbits.

Astronomers quickly worked out theory-rescuing schemes. The hot Jupiters initially formed far from their stars like they were supposed to, but got caught in spiral conveyor belts that made them migrate to their current locations (5/04/14).

Now comes a hot Jupiter detected by JPL’s Spitzer Space Telescope that throws the rescued theory into the falsification bin. HD 80806b has these unusual features:

1. It has the size of Jupiter but 4 times the mass.
2. It has a wildly eccentric orbit similar to that of a comet, with an aphelion as far from its star as the orbit of Earth is to the sun, but a perihelion that nearly touches the star. If it had migrated inward as theory predicts, its orbit should have circularized long ago.
3. On closest approach, the planet’s atmosphere heats up to 2,000° F, enough to cause hurricane force winds and intense heat in its atmosphere. If Earth came that close, its entire surface would melt into magma and its atmosphere would be stripped away.
4. Hot Jupiters are supposed to be gas planets with “squishy” interiors that circularize their orbits quickly due to tidal dissipation. For HD 80806b to have such an eccentric orbit, it cannot be as squishy as predicted, implying that it could keep its eccentric orbit for another 10 billion years.
5. The planet’s rotation rate is 90 days, “twice as long as predicted for this planet.”

The intense heat at perihelion cools back down within 10 hours, scientists calculate, and then it stays cool for the next 111 days. Still, if the planet were 100 million years old, it should have experienced 900,000 close passages. Astrobiology Magazine explains what happens each time around:

“As the planet gets closer to the star, it feels a burst of starlight, or radiation. The atmosphere becomes a cauldron of chemical reactions, and the winds ramp up far beyond hurricane force,” said Laughlin, a co-author on the Spitzer study, which is accepted for publication in The Astrophysical Journal.

Could its atmosphere survive close to a million trips like that? An embedded video clip from the Spitzer team shows infrared measurements glowing almost white-hot at perihelion. It would seem that each close passage would increase atmospheric escape, such that the entire atmosphere would be lost in relatively short order.

Quotes from the articles show theorists at a complete loss to explain this planet.

• Now an even weirder planetary system may render the puzzle more challenging. [Science Daily]
• If this is the case [that HD 80806b is not migrating as fast and is not as squishy as predicted], de Wit says scientists may have to rethink theories on how hot Jupiters are formed, as it implies that the underlying migration mechanism may not be as efficient as once believed.
• De Wit says that alternative theories for how hot Jupiters form — in which gas giants develop close to their stars or smoothly spiral inward with the help of planet-forming disks — may be more likely explanations for hot Jupiter evolution. [Ibid.]
• “This system is undoubtedly very unique as it seems to challenge in many ways our understanding of planet-star interactions and planet formation,” de Wit says. [Ibid.]
• At first, hot Jupiters were considered oddballs, since we don’t have anything like them in our own solar system. But as more were found, in addition to many other smaller planets that orbit very closely to their stars, our solar system started to seem like the real misfit. [Astrobiology Magazine]
• “We thought our solar system was normal, but that’s not so much the case,” said astronomer Greg Laughlin of the University of California, Santa Cruz, co-author of a new study from NASA’s Spitzer Space Telescope that investigates hot Jupiter formation. [Ibid.]
• As common as hot Jupiters are now known to be, they are still shrouded in mystery. How did these massive orbs form, and how did they wind up so shockingly close to their stars? [Ibid.]
• “We are starting to learn how long it may take for hot Jupiter migration to occur,” said de Wit. “Our theories said it shouldn’t take that long because we don’t see migrating hot Jupiters very often.” [Ibid.]
• “The long time scales we are observing here suggest that a leading migration mechanism may not be as efficient for hot Jupiter formation as once believed,” said Laughlin. [Ibid.]
• The Spitzer study suggests that competing theories for hot Jupiter formation — in which gas giants form “in situ,” or close to their stars, or smoothly spiral inward with the help of planet-forming disks — may be preferred. [Ibid.]
• A rotation rate of 90 hours is much slower than what is predicted for HD 80606b, puzzling astronomers, and adding to the enduring mystique of hot Jupiters. [Ibid.]

Indeed, it’s hard to find anything the experts got right about this exoplanet. And if this example is wrong, it reflects on theories of other hot Jupiters, theories of planet formation, and the structure of other stellar planetary systems. Even more important, it accentuates the uniqueness of our own solar system with its one life-friendly planet.

This is a great opportunity. If “competing theories… may be preferred,” can we suggest one? “In the beginning, God created the heavens and the Earth.” Maybe he created hot Jupiters and oddballs like HD 80806b to confound those who are wise in their own eyes. That’s something He likes to do (I Corinthians 1:26-29), because the first step toward reconciliation with the Creator is humility and shame for disobedience to His word.