July 26, 2014 | David F. Coppedge

Reality Check: Favored Planet Theory Dries Up

Three “hot Jupiters” sing “how dry I am,” casting serious doubt on the leading theory of planet formation.

Planet hunters already had their theoretical hands full when they found “hot Jupiters” around other stars.  Solar systems were not supposed to evolve that way, the gas giants closer to their host stars than Mercury is to the sun.  Now, the most precise measurements of the atmospheres of three hot Jupiters finds them much drier than expected.  Nature News explains why the findings “throw water on theory,” stating in the sub-headline that “Dry atmospheres of three exoplanets challenge ideas of how planets form.”  Mark Zastrow writes,

The atmospheres of these exoplanets, known as ‘hot Jupiters’, contain between one-tenth and one-thousandth water vapour than predicted, measurements from the Hubble Space Telescope show. The findings, published 24 July in Astrophysical Journal Letters, are at odds with theories of how planets form….

Under current theory, planets should accumulate molecules such as water faster than their host stars, write the authors. Hot Jupiters typically form in water-rich areas of solar systems and migrate toward their host stars. But Madhusudhan says the new findings suggest that these theories may have to be revised.

The theory is not dead yet.  The missing water could be too deep in the atmosphere to detect, one scientist counters.  The paper’s lead author, Nikku Madhusudhan (U of Cambridge) doesn’t think that is likely, however.  In a NASA press release, he explained:

Madhusudhan, who led the research, said that this finding presents a major challenge to exoplanet theory. “It basically opens a whole can of worms in planet formation. We expected all these planets to have lots of water in them. We have to revisit planet formation and migration models of giant planets, especially “hot Jupiters,” and investigate how they’re formed.”

In addition, the unexpected results will have “major implications in the search for water in potentially habitable Earth-sized exoplanets.”

“Core accretion” has long been the leading theory about how planets form.  Hubble’s findings have cast a wet blanket on theoretician’s confidence in understanding planets:

This theory predicts that the proportions of the different elements in the planet are enhanced relative to those in its star, especially oxygen, which is supposed to be the most enhanced. Once the giant planet forms, its atmospheric oxygen is expected to be largely encompassed within water molecules. The very low levels of water vapor found by this research raise a number of questions about the chemical ingredients that lead to planet formation.

There are so many things we still don’t know about exoplanets, so this opens up a new chapter in understanding how planets and solar systems form,” said Drake Deming of the University of Maryland, College Park, who led one of the precursor studies. “The problem is that we are assuming the water to be as abundant as in our own solar system. What our study has shown is that water features could be a lot weaker than our expectations.

Normally, this would be called “falsification” of a theory, and lead to rejection of the theory to start over.  In actual experience, though, astronomers tend to use each anomaly to tweak their models rather than toss them overboard.

We often say, but can’t say enough, that space scientists are champions at observing but buffoons at explaining.  The engineers who built the Hubble Space Telescope and launched it, and who uncovered the scientific principles by which it operates, are best in the world.  Explaining what they see in the eyepiece, though, has left a sorry trail of dashed expectations, broken promises, and falsified theories.  Exoplanets are nothing like they expected based on how our solar system is constructed.  It’s nice of them to admit that they were wrong in specific cases, but after such a failed track record, we wish they would go deeper and question the philosophical assumptions that led to their flawed models in the first place.






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