The Exoplanet Evaporation Debate

by Dr. Tahir Yaqoob on March 15, 2012

The hot Jupiter known as Osiris is famous for many things in exoplanet science (aside from the fact that it is one of the few exoplanets with a real name, it’s zip-code-like boring name being HD 209458b). One of these is that it has been at the center of a controversy about exoplanet evaporation. The question being, do giant hot Jupiters that are orbiting so close to the host star (less than 10% of the Earth-sun distance) evaporate away, leaving being a rocky core? Osiris shows evidence of a comet-like extended structure, or tail, and other evidence of mass loss, but the interpetation of the data is sufficiently complex that the issue of the degree of exoplanet evaporation has become controversial.

Sometimes it is tempting in a scientific paper to say what you really think (like, “we think that the other group’s method sucks big time”) but you have to stick to protocol and formalize your language. However, if you are in the buisiness you can read this formalized language and sometimes figure out what the authors really did want to say. The exoplanet evaparation controversy provides a great example of this: you can just feel the tension and boiling blood simply in the titles of the papers.

The detection of atmospheric escape in Osiris was first reported in a paper by Vidal-Madjar et al. (Nature 422, 2003, 143-146) and the title of the paper is:

Extended Upper Atmosphere around the Extrasolar Planet HD 209458b.

Quite plain, dry, and innocent. No tension. The word escape or evaporation doesn’t even appear in the title. Seemingly just another boring paper. But you know from the fact that it appeared in Nature that there must be more to it than that. The abstract is short and sweet and ends with the words “…escaping hydrogen atoms.” There, just quietly slipped in it at the end.

An outright refutation of the evaporation scenario appeared in a paper by Ben-Jaffel et al. (Astrophysical Journal 671, 2007, L61-L64), this time with blatent fist-waving in the title:

Exoplanet HD 209458b: Inflated Hydrogen Atmosphere but No Sign of Evaporation

To an ordinary person, this title is not in itself dagger-waving. But to a scientist this is an afront. You almost never spell out a conclusion in the title that contradicts that of your competitor. If you do, you put in some wiggle room in case you’ve made a mistake. But no, in this title there is no ambiguity, it unequivocably says that there is no sign of evaporation. Okay, there is a little bit of wiggle room by use of the phrase “no sign of” because the authors can always say they missed the sign. But on the other hand the authors are clearly saying whatever signs there were thought to be before are mistaken (by the other group).

A rebuttal of this paper by the other group soon appeared (Vidal-Madjar, et al., 2008,
Astrophysical Journal 676, 2008, L57-L60) and it had the title:

Exoplanet HD 209458b (Osiris): Evaporation Strengthened

Again, short and sweet. But, as you know, sometimes the short length of a statement in itself speaks volumes. To me, this loosely translates as “suck it!” This translation is supported by the abstract itself which criticizes the methodolgy used by the other group. A sound criticism of methodology is made when you can show precisely how the other group obtained their numbers by reproducing their method, and then showing why yours are different and why yours are more likely to reflect the truth. This is done with confidence in the abstract (and the paper). But then, in an unprecedented move (for a scientific paper), the final sentence of the abstract, quite out of the blue, thrusts the dagger deep into the already battered target:

“Because the escape of H atoms is strengthened in this paper, we renew our proposal to use the nickname ‘Osiris’ for the planet HD 209458b, which loses mass like the Egyptian god.”

Absolutely brutal! This is a double whammy. If you want to insult a scientist, you first show that his result is rubbish, then you slap him with a religious reference. To do this in the title and abstract of a paper carries triple points.

When I checked the citations today, this last paper has been cited 41 times, and the “anti-evaporation” paper has 35 citations. A decent number for both, but not a single one of these papers argued against evaporation outright.

My own take on evaporation is that the current paradigm of planet formation and the evaporation rates are ill-founded. (Yes, that’s a polite way of saying that I think they are wrong.) There is no evidence that these close-in planets are in what is known as hydrostatic equilibrium, so many of the calculations are likely to be invalid because the assumption of equilibrium is absolutely critical to many of the calculations. These planets are so close to the host star that their self-gravity competes with the gravity of the host star. It is assumed that the planets are already formed and are stable when they get to the host star, having come in from farther out. However, I think this may be back-to-front. These planets could be newly formed from mass ejections from the host star instead of coming to their death from far out. I explain this in more detail in Exoplanets and Alien Solar Systems, but in a nutshell, it is not a coincidence that the self-gravity and the host-star gravitational acceleration happen to be just on the verge of balancing, because that is precisely how you would expect an ejected blob to be able to start self-gravitating when it is far enough from the host star. And you would expect lots of mass loss, instability, and entrained material as part of this planet-formation process, just as is observed in Osiris.

Previous post:

Next post: