systemic

A recent article in Nature reports that WASP-18b has emerged victorious in the ongoing exoplanetary limbo competition.

WASP-18b is also a strong contender in the least-habitable-planet-yet-detected competition. It has a mass roughly ten times Jupiter’s and skims 2.6 stellar radii above the surface of the parent star. The orbital period is a mere 22 hours 36 minutes. A year in less than a day.

To the offhand glance, even the simple presence of the planet seems puzzling. It’s so close to its parent star that tidal orbital decay should haul it in for destruction on a timescale that’s alarmingly short in comparison to the ~1 billion year age of the parent star. Either WASP-18b has been found on the very cusp of its dénouement (which seems unlikely) or tidal dissipation in the parent star is much lower than in a star like the Sun.

Darin Ragozzine pointed me to to a recent article by Barker and Ogilvie that indicates that WASP-18 may indeed be very poor at dissipating tidal energy. It’s an F-type star, somewhat more massive than the sun, with a negligible convective envelope, and no good recourse to turning tidal waves into heat. It’s like a bell that can ring and ring without making a sound. According to Barker and Ogilvie, similarly inviscid F-type parent stars are also responsible for the survival of WASP-12 and OGLE-TR-56b. Their prediction for WASP-18b would be that changes in the orbital period will not be observable, even with the excellent precision that will be obtained by timing the orbit over periods of a decade or more.

Darin also pointed out something else that’s pretty cool. As is also the case with HD 209458b and HD 189733b, the transit of WASP-18b is readily visible in the archived photometry from the Hipparcos mission. Indeed, the planet has been sitting in open view on the web for well over a decade, assuming, of course, that one knew exactly where to look. To see it with 20-20 hindsight, use the folding applet provided at the Hipparcos web site. Enter the Hipparcos catalog number (7562) for the parent star, and fold the 130 published photometric measurements at the 0.94145299 day orbital period. Can you see the transit?

On worlds like WASP-18b, surface temperatures are well in excess of 2000 K. Under such conditions, the ionization fraction is high enough that the planetary magnetic field can affect the weather.

On Earth, where air is composed of neutral atoms and molecules, the wind blows right through magnetic field lines. By contrast, on WASP-18b, the ionization fraction is high enough that the winds will have a tendency to drag the planetary magnetic field lines along. This stretches the field lines, and like rubber bands, they offer a restoring force. Whereas ordinary exoplanetary weather can be described using the equations of hydrodynamics, on an ultra-hot Jupiter, the richer behavior of magnetohydrodynamics comes into play. As a consequence, I have little intuitive sense of what’s going on at the sub-stellar point of WASP-18b, but I’ve got little doubt that it’s interesting and complicated.

WASP-12b. Now there’s an unpleasant travel destination.

Nevertheless, this particular planet, whose transits were recently announced by the SuperWASP collaboration, is quite a remarkable world. For starters, inveterate bottle-poppers can celebrate a WASP-12b New Year on literally nine out of every ten days — the orbital period is a mere 26 hours and 11 minutes. The temperature of the planetary photosphere at the substellar point likely exceeds 2500K. Cherry orange, to be exact.

Because of its ultra-short orbital period, WASP-12b is attracting quite a bit of interest. The planet has a radius 1.8x larger than Jupiter, which should make it eminently feasible to detect secondary transits from the ground in either the optical or near-infrared. One expects, furthermore, that a planet with an orbital period just a shade over a day should have long since damped out its eccentricity, but (to better than 2-sigma) the orbit appears to be non-circular, with e=0.049 +/- 0.015. Even if another planet exists in the system, there should long since have been evolution to a tidal fixed point, followed by circularization. If the orbit really is eccentric, then GR precession of the periastron amounts to a whopping 0.2 degrees per year, nearly 2000x faster than Mercury’s stately 43” per century.

I got an opportunity to visit Harvard this month, and while I was there, David Latham remarked that he had used a remotely operated telescope in Arizona to get a high-precision light curve of a WASP-12b transit. Latham is a meticulous observer, and so, in order to get the best possible baseline, he had cued up the telescope a number of hours prior to the predicted ingress. He related that he’d been completely startled to find, upon analyzing his photometry, that the transit had occurred several hours ahead of schedule. Without a doubt, transit timing variations are going to be one of the big exoplanet stories of 2009, but they’re going to be measured in seconds, not hours. Imagine the commotion that would result if the Sun rises a few hours late tomorrow morning!

The WASP-12 mystery was solved by the amateur astronomers Veli-Pekka Hentunen and Markku Nissinen of Taurus Hill Observatory near Varkaus, Finland. Bruce Gary, who runs the Amateur Exoplanet Archive forwarded the news of their work:

AXA contributors and TransitingPlanets members,

I just received two data files for WASP-12 as observed by Veli-Pekka Hentunen and Markku Nissinen (Finland) which suggest that the discovery paper for this exoplanet has a misprint for the ephemeris. Their observations on January 1 was a “no show” (attached) whereas their observations on January 4 had a nice transit (attached). According to the discovery paper’s ephemeris there should have been a transit on January 1 but not on January 4. However, the discovery paper has a discrepancy between the stated ephemeris and the stated HJD for WASP survey observations. The Hentunen and Nissinen observations can be explained if the discovery paper’s stated WASP survey HJD is correct and their HJDo has a number transposition, such that HJDo = 4506.7961 (instead of 4506.9761). This is described on the AXA web page for WASP-12: http://brucegary.net/AXA/WASP12/wasp12.htm

[...]

We amateurs have to keep the pro’s honest! Nice work, Veli-Pekka Hentunen and Markku Nissinen.

Bruce L. Gary, webmaster
Amateur Exoplanet Archive

Indeed! The typographical error in the discovery ephemeris has now been corrected, and with it, the puzzling “early” transit was revealed to be a completely separate event in the unending sequence of near-daily occultations. It seems somehow fitting that a seemingly alarming discrepancy for the hottest planet known was resolved by a pair of dedicated amateur observers during the long, dark, and frozen Finnish nights.