We Earthlings are fascinated with weather extremes. A meteorologist friend, Harvey Leonard of WCVB in Boston, notes that residents of the Dakotas sometimes enjoy balmy temps by day only to be overrun with an arctic cold front at night — a dramatic change of as much as 75° to 85°F.
But nothing the residents of Fargo have ever experienced compares with the bizarre weather you'd encounter on the alien world known as HD 80606b.
When Swiss planet-hunters discovered HD 80606b in 2001, they could tell right away that it was an oddball. Its extremely elongated orbit carried the planet within 3 million miles (5 million km) of its host star every 111 days.
In November 2007 a team of astronomers led by Greg Laughlin (Lick Observatory, UCSC) used the Infrared Array Camera on NASA's Spitzer Space Telescope to monitor exactly what happens during this periodic roasting — and what they found boggles the mind. Temperatures on this massive planet rise by nearly 1,300°F (700°C) in the six hours it spends closest to the star.
Leonard's weather forecast for HD 80606b might go something like this: "Folks, we've had our usual few weeks of temps hovering in the 80s, but that's about to change. Next Friday you can expect to sizzle at around 1,000° when you step out for that morning commute, and by early afternoon you'll be diving for shade as the mercury tops out at just over 2,200°."
The Spitzer team knew this planet gets scorched once per orbit when planning its observations. But what the observers hadn't counted on was watching HD 80606b duck out of sight for 1.7 hours just before coming closest to the star. This eclipse was pure bonus: it let the team confirm that the planet has 4.2 times the mass of Jupiter and that its orbit is inclined 90° to the sky (the orbital plane lies in our line of sight).
This steep inclination has important implications for how HD 80606b ended up in such a strange orbit, unique among the many "hot Jupiters" discovered by exoplanet-hunters to date. HD 80606 is a close match to our Sun in mass and brightness, but it has a nearly identical companion star just 1,000 astronomical units away. That's only 25 times the Sun-Pluto distance. The companion star's gravity plays havoc with HD 80606b's orbit via what's called the Kozai mechanism. Every now and then the planet is forced into an extremely eccentric (elliptical) orbit — and it's clearly now in one of those times.
The planet's orbital eccentricity is 0.93; meaning its distance from its star ranges from 0.03 to 0.85 of Earth's distance from the Sun. So it receives 800 times as much light and heat when it's closest to the star than when it's farthest.
Aside from the orbital yo-yoing that it must endure, HD 80606b is already telling cosmic chemists a great deal about how planetary atmospheres react to being heated so dramatically and how they cool down over time. "This is the first time that we've detected weather changes in real time on a planet outside our solar system," Laughlin notes in a NASA-JPL press release. (Related press releases are here and here.)
The Spitzer observations appear in today's issue of the journal Nature.
The host star, HD 80606, 190 light-years away in southwest Ursa Major, shines at 9th magnitude — making it barely visible in large binoculars and easily visible in a good amateur telescope. Its spectral type is G5, a trace less hot than the Sun.