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.

Orbit of HD 80806b

When the Spitzer Space Telescope observed HD 80606b in November 2007, the alien planet briefly ducked behind its host star. Locations of short black bars assume the planet has a tidally locked rotation period of 40.7 hours. (Diagram not to scale.)

G. Laughlin (UCSC) & others / Nature

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.

These disks correspond to the infrared views of HD 80606b seen from Earth at the numbered positions in the orbit diagram. Note how the planet continues to radiate away from a dark-side "hot spot" long after passing through periastron.

G. Laughlin (UCSC) & others / Nature

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.

Afterglow on HD 80606b

Computer simulations show how the night side of exoplanet HD 80606b radiates away heat after a scorching pass near its parent star. The frames run from 4.4 days (upper left) to 8.9 days (lower right) after periastron.

NASA / JPL / J. Langton (UCSC)

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.


Image of Rod


January 31, 2009 at 7:26 am

Exoplanets.org has a list of 228 exoplanets, an older list. My list has a total of 230 (I don’t have the list for all 300+ exoplanets and their orbital parameters). Using a simple spreadsheet the average value for e = 0.2398, average semimajor axis (a) = 1.7735 AU, average mass = 2.775 M_Jupiter. The values for Min a = 0.0177 AU, Max a = 119 AU. The values for Min e = 0.0 and Max e = .9349. Comparing measurements like this and at HD 80606b with our solar system demonstrates how clearly our habitable Earth and its stable orbit stand out sharply in the Milky Way’s tally of exoplanets now. Is this data evidence for a cosmic accident or the Creator’s design?

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Richard Scott

January 31, 2009 at 7:50 am

If I understand the science right, our solar system would go undetected using the tools we presently have at our disposal to find planetary systems around other stars.

This doesn’t speak to anything but the fact that really big planets, with orbits that are close to their parent stars are the easiest to detect.

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January 31, 2009 at 12:18 pm

As a follow up, here are the same parameters for our solar system from Mercury out to Pluto:

Using a simple spreadsheet the average value for e = 0.081, average semimajor axis (a) = 11.902 AU, average mass = 0.157 M_Jupiter. The values for Min a = 0.387 AU, Max a = 39.48 AU. The values for Min e = 0.00677 and Max e = 0.24881. The current tally of 300+ exoplanets calls sharp attention to these differences and raises questions concerning how our solar system avoided similar orbital and mass configurations that could prove fatal to life on Earth.

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