May 24, 2010 at 5:25 PM ET
For decades, Pluto has been seen as an oddball in the planetary tribe - in part because its orbit was so much more eccentric and tipped than those of the big planets. But in recent years, more off-kilter worlds have been discovered in our own solar system. And today, astronomers are reporting that they've detected planets much bigger than Jupiter that are way more out of whack than Pluto.
Maybe Pluto, which was discovered by former Kansas farmboy Clyde Tombaugh 80 years ago, isn't so weird after all.
"We're not in Kansas any more as far as solar systems go," Barbara McArthur, an astronomer at the University of Texas' McDonald Observatory, said at a news briefing today.
McArthur and her colleagues - including Fritz Benedict of the McDonald Observatory and Rory Barnes of the University of Washington - reported their findings at the American Astronomical Society's spring meeting in Miami. They're also publishing a paper in the June 1 edition of The Astrophysical Journal.
This latest twist in planetary science is based on a combination of ground-based telescope observations and data from the Hubble Space Telescope, focusing on a yellow-white dwarf star called Upsilon Andromedae.
Working out the wobbles
Astronomers have known for years that three Jupiter-type planets orbit the star, which is 44 light-years from Earth and just a bit younger, brighter and more massive than our sun. They knew that on the basis of slight back-and-forth gravitational wobbles that were detected in the star.
But they couldn't know just how tipped those orbits were until Hubble joined the case. The space telescope's Fine Guidance Sensors were trained on Upsilon Andromedae to make ultra-high-resolution measurements of the star's up-and-down, side-to-side motion. Combined with the back-and-forth data, astronomers could figure out precisely how the planets' gravitational pulls jostled the star in three dimensions.
There were a couple of surprises: First of all, two of the outer planets were inclined by 30 degrees with respect to each other. Those are much odder orbits than Pluto's, which is tipped "only" 17 degrees from the solar system's main plane.
The fact that the planets are tipped meant that their mass had to be recalculated. Astronomers now estimate that the two planets (known as Upsilon Andromedae c and d) are 14 times and 10 times more massive than Jupiter, as opposed to the previous estimates of two and four times as massive. The new findings actually shifted the mass estimates so that c turned out to be weightier than d, rather than the other way around.
Upsilon Andromedae d is so massive that Benedict said it might be considered a failed star rather than a giant planet. "It's a brown dwarf in a bona fide planetary system," Benedict told me in an e-mail.
To top it off, there were hints that a previously undetected fourth planet, dubbed Upsilon Andromedae e, is orbiting the star much farther out. (The innermost planet, Upsilon Andromedae b, is thought to be two-thirds as massive as Jupiter but could be bigger if its orbit is inclined as well.)
Explaining orbital oddities
How did the orbits of planets c and d get so far out of whack? That question poses a challenge for theories of planetary-system evolution.
"Most probably Upsilon Andromedae had the same formation process as our own solar system, although there could have been differences in the late formation that seeded this divergent evolution," McArthur said in a news release. "The premise of planetary evolution so far has been that planetary systems form in the disk and remain relatively co-planar, like our own system, but now we have measured a significant angle between these planets that indicates this isn’t always the case."
McArthur and her team suggest that a violent event occurred to disrupt the planetary order. Perhaps some planets gradually moved inward. Planets may have pushed each other around through gravitational interaction. And one of the most likely suspects is a nearby companion star - a red dwarf that's dimmer and less massive than the sun.
“Our dynamical analysis shows that the inclined orbits probably resulted from the ejection of an original member of the planetary system," Barnes said in the news release. "However, we don’t know if the distant stellar companion forced that ejection, or if the planetary system itself formed such that some original planets were ejected. Furthermore, we find the revised configuration still lies right on the precipice of stability: The planets pull on each other so strongly that they are almost able to throw each other out of the system."
McArthur said little is known about the companion star. It appears to be thousands of times farther away from Upsilon Andromedae than Earth is from our own sun. But it may have a highly eccentric orbit that brings it close enough to send planets scrambling.
Astronomer Philip Armitage of the University of Colorado at Boulder, who was not involved in the research, said the findings support the view that "forming planetary systems are often overcrowded, if you like." As time goes on, the alien planetary system may settle into a configuration that's not so out of whack. But some oddballs might hang around even then. Thanks to Pluto and the other dwarf planets, we know that's the case in our own solar system.
Although McArthur and her colleagues say the Upsilon Andromedae system is just on the edge of instability, they expect planets c and d to stay put in their oddball orbits. What's more, Barnes told me in a follow-up phone call that it'd be "pretty unlikely" if it turned out that the first measurements of orbital inclination were made using a wildly anomalous example. Having off-kilter orbits may be the norm for planetary systems.
"Our solar system could be an outlier," Benedict chimed in.
The study demonstrates once again that as astronomers learn more about alien planetary systems, they're finding worlds that are weirder than anything in our own solar system. We're just getting into a golden age of planetary discovery, and that means we shouldn't get too persnickety about planet definitions. At least that's the way I see it. How about you?
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Correction for 6 p.m. ET: In the heat of adding in material from the press briefing, I scrambled up the name of the researcher (Barbara McArthur) with the name of the observatory (McDonald). Sorry about that!
Update for 6:45 p.m. ET: The University of Washington's Rory Barnes also discussed separate theoretical work he's been doing on the question of habitable worlds beyond our solar system. He suggests that some seemingly habitable planets might not be all that conducive to life as we know it, due to their gravitational interaction with other planets.
For example, suppose you were on a not-yet-detected Earthlike world in the Upsilon Andromedae system. That alien Earth could be sent topsy-turvy during periodic encounters with a Jupiter-scale planet in an elliptical, tipped orbit. It might be pushed outside its habitable zone, icing over and putting life in the deep freeze. It might be pulled into a tighter orbit, turning a habitable climate into a volcanic hell. It could even be spun in unconventional ways, producing chaotic variations in the length of the planet's days and its seasons.
Barnes says it's hard to tell just how many so-called "tilt-a-worlds" exist, but they're at least theoretically possible. "It's fascinating to think about how evolution occurs on such a world," Barnes said in a UW news release.
Barnes' research, presented during the AAS meeting, follows up on a paper published last year in The Astrophysical Journal Letters. The work is funded by NASA's Virtual Planetary Laboratory, and was conducted along with Brian Jackson of NASA's Goddard Space Flight Center, Richard Greenberg of the University of Arizona and Sean Raymond of the Laboratoire d'Astrophysique de Bordeaux in France.