Wrong-Way Planets

By Ken Croswell

Published in StarDate (September/October 2010, page 16)

Once upon a time, when everyone agreed the Sun had nine planets, planets obeyed certain rules.

One rule was basic: a proper planet races around its star in the same direction the star turns.

For example, view our solar system from the North Star and you would see the Sun spinning counterclockwise, with every planet--including poor Pluto--dashing about it counterclockwise as well. Furthermore, most of the planets inhabit the same plane, because they formed from a flat disk of gas and dust that revolved around the newborn Sun. Indeed, the Sun's seven largest planets--those from Venus to Neptune--all have orbits tilted at less than 3.5 degrees to Earth's orbit.

In the last two years, however, astronomers have uncovered planets beyond our solar system that have no sense of propriety. Remarkable new observations show planetary orbits tilted wildly to their stars' equators. Worse, many planets dare orbit their stars backward, opposite from how the stars turn.

Such wrong-way planets are more than just celestial freaks. They account for a substantial fraction of all "hot Jupiters," the giant extrasolar planets that orbit close to their stars. In addition, their bizarre orbits testify to strange processes that never wracked the planets of our own more orderly solar system. And the sheer abundance of backward planets unveiled in April 2010 calls into question the standard theory of how hot Jupiters arise.

"Spin-orbit angles are something that we can measure fairly precisely," says Josh Winn at the Massachusetts Institute of Technology. "And whenever you can measure something, you probably should, because there may be some surprise in store."

Extrasolar planets have sprung surprise after surprise. American astronomers found the first such planets in 1991, orbiting a pulsar, the superdense core of an exploded star. And in 1995, when Swiss astronomers found the first extrasolar planet around a Sunlike star, it was something nobody ordered: a giant world like Jupiter but so close to its star that it whirled around it once every 4.2 days.

That astronomers can measure the slant of an extrasolar planet's path is as astonishing as the odd orbits these observations are revealing. Astronomers do so by exploiting the Doppler shift, the change in wavelength caused by an object's motion. When a light source is moving toward an observer, the crests of its light waves crowd together, making their frequency appear to be higher than it actually is--a blueshift. If the light source is moving away from the observer, the wave crests arrive later and later, making their frequency seem to decrease--a redshift. This also happens to sound waves. It's why the whistle of a train seems to change pitch as the train speeds away from you.

To see how the Doppler shift works in the case of these planets, picture a star that's moving neither toward nor away from us. But the star is spinning, so one hemisphere spins toward you and is blueshifted, while the other hemisphere spins away from you, so its light is redshifted.

Imagine that a planet revolves around the star in the "proper" direction. Now imagine also that this planet's orbit takes it across the face of the star, an event called a transit. The planet first passes over the star's blueshifted hemisphere, blocking some of the blueshifted light. Then the planet passes over the star's redshifted hemisphere, blocking some redshifted light. Thus, observers first see a slight loss of blueshift, followed by a slight loss of redshift. In contrast, a transiting planet orbiting a star backward first blocks the redshifted hemisphere, then the blueshifted one, producing the opposite pattern.

Novel though the technique seems, it actually dates back almost a century, to two astronomers who were studying eclipsing binaries--double stars that pass in front of each other. In the autumn of 1922, a graduate student at the University of Michigan named Richard Rossiter discovered this behavior in the eclipsing binary star Beta Lyrae. Another Michigan astronomer, Dean McLaughlin, then observed what Rossiter called "the rotational effect" in Algol, an eclipsing binary in Perseus.

Today, the Rossiter-McLaughlin effect reveals the spin-orbit angles of extrasolar planets. However, it works only if a planet happens to transit its star. Most don't. And the few that do usually orbit close to their stars, since this increases the chance of a transit: even in our solar system, Mercury transits the Sun more often than Venus does. Astronomers spotted the first transit by an extrasolar planet--a hot Jupiter around HD 209458, a Sunlike star in Pegasus--during November 1999.

Less than three weeks later, Swiss astronomer Didier Queloz, the codiscoverer of the first hot Jupiter, led a team that recorded the star's Rossiter-McLaughlin effect. The transit of the planet had made headlines; the Rossiter-McLaughlin effect the planet induced didn't, because the planet did what proper planets do: it circled its star in the right direction near the plane of the star's equator.

Astronomers found additional transiting planets, and the planets always lay within a few degrees of the star's equator. That's the pattern in our solar system, too. For example, Earth's orbit makes an angle with the Sun's equatorial plane of 7.2 degrees. Even Pluto, often accused of nonconformity, has an orbit tilted just 12.2 degrees from the Sun's equator (and 17.1 degrees from the Earth's orbit).

Moreover, the well-aligned orbits of hot Jupiters accorded with the standard theory of how these strange planets arose. Giant planets form only in the frigid outer reaches of their solar systems, because only there does ice condense, giving the planets the bulk they need to grow into giants. Then, the theory goes, friction between the disk of gas and dust revolving around a newborn star drags the planet inward, until it becomes a hot Jupiter. This process, called disk migration, keeps the planet's orbit aligned with the disk and thus with the star's equator.

Then, in 2008, observers found the first exception.

The deviant was a hot Jupiter circling XO 3, a yellow-white star in the northern constellation Camelopardalis. Observers led by French astronomer Guillaume Hébrard had first noted the misalignment; however, their result was sufficiently uncertain that a question mark appeared in the title of the paper reporting the discovery.

Then Winn and his colleagues pointed one of the mighty Keck telescopes at XO 3. "That signal was unambiguous," says John Johnson, an astronomer at the California Institute of Technology who helped make the observations. "That was a whopping, gigantic signal, probably one of the largest Rossiter-McLaughlin signals that has ever been observed in a planetary system." XO 3's planet had a spin-orbit angle of 37 degrees, three times the tilt of Pluto's orbit.

In 2009, additional transgressors appeared. Next was the extraordinary extrasolar planet orbiting the yellow G-type star HD 80606 in Ursa Major. This planet follows an incredibly elliptical orbit. Its orbital eccentricity, a measure of the ellipticity, is 93 percent--versus 21 percent for Mercury and 25 percent for Pluto.

HD 80606 b revolves every 111 days. Its orbit is tipped about 42 degrees to the star's equator.

Strange though these worlds were, they at least traveled around their stars in the right direction. But in the summer of 2009, astronomers reported two planets that broke the final taboo. The first backward planet orbited WASP 17, a star in Scorpius; the second, HAT-P 7, a star in Cygnus.

Then this spring, Amaury Triaud, a graduate student at Geneva Observatory in Switzerland, reported a result so stunning that at first even he didn't believe it. Triaud's paper listed 25 transiting planets with spin-orbit measurements; 9 of them have tilts greater than thirty degrees, and 5 of those go around their stars backward.

"Why, why?" he says he asked himself. "What did we do different from the others? Why do we find so many misaligned planets?"

Josh Winn calls the result a significant advance. "This is a very important development," says Winn, "because it basically doubles the number of really tilted systems that we are aware of."

When astronomers knew of only one misaligned planet, they could claim that all the aligned hot Jupiters arose through disk migration, then invoke a violent process--maybe another giant planet flung the hot Jupiter into its odd orbit--to explain the lone exception.

Now, however, the theory of disk migration is in jeopardy. Disk migration can't explain misaligned planets, let alone backward ones.

Furthermore, the true tilts can be still more extreme. That's because astronomers don't actually measure the true angle between a planet's orbit and the star's equatorial plane, since they don't know the slant of the star's spin axis to our line of sight. If they happen to view a star equator-on, then the measured spin-orbit angle is correct. Otherwise, the true spin-orbit angle is closer to 90 degrees than what's measured. Thus, even a planet that seems to have a small spin-orbit angle could be quite misaligned. For example, if we view a star pole-on, and if it has a transiting planet, then that planet is passing over the star's polar regions.

Now that 25 planets have spin-orbit measurements, Triaud's team has enough data to try to average out this ambiguity. "There are way more misaligned planets than there used to be," says Triaud, who calls the high percentage of aligned planets early on a statistical fluke due to a small sample. The upshot: "I don't need disk migration. That's really what my conclusion is. I don't think it's a dominant process for making hot Jupiters."

Instead, Triaud thinks most hot Jupiters arise from the so-called Kozai mechanism, which makes a planet's orbit oscillate between two states: aligned but elliptical, and misaligned but circular. In 1962, Japanese astronomer Yoshihide Kozai calculated such behavior in asteroids responding to Jupiter's gravity.

In another solar system, a distant planet or star on a tilted orbit causes the inner planet to cycle back and forth between inclined and elliptical paths. During times when the inner planet's orbit is inclined, the planet can flip from prograde--orbiting the star in the direction of its spin--to retrograde--orbiting in the opposite direction. And during times when the orbit is elliptical, the planet ventures closer to its star, whose tides rob the planet of orbital energy and shrink the orbit. Ultimately the planet sinks into a close-in circular orbit, becoming a hot Jupiter, but it's left with whatever orbital inclination it happened to have on its last Kozai cycle. So in its final state, the hot Jupiter can be aligned or misaligned, prograde or retrograde.

The extraordinary planet around HD 80606 may be a hot Jupiter in training. Its orbit is both inclined and extremely elliptical. Furthermore, a second star about 100 billion miles from the planet may be the perturber driving the Kozai cycles.

Triaud says the Kozai mechanism could work in conjunction with other processes. For example, once a giant planet forms far from its star, disk migration or a gravitational encounter with another planet could send the giant world somewhat closer to its star, where the Kozai mechanism then takes over.

However, John Johnson points out problems with the Kozai mechanism. "The inner planet and the outer perturber have to start off very misaligned with respect to each other," says Johnson. "So if you invoke this as a universal mechanism, now we've just moved the question backward. Why are all of the outer perturbers on really highly inclined orbits?"

Johnson also worries that the few planets with measured spin-orbit angles aren't representative of all hot Jupiters. To measure the Rossiter-McLaughlin effect requires a transit; but transit searches tend to turn up planets around stars hotter than the Sun. In fact, the first known retrograde planet orbits such a star; WASP 17 is spectral type F6, so it is several hundred degrees hotter than the Sun. In contrast, hot Jupiters around cooler G, K, and M stars may have orbits that are more normal.

Still, says Johnson, "I've been pretty much flabbergasted by all of this. I have no idea how to explain all of these retrograde systems. All these observations have made me more and more realize the importance of taking a nice step back and just observing the weirdness of what the universe hands us."

In 2009, NASA launched the Kepler spacecraft, which has begun finding transiting hot Jupiters. Kepler's prime quarry, however, is grander: Earth-sized planets farther from their stars. Because these will be transiting planets, the observations will use the Rossiter-McLaughlin effect to see whether their orbits are aligned or inclined.

So, for those longing for proper planets, there's still hope. Perhaps Kepler will find plenty of planets that obey the rules. But the spacecraft could instead find that wrong-way planets are the norm--which means the real rule breakers are right here in our own solar system.

Astronomer Ken Croswell's book on extrasolar planets, Planet Quest, was a New York Times Notable Book of the Year. He has written for the StarDate radio program for nearly twenty years. His latest book is The Lives of Stars.


1991: Alex Wolszczan and Dale Frail discover the first two extrasolar planets, around PSR B1257+12, a pulsar in Virgo.

1993: Wolszczan discovers a third planet, interior to the other two, around the same pulsar.

1995: Michel Mayor and Didier Queloz discover the first extrasolar planet--and the first hot Jupiter--around a Sunlike star, 51 Pegasi.

1998: Astronomers discover the first planet around a red dwarf, Gliese 876 in Aquarius.

1999: Astronomers discover the first transiting planet, a hot Jupiter around the Sunlike star HD 209458 in Pegasus.

1999: Didier Queloz and colleagues detect the Rossiter-McLaughlin effect in HD 209458, finding the planet's orbit aligned with the star's equator.

2008: Astronomers discover the first misaligned extrasolar planet, around the F-type star XO 3 in Camelopardalis. The planet's orbit is tilted 37 degrees to the star's equator.

2009: NASA launches the Kepler spacecraft, which begins discovering transiting planets.

2009: Astronomers report the first backward planets, orbiting the stars WASP 17 in Scorpius and HAT-P 7 in Cygnus. By good luck, HAT-P 7 is in Kepler's field of view, so the spacecraft can monitor the star's light precisely.

2010: Amaury Triaud and colleagues report many retrograde planets and challenge conventional thinking about how hot Jupiters arise.