The Flip Side of Exoplanet Orbits

New research reveals the possible cause of retrograde "hot Jupiters"


It was once thought that our planet was part of a “typical” solar system. Inner rocky worlds, outlying gas giants, some asteroids and comets sprinkled in for good measure. All rotating around a central star in more or less the same direction. Typical.

But after seeing what’s actually out there, it turns out ours may not be so typical after all…

Astronomers researching exoplanetary systems – many discovered with NASA’s Kepler Observatory – have found quite a few containing “hot Jupiters” that orbit their parent star very closely. (A hot Jupiter is the term used for a gas giant – like Jupiter – that resides in an orbit very close to its star, is usually tidally locked, and thus gets very, very hot.) These worlds are like nothing seen in our own solar system…and it’s now known that some actually have retrograde orbits – that is, orbiting their star in the opposite direction.

“That’s really weird, and it’s even weirder because the planet is so close to the star. How can one be spinning one way and the other orbiting exactly the other way? It’s crazy. It so obviously violates our most basic picture of planet and star formation.”

– Frederic A. Rasio, theoretical astrophysicist, Northwestern University

Now retrograde movement does exist in our solar system. Venus rotates in a retrograde direction, so the Sun rises in the west and sets in the east, and a few moons of the outer planets orbit “backwards” relative to the other moons. But none of the planets in our system have retrograde orbits; they all move around the Sun in the same direction that the Sun rotates. This is due to the principle of conservation of angular momentum, whereby the initial motion of the disk of gas that condensed to form our Sun and afterwards the planets is reflected in the current direction of orbital motions. Bottom line: the direction they moved when they were formed is (generally) the direction they move today, 4.6 billion years later. Newtonian physics is okay with this, and so are we. So why are we now finding planets that blatantly flaunt these rules?

The answer may be: peer pressure.

Or, more accurately, powerful tidal forces created by neighboring massive planets and the star itself.

By fine-tuning existing orbital mechanics calculations and creating computer simulations out of them, researchers have been able to show that large gas planets can be affected by a neighboring massive planet in such a way as to have their orbits drastically elongated, sending them spiraling closer in toward their star, making them very hot and, eventually, even flip them around. It’s just basic physics where energy is transferred between objects over time.

It just so happens that the objects in question are huge planets and the time scale is billions of years. Eventually something has to give. In this case it’s orbital direction.

“We had thought our solar system was typical in the universe, but from day one everything has looked weird in the extrasolar planetary systems. That makes us the oddball really. Learning about these other systems provides a context for how special our system is. We certainly seem to live in a special place.”

– Frederic A. Rasio

Yes, it certainly does seem that way.

The research was funded by the National Science Foundation. Details of the discovery are published in the May 12th issue of the journal Nature.

Read the press release here.

Main image credit: Jason Major. Created from SDO (AIA 304) image of the Sun from October 17, 2010 (NASA/SDO and the AIA science team) and an image of Jupiter taken by the Cassini-Huygens spacecraft on October 23, 2000 (NASA/JPL/SSI).


Neptune's largest Moon, Triton. Astronomers think that Triton is a captured Kuiper Belt Object. Credit: NASA/JPL

When objects in the Solar System orbit other objects, they can either go in a regular prograde direction, or in a retrograde direction.

Almost all of the orbits in the Solar System are caused by the initial collapse of the Solar System 4.6 billion years ago from the solar nebula. As the cloud of gas and dust collapsed down into the stellar disk, the conservation of angular momentum caused the disk to rotate. The Sun formed out of a bulge in the center of the Solar System, and the planets formed out of lumps in the protoplanetary disk.

And so, all of the planets in the Solar System orbit in a prograde direction. And then the planets themselves also collapsed down, and started rotating because of the conservation of angular momentum. And again, almost all of the planets rotate in a prograde direction; except one: Venus. When seen from above their north pole, all the planets rotate in a counter-clockwise direction. But Venus is actually rotating in a clockwise direction.

It’s believed that most of the moons in the Solar System formed in place around their planets. And so they orbit in a prograde direction as well, orbiting in the same direction that their planet turns. There are a few exceptions; however, like Neptune’s moon Titan, which orbits in a retrograde direction.

Because the Earth and the planets are orbiting the Sun, we get a changing perspective of their position as we go around the Sun. The planets can seem to slow down, stop, and then move backwards in the sky. Of course, they’re not actually going backwards in their orbit, but we’re seeing that from our perspective. When the planets move in this backwards direction, they’re said to be “in retrograde”. And then they start moving forward again and come out of retrograde.

We’ve written a few articles about retrograde orbits for Universe Today. Here’s an article about Mercury in retrograde, the 2009 Mercury retrograde dates, and here’s an article about Venus in retrograde.

If you’d like more information on orbits, check out this cool list of orbit diagrams. And here’s more info on Neptune’s moon Triton, which follows a retrograde orbit.

We’ve also done an episode of Astronomy Cast about Neptune. Listen here, Episode 63: Neptune.

Second Exoplanet with Retrograde Orbit Discovered

The exoplanet HAT-P-7b has been observed to have a very curious orbit. It either has a highly tilted orbit – passing almost over the poles of its parent star, HAT-P-7 –  or a retrograde orbit; that is, orbiting in the opposite direction of its parent star. Two teams of researchers, both using the Subaru Telescope in Japan, have published papers on the bizarre properties of this planet, the second exoplanet ever observed to have a retrograde orbit.

In our Solar System, the planets calmly rotate in the same direction as that of their parent star, in our case the Sun. This is called a prograde orbit, and the Earth has the most inclined orbit with regard to the equator of the Sun, of 7.15 degrees. The planet HAT-P-7b, however, has an orbit that is the opposite of the rotation of its parent star but in the same plane as the equator (effectively a 180 degree incline). This is called a retrograde orbit. It may also be the case that it is inclined to at least 86 degrees of the equator of its Sun, so as to have almost a polar orbit. The researchers have yet to determine the true rotation of the star HAT-P-7, and thus which scenario is true for the exoplanet.

“There is a large range of uncertainty because we have not measured the true angle between the orbit and the stellar equator. Instead we can only measure the angle that we see from our perspective on Earth,” said Winn in a MIT press release.

HAT-P-7b is about 1.4 times as wide and 1.8 times as massive as Jupiter, and lies approximately 1,000 light years from the Earth.

A Japanese collaboration led by Norio Narita of the National Astronomical Observatory of Japan, and a team led by MIT assistant professor of physics Joshua Winn both published papers detailing their studies of HAT-P-7b. These studies were published in the Publications of Astronomical Society of Japan Letters October 25, 2009 and the Astrophysical Journal Letters for October 1, 2009, respectively. The paper by the Japanese team is available for your perusal on Arxiv here.

Both research teams used the Subaru Telescope’s High Dispersion Spectrograph instrument to observe the star HAT-P-7. The spectrograph allowed the researchers to monitor the redshift or blueshift of light as the planet orbited the star. In planets with a prograde orbit, their transit in front of the star blocks the blue shifting of the light from the star first, then blocks the redshift of the light, making the star appear to move more that it actually is.

In the case of HAT-P-7b the effect was reversed – that is, the redshifted light appeared bluer, then the blueshifted light appeared redder, making it apparent that the orbit of the planet was not in the same direction of that of HAT-P-7. This effect is called the Rossiter-McLaughlin effect, illustrated below.

The Rossiter McLaughlin effect makes a star appear to have a greater radial velocity than it actually does because of a transiting planet. Image Credit: Nicholas Shanks, WikiMedia Commons
The Rossiter McLaughlin effect makes a star appear to have a greater radial velocity than it actually does because of a transiting planet. Image Credit: Nicholas Shanks, WikiMedia Commons

The odd orbit of HAT-P-7b could have been caused by a number of different factors, and theorists that model the formation of exoplanetary systems will not have to “go back to the drawing boards”. The general consensus is that planets form out of a large disk of material orbiting the star, and thus all orbit in the same direction as the disk out of which they formed.

Multiple planets could have formed in an unstable configuration around the star, and their proximity to each other could have caused a rather chaotic series of gravitational billiards to boot HAT-P-7b into its current orbit. Another explanation is the presence of a third object in the system, such as another massive planet or companion star, that is tilting the orbit of HAT-P-7b due to what’s known as the Kozai effect.

The announcement of the retrograde orbit of HAT-P-7b came only one day after the announcement on August 12th, 2009 that the planet WASP-17b orbits opposite its parent star. HAT-P-7b is also one of the first exoplanets to be studied by the Kepler mission, which studied the planet’s orbit over 10 days. Kepler will take further images of the star during its mission, and by observing the rotation of spots on the surface of the star, nail down the orbital direction, after which we’ll know whether HAT-P-7b is orbiting “backwards” or around the poles of the star.

Source: Subaru Telescope, MIT