Targeting Planetary Migration

by Paul Gilster on November 16, 2009

When the stars are properly aligned, expect remarkable things. How useful, for example, to find that a planet we would like to know much more about — HAT-P-7b, about 1000 light years from Earth — is not only providing intriguing transit information right now, but is also in Kepler’s field of view. We’d like to know whether there are massive outer planets in this system, or possibly a binary companion. These are questions that the Kepler observatory may be able to answer.

Any transiting exoplanet is obviously of high interest, but HAT-P-7b stands out a bit more following the publication of two recent papers in separate journals. Both used the Subaru Telescope to examine the planet’s unusual orbit, which appears to be retrograde or polar. This is useful stuff, because it’s telling us something about how planetary systems form, offering useful evidence about planetary migration models.

What we would expect is that planets that form in protoplanetary disks around young stars would have an orbital axis aligned with the stellar spin axis — this is certainly what we see in our own Solar System. But what we are finding around other stars compels us to look at planetary migration models that can disrupt this pattern. Interactions between giant planets or planets and a nearby companion star can cause tilted or retrograde orbits to occur, the latter being defined as orbits that are tilted by more than ninety degrees from the stellar spin axis.

A Japanese team led by Norio Narita (National Astronomical Observatory of Japan) worked with the Rossiter-McLaughlin effect, which produces a distortion in radial velocity data during a planetary transit. The diagram shows the effect, which is perceived as a change in the velocity of the star, but is actually the effect of the transiting planet on the star’s light. This is thorny, but bear with me: If a planet occults part of the approaching (blue-shifted) part of the star’s disk, the radial velocity of the star will appear to be slightly red-shifted, and vice-versa. The Rossiter-McLaughlin effect depends on the spin-orbit alignment of the system, which is why it is so useful.

rossiter_mclaughlin_1

Image: An illustration of the Rossiter-McLaughlin effect. As a star rotates, one part appears to be approaching, the other receding. During a planetary transit, we can see the Rossiter-McLaughlin effect as an apparent anomaly in the stellar radial velocity. The star appears to be receding if the transiting planet hides an approaching part and vice versa. We can observe this effect by precise radial velocity measurements. Note that if the planet orbits in a prograde manner, the planet first hides an approaching side and subsequently hides a receding side. Inversely, if the planet orbits in a retrograde manner, the effect occurs in reverse. Credit: National Astronomical Observatory of Japan.

Narita’s team found that HAT-P-7b shows clear evidence of a retrograde orbit, an observation confirmed by a US team led by Joshua N. Winn (MIT). So we have information about an unusual orbit but no migration model for this system, which is why finding other planets or a binary companion would be useful. Kepler’s help will be invaluable as we work to understand what appears to be a wide variety of planetary orbits, fitting these into theoretical models that explain the origin of each.

rossiter_mclaughlin_2

Image: The observational result of the Rossiter-McLaughlin effect on UT May 30, 2008 taken with the Subaru HDS (Narita et al. 2009). This figure shows that HAT-P-7b first hides a receding part of the star HAT-P-7 and subsequently hides an approaching side. National Astronomical Observatory of Japan.

The first paper is Narita et al., “First Evidence of a Retrograde Orbit of a Transiting Exoplanet HAT-P-7b,” Publications of the Astronomical Society of Japan Letters, Vol 61, No. 5 (2009), pp. L35-L40 (abstract). The second is Winn et al., “HAT-P-7: A Retrograde or Polar Orbit, and a Third Body,” The Astrophysical Journal Letters, Vol. 703, Issue 2 (2009), pp. L99-L103 (abstract).

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{ 4 comments }

Zen Blade November 16, 2009 at 17:20

I feel a bit slow today, but can someone expand upon the following paragraph:

“What we would expect is that planets that form in protoplanetary disks around young stars would have an orbital axis aligned with the stellar spin axis — this is certainly what we see in our own Solar System. But what we are finding around other stars compels us to look at planetary migration models that can disrupt this pattern. Interactions between giant planets or planets and a nearby companion star can cause tilted or retrograde orbits to occur, the latter being defined as orbits that are tilted by more than ninety degrees from the stellar spin axis.”

What is our sun’s stellar spin (directionality and axis)?
What determines the stellar spin in the first place?
Do different stars have dramatically different stellar spin axes?

thanks,
-Zen Blade

andy November 16, 2009 at 17:41

It’s not entirely clear why this is coming out now – HAT-P-7b’s retrograde nature was mentioned in New Scientist a while back. Let’s also not forget the previously-announced case of WASP-17b. These systems are clearly not vanishingly rare.

Ron S November 19, 2009 at 1:29

Zen, no one answered your questions, so let me try to do so, but briefly.

1) Cloud of gas and dust is pushed inward from one side by a flow of matter from, for example, a reasonably nearby supernova or protostar. This increasing the density of the cloud enough to get it self-gravitating, so it collapses.
2) That initial push is the source of the newly-forming system’s angular momentum. It will be non-zero since symmetry in the push and subsequent collapse cannot be exactly spherical. This angular momentum will be conserved in the evolving system. The direction of the angular momentum, while determined by that push, is uncorrelated with any other stellar system: you can treat it as being determined by a random process.
3) Interactions (friction, etc.) will transform the cloud into a roughly 2-dimensional disk. This plane will contain the planets that will form from the disk, with the star at the center.
4) Each body, including the star, will partake of the total system’s angular momentum. Therefore each will have its spin axis perpendicular to the disk. Each body will rotate about that axis with the same spin direction as the disk, as will the directions of the planets’ orbits.
5) Continuing interactions during formation and throughout the life of the stellar system will perturb these exact alignments, though unless something spectacular happens the perturbations will be fairly small. One exception may be planet spin axes, which seem to be more changeable. Also, low-mass bodies can end up in orbits with high inclinations and eccentricities since they’re more affected by encounters with planets and other large bodies. The Oort cloud remains roughly spherical since it is largely unaffected by the dynamics that formed the disk.
6) Finding a planet with an orbit far out of the plane where others orbit is a strong indicator that something spectacular happened. If true of more than one planet, perhaps another star passed the system at a close distance. This might also explain a case where the star’s spin axis is not perpendicular to the plane containing most of the planets’ orbits.

This list may suffer from some over-simplification and omissions, but I hope that it helps answer your questions.

ljk January 3, 2010 at 0:39

Emergence of Protoplanetary Disks and Successive Formation of Gaseous Planets by Gravitational Instability

Authors: Shu-ichiro Inutsuka, Masahiro N. Machida, Tomoaki Matsumoto

(Submitted on 30 Dec 2009)

Abstract: We use resistive magnetohydrodynamical simulations with the nested grid technique to study the formation of protoplanetary disks around protostars from molecular cloud cores that provide the realistic environments for planet formation.

We find that gaseous planetary-mass objects are formed much earlier than previously thought, by gravitational instability in regions that are de-coupled from the magnetic field and surrounded by the injection points of the magnetohydrodynamical outflows during the formation phase of protoplanetary disks. Magnetic de-coupling enables massive disks to form and these are subject to gravitational instability, even at ~10 AU.

The frequent formation of planetary mass objects in the disk suggests the possibility of constructing a hybrid planet formation scenario, where the rocky planets form later under the influence of the giant planets in the protoplanetary disk.

Comments: 10 pages, 3 figures

Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)

Cite as: arXiv:0912.5439v1 [astro-ph.EP]

Submission history

From: Shu-ichiro Inutsuka [view email]

[v1] Wed, 30 Dec 2009 10:18:42 GMT (1118kb)

http://arxiv.org/abs/0912.5439

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