Planet Discovery Through Disk Structure

by Paul Gilster on December 28, 2012

As the number of confirmed planets and planet candidates has grown, we’ve gone through a variety of techniques for exoplanet hunting, as Michael Lemonick’s new book Mirror Earth: The Search for Our Planet’s Twin (Walker & Co., 2012) makes clear. I’m only a third of the way into the book but I bring it up because it’s germane to today’s discussion in two ways. The first is purely administrative. Readers of Centauri Dreams are used to seeing information about the book I’m reading on the front page, but as many emails have reminded me, lately it’s been absent.

What’s happening is this: The software I use to display the book cover and progress bar is no longer being maintained by its creator, and the program has become flaky. I’ve discovered more and more that certain books will not display properly, so that although I can enter them in the configuration file, nothing shows up in the sidebar on the home page. As a result, I’m searching for alternatives that will display titles like Mirror Earth and anything else I’m currently reading.

But even though I’m early on in my progress through Lemonick’s book, I can already recommend it for the overview it provides. Lemonick writes for TIME and has written skillfully about astronomy’s history (William and Caroline Herschel were the topic of a recent book) and cosmology. He was an early entrant into exoplanet books with 1998’s Other Worlds. In short, Lemonick knows what he’s up to, and he offers the general reader a concise set of discussions with major exoplanet figures and a cogent history of how the field developed.

Astrometry was an early entrant in the exoplanet hunt as astronomers hoped to identify the side-to-side motion of a star being tugged by its planet, but early ‘discoveries’ made with the method proved bogus, and radial velocity emerged as the technique of choice, focusing on the movement of the star to and away from the Earth as measured by Doppler shift. We soon learned the effectiveness of transits (Kepler uses this method) and investigated gravitational lensing and, in rare cases, succeeded at direct imaging. What other methods will emerge?

As I proceed with the book, I’m also looking at recent news from the planet hunting HiCIAO ( High Contrast Instrument for the Subaru Next Generation Optics), used by an international team with the Subaru Telescope on Mauna Kea. HiCIAO has ways of blocking out the central star to help astronomers detect faint objects like planets or dust disks around it. The new work presents studies of the disk around the young star (about 9 million years old) HD 135344B, some 460 light years from Earth in the constellation Lupus. The disk under measurement is 20 billion kilometers in radius, which is about five times greater than Neptune’s distance from the Sun.

This is intriguing stuff because the team is looking at a spiral disk structure with two discernible arms. Applying density wave theory to the data, the researchers conclude that planets embedded within the disk may account for the spiral shape, and in doing so, they may be revealing another way of detecting exoplanets. The idea here is that regions of enhanced density develop inside a rotating disk because of differential rotation. A planet within the disk could produce the kind of density wave that results in the formation of just this kind of spiral structure. From the paper: “While we cannot uniquely identify the origin of these spirals, planets embedded in the disk may be capable of exciting the observed morphology. Assuming that this is the case, we can make predictions on the locations and, possibly, the masses of the unseen planets.”

Image: A comparison of the fit between the theoretical model and observational data. The red dashed line represents the shape of the disk based on modeling from density wave theory. The image shows that the data conform to the predictions of the theory and supports an explanation for the development of the structure in terms of this theory’s model. (Credit: NAOJ).

So far no planet candidate has emerged — we are simply considering the possibility that a planet is the cause of the density wave. But the use of density wave theory in measuring a protoplanetary disk helps us to understand how spiral disks form, and it may develop into a more precise way to detect exoplanets. The paper on this work suggests that HiCIAO may be capable of detecting the indirect signatures of planets down to 0.05 Jupiter mass, but it will take future observations at other wavelengths to see whether such predictions are borne out in fact.

The paper is Muto et al., “Discovery of Small-Scale Spiral Structures in the Disk of SAO 206462 (HD 135344B): Implications for the Physical State of the Disk from Spiral Density Wave Theory,” in Astrophysical Journal Letters 748 (April 2012), L22, 2012 (abstract).

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{ 8 comments }

Michael December 28, 2012 at 12:21

I wonder if these inward spirals are also responsiable for hot planets found close to their Stars, maybe the outer planets shape the spirals and the spirals drag the planets in gravitationally or through friction towards the Star.

coacervate December 28, 2012 at 22:49

Is there now or could there be a list of books read, say year by year?

Connie McManus December 29, 2012 at 11:34

As to the problem with displaying the book cover of what you are currently reading – are you familiar with Good Reads? I really love that site and I think you might be able to put it to good use here. I agree it’s not quite the same as having a nice cover display with a progress bar, but, hey, it’s better than nothing. Maybe. Food for thought. :-)

CVM

Paul Gilster December 29, 2012 at 14:11

Connie, I’m not sure how to adapt Good Reads here, but I’m looking around at various plugin options otherwise. Thanks for the idea.

coacervate: I have no list of books read year by year, but I can start compiling one. Let me see what I can find in terms of a new plugin to take over from the ‘I Am Reading’ app first.

Sturla December 29, 2012 at 20:34

You could try http://www.shelfari.com. They have somewhat customizable widgets to show your bookshelf on your page.

Paul Gilster December 30, 2012 at 16:53

Good idea, Sturla. I’ll check out this option.

andy January 2, 2013 at 19:14

Well Beta Pictoris b is an example of the planet predictions from disk morphology turning out to be correct.

And on a similar subject, it seems streams of material across a gap in a circumstellar disc have been detected, perhaps indicating the presence of giant planets in formation.

ljk January 3, 2013 at 12:40

2 January 2013

** Contact information appears below. **

Text & Image:

http://www.caltech.edu/content/planets-abound

PLANETS ABOUND:
CALTECH-LED ASTRONOMERS ESTIMATE THAT
AT LEAST 100 BILLION PLANETS POPULATE THE GALAXY

Look up at the night sky and you’ll see stars, sure. But you’re also seeing planets — billions and billions of them. At least.

That’s the conclusion of a new study by astronomers at the California Institute of Technology (Caltech) that provides yet more evidence that planetary systems are the cosmic norm. The team made their estimate while analyzing planets orbiting a star called Kepler-32 — planets that are representative, they say, of the vast majority in the galaxy and thus serve as a perfect case study for understanding how most planets form.

“There’s at least 100 billion planets in the galaxy — just our galaxy,” says John Johnson, assistant professor of planetary astronomy at Caltech and coauthor of the study, which was recently accepted for publication in the Astrophysical Journal. “That’s mind-boggling.”

“It’s a staggering number, if you think about it,” adds Jonathan Swift, a postdoc at Caltech and lead author of the paper. “Basically there’s one of these planets per star.”

The planetary system in question, which was detected by the Kepler space telescope, contains five planets. The existence of two of those planets have already been confirmed by other astronomers. The Caltech team confirmed the remaining three, then analyzed the five-planet system and compared it to other systems found by the Kepler mission.

The planets orbit a star that is an M dwarf — a type that accounts for about three-quarters of all stars in the Milky Way. The five planets, which are similar in size to Earth and orbit close to their star, are also typical of the class of planets that the telescope has discovered orbiting other M dwarfs, Swift says. Therefore, the majority of planets in the galaxy probably have characteristics comparable to those of the five planets.

While this particular system may not be unique, what does set it apart is its coincidental orientation: the orbits of the planets lie in a plane that’s positioned such that Kepler views the system edge-on. Due to this rare orientation, each planet blocks Kepler-32′s starlight as it passes between the star and the Kepler telescope.

By analyzing changes in the star’s brightness, the astronomers were able to determine the planets’ characteristics, such as their sizes and orbital periods. This orientation therefore provides an opportunity to study the system in great detail — and because the planets represent the vast majority of planets that are thought to populate the galaxy, the team says, the system also can help astronomers better understand planet formation in general.

“I usually try not to call things ‘Rosetta stones,’ but this is as close to a Rosetta stone as anything I’ve seen,” Johnson says. “It’s like unlocking a language that we’re trying to understand — the language of planet formation.”

One of the fundamental questions regarding the origin of planets is how many of them there are. Like the Caltech group, other teams of astronomers have estimated that there is roughly one planet per star, but this is the first time researchers have made such an estimate by studying M-dwarf systems, the most numerous population of planets known.

To do that calculation, the Caltech team determined the probability that an M-dwarf system would provide Kepler-32′s edge-on orientation. Combining that probability with the number of planetary systems Kepler is able to detect, the astronomers calculated that there is, on average, one planet for every one of the approximately 100 billion stars in the galaxy. But their analysis only considers planets that are in close orbits around M dwarfs — not the outer planets of an M-dwarf system, or those orbiting other kinds of stars. As a result, they say, their estimate is conservative. In fact, says Swift, a more accurate estimate that includes data from other analyzes could lead to an average of two planets per star.

M-dwarf systems like Kepler-32′s are quite different from our own solar system. For one, M dwarfs are cooler and much smaller than the Sun. Kepler-32, for example, has half the mass of the Sun and half its radius. The radii of its five planets range from 0.8 to 2.7 times that of Earth, and those planets orbit extremely close to their star. The whole system fits within just over a tenth of an astronomical unit (the average distance between Earth and the Sun) — a distance that is about a third of the radius of Mercury’s orbit around the Sun. The fact that M-dwarf systems vastly outnumber other kinds of systems carries a profound implication, according to Johnson, which is that our solar system is extremely rare. “It’s just a weirdo,” he says.

The fact that the planets in M-dwarf systems are so close to their stars doesn’t necessarily mean that they’re fiery, hellish worlds unsuitable for life, the astronomers say. Indeed, because M dwarfs are small and cool, their temperate zone — also known as the “habitable zone,” the region where liquid water might exist — is also further inward. Even though only the outermost of Kepler-32′s five planets lies in its temperate zone, many other M dwarf systems have more planets that sit right in their temperate zones.

As for how the Kepler-32 system formed, no one knows yet. But the team says its analysis places constraints on possible mechanisms. For example, the results suggest that the planets all formed farther away from the star than they are now, and migrated inward over time.

Like all planets, the ones around Kepler-32 formed from a proto-planetary disk — a disk of dust and gas that clumped up into planets around the star. The astronomers estimated that the mass of the disk within the region of the five planets was about as much as that of three Jupiters. But other studies of proto-planetary disks have shown that three Jupiter masses can’t be squeezed into such a tiny area so close to a star, suggesting to the Caltech team that the planets around Kepler-32 initially formed farther out.

Another line of evidence relates to the fact that M dwarfs shine brighter and hotter when they are young, when planets would be forming. Kepler-32 would have been too hot for dust — a key planet-building ingredient — to even exist in such close proximity to the star. Previously, other astronomers had determined that the third and fourth planets from the star are not very dense, meaning that they are likely made of volatile compounds such as carbon dioxide, methane, or other ices and gases, the Caltech team says. However, those volatile compounds could not have existed in the hotter zones close to the star.

Finally, the Caltech astronomers discovered that three of the planets have orbits that are related to one another in a very specific way. One planet’s orbital period lasts twice as long as another’s, and the third planet’s lasts three times as long as the latter’s. Planets don’t fall into this kind of arrangement immediately upon forming, Johnson says. Instead, the planets must have started their orbits farther away from the star before moving inward over time and settling into their current configuration.

“You look in detail at the architecture of this very special planetary system, and you’re forced into saying these planets formed farther out and moved in,” Johnson explains.

The implications of a galaxy chock full of planets are far-reaching, the researchers say. “It’s really fundamental from an origins standpoint,” says Swift, who notes that because M dwarfs shine mainly in infrared light, the stars are invisible to the naked eye. “Kepler has enabled us to look up at the sky and know that there are more planets out there than stars we can see.”

Contact:

Brian Bell
Caltech Media Relations
+1 (626) 395-5832
bpbell@caltech.edu

In addition to Swift and Johnson, the other authors on the Astrophysical Journal paper are Caltech graduate students Timothy Morton and Benjamin Montet; Caltech postdoc Philip Muirhead; former Caltech postdoc Justin Crepp of the University of Notre Dame; and Caltech alumnus Daniel Fabrycky (BS ’03) of the University of Chicago. The title of the paper is, “Characterizing the cool KOIS IV: Kepler-32 as a prototype for the formation of compact planetary systems throughout the galaxy.”

In addition to using Kepler, the astronomers made observations at the W. M. Keck Observatory and with the Robo-AO system at Palomar Observatory. Support for all of the telescopes was provided by the W. M. Keck Foundation, NASA, Caltech, the Inter-University Center for Astronomy and Astrophysics, the National Science Foundation, the Mt. Cuba Astronomical Foundation, and Samuel Oschin.

Preprint: http://arxiv.org/abs/1301.0023

[ Written by Marcus Woo ]

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