Beyond the Kepler Planets

by Paul Gilster on May 25, 2011

Kepler is a telescope that does nothing more than stare at a single patch of sky, described by its principal investigator, with a touch of whimsy, as the most boring space mission in history. William Borucki is referring to the fact that about the only thing that changes on Kepler is the occasional alignment of its solar panels. But of course Borucki’s jest belies the fact that the mission in question is finding planets by the bushel, with more than 1200 candidates already reported, and who knows how many other interesting objects ripe for discovery. Not all of these are planets, to be sure, and as we’ll see in a moment, many are intriguing in their own right.

But the planets have center stage, and the talk at the American Astronomical Society’s 218th meeting has been of multiple planet systems found by Kepler, after a presentation by David Latham (Harvard-Smithsonian Center for Astrophysics). Of Kepler’s 1200 candidates, fully 408 are found in multiple planet systems. Latham told the conference that finding so many multiple systems was a surprise to a team that had expected to find no more than two or three.

To discover this many multiple systems requires planetary orbits to be relatively flat in relation to each other. In our Solar System, for example, some planetary orbits are tilted up to seven degrees, meaning that no one observing the system from outside would be able to detect all eight planets by the transit method. What Kepler has uncovered are numerous multiple systems whose planetary orbits are much flatter than our own, tilted less than a single degree.

Image (click to enlarge and animate): Multiple-planet systems discovered by Kepler as of 2/2/2011; orbits go through the entire mission (3.5 years). Hot colors to cool colors (red to yellow to green to cyan to blue to gray) are big planets to smaller planets, relative to the other planets in the system. Credit: Daniel Fabrycky.

Interestingly, multiple planet systems may give us the help we need to detect small, rocky worlds. While the radial velocity method helps us find larger objects orbiting a star, terrestrial-class worlds are small enough that their radial velocity signal is hard to detect. With a multiple planet systems, astronomers will be able to use transit timing variations, measuring how gravitational interactions between the planets cause tiny changes to the time between transits. Latham’s colleague Matthew Holman notes the power of such a signal:

“These planets are pulling and pushing on each other, and we can measure that. Dozens of the systems Kepler found show signs of transit timing variations.”

Using the transit method, Kepler should be able to identify small planets in wider orbits around their stars, including those that may be in the habitable zone, but transit timing variations may flag the presence of such planets and play a role in the intense follow-up that will produce a confirmation. We have exciting times ahead of us as Kepler continues its mission. Meanwhile, what accounts for the flatness of the planetary orbits in these multiple planet systems? Latham gives a nod to the fact that most of them are dominated by planets smaller than Neptune. Jupiter-class worlds cause system disruptions that can result in tilted orbits for smaller planets.

“Jupiters are the 800-pound gorillas stirring things up during the early history of these systems,” said Latham. “Other studies have found plenty of systems with big planets, but they’re not flat.”

A Catalog of Eccentric Objects

Kepler’s treasure trove includes far more than planets, as an interesting article in Science News points out (thanks to Antonio Tavani for the pointer to this one). After all, the observatory is looking at tens of thousands of stars to produce its planetary finds, and in most cases, planets aren’t lined up in such a way that they can be seen from Earth, if they exist. But Science News quotes Geoff Marcy (UC-Berkeley) on the variety of stars being seen: “There are so many stars that show bizarre, utterly unexplainable brightness variations that I don’t know where to begin.”

Consider the English amateur Kevin Apps, who became curious about a red dwarf in the Kepler field that was not among the 156,000 chosen for full investigation. Apps discovered that he could get light curves for the system from data produced by Kepler’s initial commissioning phase. The light curve showed dips spaced 12.71 days apart, an intriguing find that led him to contact professional astronomers who went to work on the system themselves. The result: The red dwarf turned out to be not a single star but a widely spaced binary of two M-dwarfs, with a massive object in orbit around the larger of the stars, evidently a brown dwarf.

Kepler keeps turning up oddities. A star called KIC 10195926, for example, twice the mass of the Sun, shows ‘torsional modes’ in its rotation — the star’s northern and southern halves spin at different rates, trading off which spins fastest. This is the first time such torsional modes have been seen. The star has now been classed as an Ap star — A-peculiar — with a strong magnetic field. It’s the subject of a paper in Monthly Notices of the Royal Astronomical Society.

The star HD 187091, about a thousand light years from the Earth, is twice as massive as the Sun. Kepler’s light curve showed a 42-day cycle, with the star’s brightness rising to a peak and then quickly subsiding, with numerous secondary brightness variations between the peaks. It turns out this is not a single A-class star, as previously believed, but two stars of nearly the same size in a highly elliptical orbit. Let me quote from Science News on this, drawing on the magazine’s interview with William Welsh (San Diego State University):

The brightening occurs as the stars, tidally warped by their gravity at closest approach into slight egg shapes, roast one another on their facing sides and heat up. And that explains the spike in brightness, the team reported online in February at arXiv.org. The more surprising revelation of Kepler’s data is that one, and perhaps both, pulsate furiously at rates that are precise multiples of their rate of close encounters, in some cases pulsing exactly 90 and 91 times for each orbit. “Nobody had ever seen, or even thought, something like this could happen,” Welsh says. Discovering that a star’s rapid pulsations are not always driven by internal processes, but can be paced by a tidal metronome from a partner star, offers a new window into stellar dynamics and structure.

How many more such surprises will Kepler give us? An extended Kepler mission (and we might be able to get an additional four or five years beyond the 2013 original mission end date) should yield interesting objects galore. The follow-up to Kepler might be the European Space Agency’s Plato (Planetary Transits and Oscillations of Stars), which would, like Kepler, examine star fields for lengthy periods of time, but would also be able to swivel and look at different stellar fields. Perhaps the success of Kepler will give Plato the boost it needs for a liftoff in this decade.

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{ 21 comments }

Parker Shaw May 25, 2011 at 12:49

Wow, HD 187091 reminds me Rocheworld:

http://en.wikipedia.org/wiki/Rocheworld

Ashley May 25, 2011 at 15:21

The present excitement around exoplanetology reminds me the one cosmology knew a few years ago. Obviously, Kepler will keep feeding the interest for alien worlds for some time at least: let’s hope it lasts as long as it can!

Rob Henry May 25, 2011 at 19:36

I’m confused. Previously the presented evidence that I heard seemed indicate that most systems of planets were much worse behaved than ours, being more eccentric, higher inclinations, less stable etc. Here I hear that most could much more orderly than ours. Which view is the more correct??

andy May 26, 2011 at 2:29

Rob Henry: seems to be that most systems containing gas giants are less well-behaved than our own system. These are obviously the systems that are easiest to find due to large RV amplitudes. On the other hand systems without gas giants (which are probably in the majority) have only recently started to show up, and turns out the rules may be somewhat different in these cases.

Mongeeses May 26, 2011 at 3:59

Now the question I have is that while we have up to 7 degrees of tilt in our planets orbits how many of them stack to within the 1 degree required to see us from a similar mission based on one of these multiple planet systems, is it at least two? that would make it look to them like they do to us maybe.

Holger May 26, 2011 at 7:55

@Rob Henry:
Actually I think both views are rather incorrect. The “old evidence” was biased by the fact that e.g. Hot Jupiters are easiest to detect with radial velocity methods, and could not provide statistical evidence for the “average” planetary system. Kepler now provides statistics, showing that a substantial part (>10%) of all planetary systems has several planets orbiting in almost exactly the same plane (tilted 10 years) and from the right direction.

Danonino May 26, 2011 at 9:10

What I dont understand is why whe always look at stars that lay hundreds, thousands or millions of light years away.. Whats the point? Why not look at the ones that are much closer, like Alpha Centauri for instance..?

Paul Gilster May 26, 2011 at 9:26

Danonino, the point about Kepler is to get a statistical sampling of a large number of stars so we can get an idea of how common planets are and learn more about where they are found. There are proposals for targeted missions to study nearby stars as well but they haven’t yet flown. Intense study of Alpha Centauri is ongoing, though, through three different ground-based projects.

Holger May 26, 2011 at 10:03

Apparently my last post was cut off in the middle, so I’ll continue it here:
(also answering Mongeeses’ question):
Kepler now provides statistics, showing that a substantial part (>10%) of all planetary systems has several planets orbiting in almost exactly the same plane (tilted <1 degree). But, contrary to what this article implies, the Solar System does have several planets lying within 1 degree of the same plane, namely the four gas giants. So the Solar System isn't at all atypical compared with the Kepler systems (which could have some additional planets which are more tilted as well).

tesh May 26, 2011 at 10:11

Any news /murmurs/rumours on that search?

Paul Gilster May 26, 2011 at 14:17

tesh, no news yet but we should have something one way or the other within the next two years, from everything I’ve heard. As to rumors, I have to stay out of that game…

tesh May 26, 2011 at 14:40

I seem to remember an interview with Dr. Debra Fischer about the ongoing search around the A Cen system a few years back and she said then that if there were Jupiter sized planets, then they would find them in the first few years of observations, i.e. by now. Maybe no news is good news…

henk May 26, 2011 at 15:11

alpha centauri is the best chance to find life. That is what some Astronomer say.

Paul Gilster May 26, 2011 at 15:53

Not necessarily, henk. We have no idea yet whether there are planets around Centauri A or B. And there’s no reason why Alpha Centauri would present a better chance for finding life on a planet than numerous other systems. We simply have to wait to find out whether there are rocky planets around these stars, and then learn whether any are favorable for life.

andy May 26, 2011 at 16:24

Regarding the Alpha Centauri campaigns, the Extrasolar Planets Encyclopaedia lists a meeting tomorrow titled “The Next 40 Years of Exoplanets”. The agenda (pdf) includes a presentation by Prof. Debra Fischer titled “Alpha Centauri Update”. The meeting is being webcast here.

henk May 26, 2011 at 17:12

Paul Gilster

a few astromers make a model of alpha centauri a few years back. And they were very hopeful to find a planet with life. Is it because those stars are so close. Or is it realy a great system to find life?

Paul Gilster May 26, 2011 at 17:56

henk, I don’t know which astronomers you’re thinking of, but if you run a search here on Alpha Centauri, you’ll find results from papers in the past five years that have looked at this system closely. Alpha Centauri is particularly interesting because it’s so close, but I don’t think we can say anything about life at this point because we don’t know whether rocky planets are orbiting these stars. We still have much to learn about how close binary systems like this work.

henk May 26, 2011 at 18:24

Paul Gilster

yes i agree with you about that. We humans have much to learn about our place in the universe

ljk May 27, 2011 at 11:56

James Cameron says there are several gas giants in the Alpha Centauri system and at least one of them has a moon with all kinds of life forms and some mineral resources. The man’s films make billions of dollars so I would listen to him.

Planets or not, we will probably send our first probes there just because it is the closest.

Rob Henry May 28, 2011 at 19:40

Ljk, Jonathan Swift’s book did it why not James Cameron’s movie. You skeptic you!

SakeBito-san April 9, 2012 at 22:36

I’ve wondered the same thing as Danonino. A couple of months ago I attended a talk by one of the Kepler people at JPL who – atop a rather disgusting, now unfortunately typical, undercurrent of casual misanthropy – explained that every second any piece of space hardware observes the sky is costly, hence the decision to point it at a chunk of sky chosen specifically for its high concentration of likely planet-bearing star systems. There’s also the fact that in order to perceive exoplanets, Kepler has to remain locked onto the exact same field of view for an extended period of time.

I understand all of that, but one hopes that at some point the quest for purely Platonic knowledge will give way to a refocus on exploration within the conceivable reach of humanity. I don’t remember what Kepler’s mission life is projected to be, but hopefully once the current survey is concluded it will be turned to nearby systems.

If we could achieve half-light-speed propulsion, we could do a round trip to the Centauri system in a little over 17 years and Barnard’s Star in under 25 – plus whatever time is spent at the destination – all well within the human time-frame. Even quarter-light-speed would be feasible, particularly for an unmanned probe. Granted, per “Centauri Dreams” either such propulsion velocity is a long, long way off, but reason nonetheless would suggest that acquiring knowledge of the nearest systems should be the priority.

As for Cameron, I’m likely an oddball, but I’m not understanding how the views of one of the most violently misanthropic moviemakers in existence can have anything useful to say about the expansion of humanity into space, or anywhere else. One cannot simultaneously be pro-human and anti-human.

/soapbox

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