New Multiple Planet Systems Verified

by Paul Gilster on January 27, 2012

Confirming Kepler’s planet candidates is a crucial part of the process, because no matter how tantalizing a candidate appears to be, its existence needs to be verified. We have more than 60 confirmed Kepler planets and over 2300 candidates, many of which will eventually get confirmed, but it’s interesting to see that the mission’s latest announcements relate to multiple planet systems and how their presence can itself speed up the verification process.

In today’s focus are the eleven new planetary systems just announced, 26 confirmed planets in all, which actually triples the number of stars known to have more than one transiting planet. One of the systems, Kepler-33, has been demonstrated to have five planets. We also have five systems (Kepler-25, Kepler-27, Kepler-30, Kepler-31 and Kepler-33) showing a 1:2 orbital resonance — the outer planet orbits the star once for every two orbits of the inner planet — and four systems with a 2:3 resonance, with the outer planet orbiting twice for every three times the inner planet completes its orbit.

Image (click to enlarge): The artist’s rendering depicts the multiple planet systems discovered by NASA’s Kepler mission. Out of hundreds of candidate planetary systems, scientists had previously verified six systems with multiple transiting planets (denoted here in red). Now, Kepler observations have verified planets (shown here in green) in 11 new planetary systems. Many of these systems contain additional planet candidates that are yet to be verified (shown here in dark purple). For reference, the eight planets of the solar system are shown in blue. Credit: NASA Ames/Jason Steffen, Fermilab Center for Particle Astrophysics

Usefully for verification purposes, these are systems in which the planets are relatively close to their host stars, with orbital periods between six and 143 days, the tight configuration creating a clear Transit Timing Variation (TTV) signature as the planets tug and pull on each other. TTV makes verification much simpler and eases the need for backup observations from ground-based telescopes. We’ve looked at Transit Timing before in its application for possible detection of exomoons, but it’s also useful for analyzing planetary systems around fainter, more distant stars.

Eric Ford (University of Florida) and colleagues discuss the utility of TTVs in the paper on their work on Kepler-23 and Kepler-24:

For systems with MTPCs [multiple transiting planet candidates], correlated TTVs provide strong evidence that both transiting objects are in the same system. Dynamical stability provides an upper limit on the masses of the transiting bodies. For closely-spaced pairs, the mass upper limit is often in the planetary regime, allowing planets to be confirmed by the combination of correlated TTVs and the constraint of dynamical stability.

And Jack Lissauer (NASA Ames) and team discuss the validity of Kepler’s multiple planet candidates in a separate paper. The italics are mine:

Roughly one-third of Kepler’s planet candidates announced by Borucki et al. (2011) are associated with targets that have more than one candidate planet. False positives (FPs) plague ground-based transit searches, but the exquisite quality of Kepler photometry, combined with the ability to measure small deviations in center of light during transits (Jenkins et al. 2010; Batalha et al. 2010), have been used to cleanse the sample prior to presentation in Borucki et al. (2011). Accounting for candidates on each one’s individual merit, Morton & Johnson (2011) estimated the fidelity of Kepler’s planet candidates (fraction of the candidates expected to be actual planets) to be above 90%. Yet the fidelity of multiple planet candidates is likely to be higher than that for singles (Latham et al. 2011; Lissauer et al. 2011a). We show herein that the vast majority of Kepler’s multiple planet candidates are true multiple planet systems.

Find multiple planets in the same system, then, and the odds on their being verified are excellent, what Lissauer calls ‘validation by multiplicity,’ based on our knowledge of the properties of the host star and examination of planetary transits that show similar signatures around the same star. Thus the gravitational dance of multiple planets leads to faster verifications as the orbital period of each planet changes through the slightest of variations in its transit timing. Now we have yet another crop of exoplanets, fifteen of them between Earth and Neptune in size. But whether these smaller planets are rocky worlds or gaseous ‘Neptunes’ will have to be the subject of further study.

The papers are Lissauer et al., “Almost All of Kepler’s Multiple Planet Candidates are Planets” (preprint); Ford et al., “Transit Timing Observations from Kepler: II. Confirmation of Two Multiplanet Systems via a Non-parametric Correlation Analysis,” accepted at the Astrophysical Journal (preprint); Steffen et al., “Transit Timing Observations from Kepler: III. Confirmation of 4 Multiple Planet Systems by a Fourier-Domain Study of Anti-correlated Transit Timing Variations,” accepted by MNRAS (preprint); and Febrycky et al., “Transit Timing Observations from Kepler: IV. Confirmation of 4 Multiple Planet Systems by Simple Physical Models,” in press at the Astrophysical Journal (preprint).

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{ 18 comments }

Scott G. January 27, 2012 at 12:17

Since Kepler went operational on May 12, 2009 and requires a minimum of 3 transits to flag a candidate planet, we should be aware that any planet with an orbital period of 1 year will not become “visible” until sometime between May of 2012 at the earliest and May of 2013 at the latest. So, we still have a strong bias for detecting only the planets that lie very close to their parent stars. Also, unless the mission is extended past mid-2015, then all planets within Kepler’s field-of-view with a period of 2 or more years will never be detected.

Mike January 27, 2012 at 14:48

The Kepler space telescope is really proving it’s worth and more. Plus the ingenuity of the astronomers who know how to get the best possible use out of the observations. What a success this project has been so far. What future discoveries it may reveal.

Tom W January 27, 2012 at 17:14

Brilliant stuff. Think the next great milestone for Kepler is finding an exomoon, it would be amazing if it could be done. It might also up the ante for Terrestrial Planet Finder to get the funding go-ahead which would do wonders for detecting exoplanets in closer star systems(i.e. Alpha Centauri/Bernards Star).

Tony P January 27, 2012 at 17:32

It’ll be interesting to see the fully fleshed out star systems that we’ll have a better idea of in the coming years. I wouldn’t be surprised to see systems with 20+ planets, although for the more out there planets, they will not really be detectable without centuries of continuous observation.

Dave January 27, 2012 at 23:51

Tony P has a good point about the Kepler observation time. Once the Kepler mission is over there will be many planets around the stars being observed that we never detected because their orbital period was too long. If Kepler is observing a system with 8 planets exactly like our own it would only get enough transits to confirm the 3 closest planets.

Enzo January 28, 2012 at 3:00

@ Scott G.,

Good point : I hope that all this abundance of “compact” systems is an observational artifact due to the short observation time.
Otherwise terrestrial planets might very well be common but mostly inhabitable.

bigdan201 January 30, 2012 at 9:00

Good point : I hope that all this abundance of “compact” systems is an observational artifact due to the short observation time.
Otherwise terrestrial planets might very well be common but mostly inhabitable.

I’m sure it’s an artifact. Finding exoplanets is a definite challenge, and those that are easiest to see will be found first. Thus, it’s no surprise that we’ve found an abundance of Hot Jupiters. That doesn’t mean that they’re the majority of planets, or even of gas giants – rather, there is observation bias at work. Logically, finding terrestrial planets at 1+ year periods is going to be more difficult.

I recall that we’ve found small and distant pulsar planets, which makes sense because the rapid signal makes detection so much easier.

In general, from what I’ve seen, the universe has a great deal of variety. I’m sure that’ll be the case with exoplanets.

Bob January 30, 2012 at 13:37

“If Kepler is observing a system with 8 planets exactly like our own it would only get enough transits to confirm the 3 closest planets.”

I though it would observe all the transits but not consider one a verified planet without at least three transits. You would have left a lot of unconfirmed candidates.

kittlej January 30, 2012 at 13:43

I have been thinking about a project I would call ISSIS for discovering planets of more nearby stars using the transient method. This would involve placing a satellite in geosynchronis orbit that carries 80 50 megapixel cameras each with a 35X35 degree Field of view. The rig would be able to map the ENTIRE sky with a greater than 2X redundancy WITH EACH EXPOSURE. if the array of Cameras were using about one minute exposures and simple optic lenses this results in about 4, gigapixels of data per exposure with a resolution of about 500K pixels per square degree, plenty of resolution for either tracking objects in the solar system or watching for planet transient of nearby stars. cost is actually not a lot since the cmeras ras are similar to off the shelf commercial products and data from geosynchronous orbit are reasonable. the optics are no sweat.
jdk

philw1776 January 30, 2012 at 16:20

@Scott

It’s even worse. Kepler has discovered that main sequence dwarfs are “noisier” than expected such that the signal to noise ratio is lower than expected for smaller terrestrial planets and that ~5 transits will be required in many instances to be confident that there is a planet in the haystack. Thus the push for an extended mission.

As someone following the space age since boyhood with Sputnik, to me Kepler has been THE most exciting and rewarding scientific mission. Kudos to all involved.

andy January 30, 2012 at 17:28

That we have so many Kepler multiples was actually fairly unexpected because it requires very flat planetary systems: these systems have far less variation in inclination than our own solar system. There is a bias in the planetary system architectures that Kepler can find: systems which ended up more like Upsilon Andromedae would not show multiple transiting planets unless we happened to lie along the line of nodes, which is extremely unlikely. So as with all results in science, we have to be very careful about the systematics: the Kepler multiples may or may not be representative of typical planetary systems.

Holger January 31, 2012 at 10:21

“these systems have far less variation in inclination than our own solar system.”

That’s not true, resp. it’s only observation bias:
The Solar system has 4 planets with orbits within 1 degree inclination (namely the four gas giants). Only one of the 17 Kepler multiple systems has more planets within one degree of inclination, namely Kepler-11c to g (5 planets). All the Kepler systems may still have some more inclined undiscovered planets just like the Solar System’s inner planets. So the Solar System is still among the 10% most orderly systems even when only compared to Kepler’s multiple systems!

andy January 31, 2012 at 15:22

The Solar system has 4 planets with orbits within 1 degree inclination (namely the four gas giants).

Quickly playing around with the orbital elements…

Saturn-Jupiter mutual inclination: 1.249 degrees
Saturn-Uranus mutual inclination: 1.953 degrees
Uranus-Neptune mutual inclination: 1.503 degrees

Hardly all within 1 degree!

Rob Henry January 31, 2012 at 17:36

Andy, Holger wrote “4 planets with orbits within 1 degree inclination” NOT “4 planets with orbits within 1 degree inclination of each other”. You need a reference plane.

ljk February 1, 2012 at 14:16

Casting Swords into Space Observatories

by Bruce Dorminey on February 1, 2012

Editor’s note – Bruce Dorminey is a science journalist and author of Distant Wanderers: The Search for Planets beyond the Solar System.

Planet hunter extraordinaire Geoff Marcy recently let his frustration surface about the current state of the search for other habitable solar systems. Despite the phenomenal planet-finding success of NASA’s Kepler mission, Marcy, an astronomer at the University of California at Berkeley, correctly pointed out that NASA budget cuts have severely hampered the hunt for extrasolar life.

A decade ago, only a few dozen extrasolar planets had been detected. Today, by some recent gravitational microlensing estimates, there are more planets than stars in the Milky Way. But without the ability to characterize these extrasolar planetary atmospheres from space, we are astrobiologically hamstrung.

NASA’s goal had been that by 2020, we would have a pretty good idea about how frequently terrestrial Earth-mass planets orbit other stars — whether those planets have atmospheres that resemble our own; and, more crucially, whether those atmospheres exhibit the telltale signs of planets harboring life.

But consider how the federal government spends our tax dollars on a daily basis. Each and every day for more than a decade, the U.S. military spent roughly $1 billion a day funding congressionally-undeclared wars in Iraq and Afghanistan.

In contrast, NASA’s cancelled SIM and TPF missions were both originally estimated to have cost less than $1.5 billion dollars each.

Full article here:

http://www.universetoday.com/93211/casting-swords-into-space-observatories/

Another answer to Fermi’s Paradox: Cultures that are too backwards and self-centered to focus on the wider Universe.

andy February 1, 2012 at 15:00

Yeap, ambiguity in the statement. I stand corrected.

Nevertheless I stand by the point that Kepler multiples do constitute a biased sample of planetary systems, even if the ones it is detecting are less flat than the four gas giants of our own system (which wouldn’t be a multi-transit system anyway due to scale).

Holger February 2, 2012 at 10:38

Andy, I completely support that point. I was just pointing out that even with this bias, we have hardly detected any “flatter” systems (i.e. with more planets sharing a orbital plane) than our own.

(Sorry for the ambiguity in my previous statement. But the argument would work almost identically with 1 degree replaced e.g. by 2 degrees.)

@Dave, Bob above:
It would depend on the plane from which the Solar System is observed. The observers would either find up to four unconfirmed “single-transit” planets (the gas giants) or one or two confirmed planets (Venus and/or Earth). Mercury is too small to be discovered even if transiting, I think.

Eniac February 2, 2012 at 13:04

Note that Jupiter would need about 35 years of observation to get three transits, so realistically is not detectable by a Kepler like program. All the other gas giants more so.

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