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Kepler’s Astounding Haul of Multiple Planet Systems

Presentation Slides (PPTX, 1.2 MB) from 2011 May 23 Press Conference at American Astronomical Society in Boston.

Text below from Harvard Smithsonian Center for Astrophysics Press Release:

Numbers of single planet and multiple planet system candidates

Cambridge, MA — NASA’s Kepler spacecraft is proving itself to be a prolific planet hunter. Within just the first four months of data, astronomers have found evidence for more than 1,200 planetary candidates. Of those, 408 reside in systems containing two or more planets, and most of those look very different than our solar system.

In particular, the Kepler systems with multiple planets are much flatter than our solar system. They have to be for Kepler to spot them. Kepler watches for a planet to cross in front of its star, blocking a tiny fraction of the star’s light. By measuring how much the star dims during such a transit, astronomers can calculate the planet’s size, and by observing the time between successive events they can derive the orbital period – how long it takes the planet to revolve around its star.

To see a transit, the planet’s orbit must be edge-on to our line of sight. To see multiple transiting planets, they all must be edge-on (or nearly so).

David Latham, Kepler Co-Investigator
David Latham, Kepler Co-Investigator

“We didn’t anticipate that we would find so many multiple-transit systems. We thought we might see two or three. Instead, we found more than 100,” said Smithsonian astronomer David Latham (Harvard-Smithsonian Center for Astrophysics).

Latham presented the findings today in a press conference at the 218th meeting of the American Astronomical Society.

In our solar system, some planet orbits are tilted by up to 7 degrees, meaning that an alien astronomer looking for transits wouldn’t be able to detect all eight planets. (In particular, they would miss Mercury and Venus.) The systems spotted by Kepler are much flatter, with orbits tilted less than 1 degree.

Why are they so flat? One clue comes from the planets themselves. The multiplanet systems found by Kepler are dominated by planets smaller than Neptune. They lack Jupiter-sized gas giants. Scientists believe that a gas giant’s powerful gravity tends to disrupt planetary systems, tilting the orbits of neighboring worlds.

Kepler multiple planet (candidate) systems (blue dots).
Kepler multiple planet (candidate) systems (blue dots).

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

Multiple-planet systems may offer a chance for confirming the densities of small, rocky worlds. The more massive a planet, the easier it is to detect using radial velocity measurements (essentially the star’s wobble as a planet’s gravity tugs it). Earth-sized worlds in Earth-sized orbits aren’t massive enough to make a radial velocity signal that’s detectable with present technology.

In systems with more than one transiting planet, astronomers have another option: transit timing variations. They can measure how the time between successive transits changes from orbit to orbit due to mutual gravitational interactions between the planets. The size of the effect depends on the planets’ masses.

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

As Kepler continues to gather data, it will be able to spot planets with wider orbits, including some in the habitable zones of their stars. Transit timing variations may play a key role in confirming the first rocky planets with the right temperature for water to be liquid on their surfaces.

NASA's Kepler spacecraft has discovered 170 planetary systems
containing between two and six transiting planets.
The animation below shows all the 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.

Click on the above image to play 30 Mb animation in new tab/window,
or play 122 Mb animation in new tab/window
or YouTube version with music.

Credit: Daniel Fabrycky

Ames Research Center is responsible for the ground system development, mission operations and science data analysis. NASA’s Jet Propulsion Laboratory in Pasadena, Calif., managed the Kepler mission development. Ball Aerospace and Technologies Corp. in Boulder, Colo., developed the Kepler flight system, and supports mission operations with the Laboratory for Atmospheric and Space Physics at the University of Colorado, Boulder. The Space Telescope Science Institute in Baltimore archives, hosts and distributes the Kepler science data.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, contact:

David Aguilar

Christine Pulliam

See Astrophysical Journal Article:
A First Comparison of Kepler Planet Candidates in Single and Multiple Systems by David W. Latham (Harvard-Smithsonian Center for Astrophysics), Jason F. Rowe (NASA Ames Research Center), Samuel N. Quinn (Harvard-Smithsonian Center for Astrophysics) Natalie M. Batalha (San Jose State University), William J. Borucki (NASA Ames Research Center), Timothy M. Brown (Las Cumbres Observatory Global Telescope), Stephen T. Bryson (NASA Ames Research Center), Lars A. Buchhave (Neils Bohr Institute, Copenhagen University), Douglas A. Caldwell (SETI Institute, NASA Ames Research Center), Joshua A. Carter (Harvard-Smithsonian Center for Astrophysics), Jesse L. Christiansen (SETI Institute, NASA Ames Research Center), David R. Ciardi (NASA Exoplanet Science Institute/California Institute of Technology), William D. Cochran (University of Texas, Austin), Edward W. Dunham (Lowell Observatory), et al. (22 additional authors not shown)
(Submitted on 20 Mar 2011)
ABSTRACT: In this letter we present an overview of the rich population of systems with multiple candidate transiting planets found in the first four months of Kepler data. The census of multiples includes 115 targets that show 2 candidate planets, 45 with 3, 8 with 4, and 1 each with 5 and 6, for a total of 170 systems with 408 candidates. When compared to the 827 systems with only one candidate, the multiples account for 17 percent of the total number of systems, and a third of all the planet candidates. We compare the characteristics of candidates found in multiples with those found in singles. False positives due to eclipsing binaries are much less common for the multiples, as expected. Singles and multiples are both dominated by planets smaller than Neptune; 69 +2/-3 percent for singles and 86 +2/-5 percent for multiples. This result, that systems with multiple transiting planets are less likely to include a transiting giant planet, suggests that close-in giant planets tend to disrupt the orbital inclinations of small planets in flat systems, or maybe even to prevent the formation of such systems in the first place.
Cite as: arXiv:1103.3896v1 [astro-ph.EP]

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