Kepler-56: Misaligned Planets Around a Swelling Star

by Paul Gilster on June 6, 2014

Although I’m sure I’ll refer to various papers presented at the American Astronomical Society this week in future entries, I’ll close our current look at the Boston meeting with word of two planets that will be falling into their star in short order (at least as astronomers measure time). Kepler-56b and Kepler-56c have a predicted era of death, some 130 million and 155 million years from now respectively. I can’t think of any other exoplanets about which we’ve been able to make such statements, making this a system worth watching as we ponder our own Sun’s future.

Just as the Sun will one day enlarge to red giant status, threatening the inner planets, so Kepler-56 is growing, already reaching four times the size of the Sun. The star has a long way to go as it continues its outward expansion, and the two planets in question are in a perilous position, with Kepler-56b orbiting the host star every 10.5 days, and Kepler-56c every 21.4. Gongjie Li (Harvard-Smithsonian Center for Astrophysics) and team have worked the number’s on the star’s evolution, leading to this prediction about the inner worlds’ demise.

You may recall this system as the first multi-planet system to be found with a tilt, as the orbits of the two inner planets are tipped from the star’s equator. Li told her session at the AAS conference that while the inner planets will be gradually destroyed by the star, heated enough to boil off any atmospheres and stretched by stellar tides along the way, a third known planet in this system, Kepler-56d, will be far enough out to survive. The latter is a gas giant in a 3.3 year orbit, and the researchers now believe that its orbit is tilted relative to the star as well. Such systems demand explanation since we would expect that planets formed from the same gas and dust disk as their host star would orbit in the plane of the star’s equator, as we see in our own system.


Image: Graphical sketch of the Kepler-56 system. The line of sight from Earth is illustrated by the dashed line, and dotted lines show the orbits of three detected companions in the system. The solid arrow marks the rotation axis of the host star, and the thin solid line marks the host star equator. Credit: NASA GSFC/Ames/D Huber.

Li pointed out in Boston that finding the inclination of the outer companion was important in helping us learn about the processes that produced the planetary misalignments. Last October I wrote about Daniel Huber’s work at NASA Ames involving the misalignment of the Kepler-56 inner planets with the host star. We do know of systems where ‘hot Jupiters’ are found that are not aligned with the equatorial plane of their hosts. Is this the result of planetary migration and chaotic close encounters with other planets in the system? If that is the case, then multi-planet systems without hot Jupiters should not show misalignment with the host star.

But we have no hot Jupiters around Kepler-56. Huber’s team used radial velocity measurements from the HIRES instrument at the Keck 10-meter telescope to study the system, seeing the signature of a third planet in a wide orbit (Kepler-56d) that was likewise misaligned from the equatorial plane of the star. It’s that outer planet that could in Huber’s view, have torqued the orbits of the inner planets out of the star’s equatorial plane. From the paper:

The inner planetary orbits would stay aligned with one another because of strong coupling between their orbits, resulting in a misalignment of the two co-planar transiting planets with the host star. Dynamical simulations that include a third companion in an eccentric orbit inclined to the equatorial plane of the host star confirm that such a mechanism can reproduce the architecture of the Kepler-56 system.

At 3000 light years from the Earth, Kepler-56 turns out to be a fascinating system. Huber found the rotation axis of the star tilted about 45 degrees to our line of sight at the same time that we were seeing the two transiting inner planets (Kepler-56d was detected solely by radial velocity methods). Both Li’s and Huber’s earlier work point to misalignment like this not being caused by hot Jupiters, and leave open for study how Kepler-56’s tilting mechanism could be at play around other stars. The important work ahead is to further constrain the inclination of the outer planet, Kepler-56d.

The Huber paper is “Stellar Spin-Orbit Misalignment in a Multiplanet System,” Science Vol. 342, No. 6156 (18 October 2013), pp. 331-334 (abstract).


Michael June 6, 2014 at 14:18

Hi Paul

Is it possible to have a catalogue (link) attached to an article on planets such as this one below when referring to planets, anyone can have a look at the system in motion as well. I think it is very much work in progress but it does look good. Maybe others who frequent this site and have more in-depth knowledge of astronomy could aid them as well.

Paul Gilster June 6, 2014 at 14:59

Looks like a good place to link to, though I’m not familiar with it enough to know how comprehensive it is. Let me do some scouting — I wonder, though, if the site (The Extrasolar Planets Encyclopaedia) wouldn’t be the primary place to link? Opinions welcome from other readers.

ljk June 17, 2014 at 14:40

NASA Dark-Energy Mission Could Spot 3,000 New Alien Planets

By Nola Taylor Redd, Contributor | June 17, 2014 10:00 am ET

A mission NASA is designing to probe the nature of mysterious dark energy could discover thousands of alien planets as well.

NASA’s proposed Wide-field Infrared Survey Telescope (WFIRST) mission aims to help researchers better understand dark energy, the puzzling stuff that makes up about three-quarters of the universe and drives its accelerating expansion.

But WFIRST — which is tentatively scheduled to launch in the early to mid-2020s — should also prove to be an adept planet hunter, complementing the activities of the space agency’s prolific Kepler space telescope, researchers say.

Full article here:

To quote:

“We predict WFIRST will have 3,000 individual planet detections, the same order of magnitude as Kepler,” Scott Gaudi, of Ohio State University, said in April during the Space Telescope Science Institute’s Habitable Worlds Across Time and Space Symposium in Baltimore.

In addition, WFIRST should be able to detect smaller distant planets, as well as free-floating “rogue planets” that have been ejected from their systems. Together, Kepler and WFIRST will cover virtually the entire plausible spectrum of planets in mass and orbits.

WFIRST will be able to capture information about Earth-size planets that lie farther from their suns than Earth does, as well as unbound planets the size of Mars. According to Gaudi, in favorable cases, the instrument should be able to detect a terrestrial moon orbiting a distant Earth, or a gas-giant satellite as large as Ganymede (Jupiter’s largest moon), though both observations would be challenging. Unbound moons, like unbound planets, would also be detectable.

Of the 3,000 new planets expected to be found by WFIRST, scientists think about 300 will be Earth-size worlds and 1,000 will be “super-Earths,” possibly rocky planets up to 10 times the mass of our own. Such predictions are based on present-day understanding of the distribution of types of planets, knowledge that may be either strengthened or challenged by the wealth of data that WFIRST will bring.

With WFIRST, Gaudi said, “we’ll measure the galactic distribution of the planets.”

ljk July 10, 2014 at 9:58

IAU is allowing the public to name exoplanets. There are likely more planets than stars in the Milky Way galaxy, so why not – we’ll run out of classical Greek and Roman name in short order:

Friction might help Earthlike planets in cosmic trouble:

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