It was always a good bet that we’d get plenty of surprises as data from Kepler began to come in, but the odd world known as Kepler-413b really does stand out. The transit method seems made to order for a certain regularity — Kepler looks at how the light from a given star dims when a planet passes in front of it as seen from Earth. Slight changes in these transits can help us detect other worlds in the system or, perhaps, help us make future discoveries of exomoons. But what happens when the transit is so erratic that both these scenarios can be ruled out?
Veselin Kostov and team have exactly that situation on their hands. According to Kostov (Space Telescope Science Institute and Johns Hopkins University), the data for Kepler-413b show, over a period of 1500 days, three transits in the first 180 days, followed by 800 days with no transits at all. Following that, the researchers noted five more transits in a row. And according to their analysis, the next transit is not going to happen until 2020.
Intriguing, no? The planet is a Neptune class world in a circumbinary 66-day orbit around a close eclipsing binary, K- and M-class dwarf stars that eclipse each other every 10 days. The system is 2300 light years out in the constellation Cygnus, and the image below depicts its complexities.
Image: This illustration shows the unusual orbit of planet Kepler-413b around a close pair of orange and red dwarf stars. The planet’s 66-day orbit is tilted 2.5 degrees with respect to the plane of the binary star’s orbit. The orbit of the planet wobbles around the central stars over 11 years, an effect called precession. This planet is also very unusual in that it can potentially precess wildly on its spin axis, much like a child’s top. The tilt of the spin axis of the planet can vary by as much as 30 degrees over 11 years, presumably leading to the rapid and erratic changes in seasons on the planet and any accompanying large moons. As Kepler views the system nearly edge on, sometimes the planet passes in front of the binary pair, and sometimes it does not. The next transit is not predicted to occur until 2020. This is due not only to the orbital wobble, but also to the small diameters of the stars and the fact that the orbital plane of the stars is not exactly edge-on to Kepler’s line of sight. (The vertical axis on the right panel is exaggerated by a factor of 10, for viewing purposes only.) Credit: NASA, ESA, and A. Feild (STScI).
The orbital plane of the stars is not edge-on, and as the caption notes, the planet’s orbit is titled 2.5 degrees with respect to the plane of the binary stars’ orbit. From Kepler’s vantage point, Kepler-413b’s orbit moves up and down continuously, accounting for why it often fails to transit its host stars. The wobble on the spin axis we see here is quite a contrast with the Earth, where the rotational precession amounts to 23.5 degrees over 26,000 years. The planet appears to be closer to its host stars than the inner edge of the habitable zone, with complex seasonal cycles and what the paper describes dryly as “interesting climate patterns.” Interesting indeed.
A key question now is whether there are other planets in this system that account for the tilt in Kepler-413b’s orbit, or whether a third star may be gravitationally bound to the system, thus exerting its own influence on the wandering world. That would give this unusual world the further distinction of being a circumbinary planet in a triple star system. We can also wonder whether there is a population of planets like this one whose transits we may be missing because we simply aren’t looking at their stars at the right time. Whatever the case, Kepler-413b gives us a useful circumbinary planet whose characteristics may prove helpful not only in terms of planet formation but questions of habitability outside of single-star planetary systems.
The paper is Kostov et al., “Kepler-413b: a slightly misaligned, Neptune-size transiting circumbinary planet,” accepted at the Astrophysical Journal (preprint).
Regarding the possibility of additional objects in the system, the discovery paper does note that there is an additional star contributing to the light in the pixel, and the authors argue that it is likely to be a gravitationally-bound member of the system.
Paul: I think you’re a bit confused about the precession of the earth’s rotational axis, which is 360 degrees over about 26,000 years, and the change in the angle between the earth’s rotational axis and the perpendicular to its orbit, which is about 23.5 degrees now. This “obliquity of the ecliptic” varies from about 22.1° to 24.5° and is roughly periodic with a period of around 41,000 years. Every single press release I’ve seen about this paper has made similar errors, so don’t feel too badly!
I haven’t read this paper yet, but it would appear that the orbit of this planet is precessing, hence the fact that transits don’t always occur every 66 days (there are eclipsing binaries where this happens, due to the gravity of a third body in the system ) AND that the rotational axis of the planet is wobbling.
I may well have got it wrong — thanks for this. My understanding, though, is that the orbit is precessing and the rotational axis is wobbling — as you say here — with the potential third body being what had been interpreted as a field star but is now being suggested as part of the system. But I’m going to need to read up on precession of the rotational axis and the change in angle between rotational axis and perpendicular to the orbit. Further clarifications welcome, because I’m still having trouble puzzling this out! Coolstar, what do I need to re-phrase in the article to get it right?
Hi Paul: well, your article is I think the best I’ve seen. I’d just phrase it as you did here, but perhaps make it more explicit about the difference between orbital precession, obliquity precession, and obliquity wobble. and my apologies for STILL not having found time to read the paper, so it’s possible I have things wrong myself (it’s been a busy semester, is my only excuse).
Jack Lissauer and colleagues just did an interesting paper about how the absence of our moon would increase the changes in the earth’s obliquity over time, but not to the extreme that many had thought. (and once again, I don’t have the reference handy….should be easy to find thru the NASA ADS though).
Good points. I need to do some boning up on the distinctions here in any case! Haven’t heard about the Lissauer paper but it’s one I’ll look for.
Rife with hype, exoplanet study needs patience and refinement
8 hours ago on February 18, 2014 by Morgan Kelly
(Phys.org) —Imagine someone spent months researching new cities to call home using low-resolution images of unidentified skylines. The pictures were taken from several miles away with a camera intended for portraits, and at sunset. From these fuzzy snapshots, that person claims to know the city’s air quality, the appearance of its buildings, and how often it rains.
Full article here:
More Kepler Discoveries
By Keith Cowing on February 24, 2014 8:59 AM. 1 Comments
NASA Media Telecon to Announce Latest Kepler Discoveries, 26 Feb.
Keith’s note: These are recent papers by several of the press conference participants. They seem to be interested in Earth-sized panets and habitable zones. They also proposed to do observations using Kepler in two-wheel or K2 mode. Stay tuned.
Two-Wheel Kepler Mission Invited to 2014 Senior Review
“The Kepler team’s proposal, dubbed K2, demonstrated a clever and feasible methodology for accurately controlling the Kepler spacecraft at the level of precision required for scientifically valuable data collection. The team must now further validate the concept and submit a Senior Review proposal that requests the funding necessary to continue the Kepler mission, with sufficient scientific justification to make it a viable option for the use of NASA’s limited resources.”
Searching for Terrestrial Planets Orbiting in the Habitable Zone of Ultra-Cool Stars and Brown Dwarfs
“We propose to use Kepler in 2-wheel mode to conduct a detailed search for Earth-sized planets orbiting ultra-cool stars and brown dwarfs (spectral types from M7 to L3). This population of objects presents several advantages for exoplanet surveys.”
A Habitable Zone Census via Transit Timing and the Imperative for Continuing to Observe the Kepler Field
“We propose a scientific program to complete a census of planets, characterizing their masses, orbital properties, and dynamical histories using continued observations of the Kepler field of view with the Kepler spacecraft in a two reaction wheel mission.”