Centauri Dreams

The confirmation of a planet circling two stars, recounted in these pages yesterday, is actually the result of a long process. Jean Schneider (CNRS/LUTH – Paris Observatory) noted in a follow-up comment to the Kepler-16b story that investigation of such systems dates back to 1990 (see citation below), while Alex Tolley has pointed out that the great space artist Chesley Bonestell was painting imaginary planets orbiting binary stars fully sixty years ago. So the idea isn’t new, but the confirmation was obviously useful, and in more ways than we might have expected.

For one thing emerging from the Kepler-16b paper is that the smaller of the two stars in this binary system, an M-dwarf, is now the smallest low-mass star to have both its mass and radius measured at such precision. The question of stellar mass and M-dwarfs is significant because a new paper by Philip Muirhead (Cornell University) and colleagues goes to work on the parameters of low-temperature Kepler planetary host stars and finds stellar radii that are roughly half the values reported in the Kepler Input Catalogue. The authors believe these values correlate better with the estimated effective temperatures (T_eff) of these stars and suggest a striking possibility:

The effective temperatures, radii and masses of the KOIs imply different planet-candidate equilibrium temperature estimates, such that 6 planet-candidates are terrestrial-sized and have equilibrium temperatures which may permit liquid water to reside on the planet surface, assuming Earth-like albedos and re-radiation fractions. Scaling the Earth’s equilibrium temperature of 255 K by the orbital semi-major axis, stellar T_eff and stellar radius of the KOIs in this letter, we find that KOIs 463.01, 1422.02, 947.01, 812.03, 448.02 and 1361.01 all have equilibrium temperatures between 217 K and 261 K: the limits of the habitable zone as described in Kasting et al. (1993).

This one has struck a nerve and it’s easy to see why, as we are suddenly looking at six Earth-like planets in the habitable zone of their stars. I’ve received quite a few links to the paper (and thanks to all who sent them, as this is often how I find interesting work!), but we first have to note a few qualifiers. The authors point out, for example, that this work assumes “the same albedo, re-radiation fraction and greenhouse effect” as are found in our own system, an assumption that may well be challenged for a terrestrial planet orbiting a red dwarf star.

I’m also cautious because the physical parameters of exoplanet-hosting stars are so crucial to our understanding of the detected exoplanets themselves. Here we run into issues, and the authors are quick to point this out. We have detailed information about the Sun, for example, that helps us calibrate models for Sun-like stars, so our analyses of mass, effective temperature, radius and other values seems logical and well-founded. But M-dwarfs are a different story because few such stars are both bright enough and close enough for us to obtain accurate parallaxes and direct measurement of their radius. The authors also note a discrepancy between radii as measured in eclipsing binaries and the predictions of at least some stellar evolution models.

The authors go on to say this:

Although there remains a monotonic correspondence between spectral type (the observational parameter) and effective temperature, T_eff, the calibration of this relationship is not as advanced as it is for solar-type stars. M dwarf atmospheres are fully convective, rich in molecular absorption features and depart substantially from blackbody emission at all wavelengths…, so the empirical effective temperature scale is particularly challenging.

Muirhead and team went at their work using the TripleSpec Spectrograph at Palomar, observing 84 Kepler ‘objects of interest’ (KOIs) with effective temperatures (as described by the Kepler Input Catalogue) of less than 4400 K. The resultant mass and radius estimates derived in this paper reduce the size of the planet candidates to the Earth-analogue worlds reported here. This would obviously be a significant finding, but I think we have to wait for a response from the Kepler team, and in particular those involved with the Kepler Input Catalogue, to put the work into perspective. An error of this size would be extreme and, as at least one commenter has noted here, such an error should have shown up in the work on Kepler-16b, yet evidently did not.

I’m a writer, not an astrophysicist, so I’m intrigued but waiting for follow-up work to sort this out. This is, after all, how science works, an interplay of data and analysis that is adjusted as new data emerge. We’ll soon learn whether we have to modify our views of other Kepler candidates to match this result. In the meantime, I’m interested to learn what readers think of the Muirhead team’s analysis.

The paper is Muirhead et al., “Near-Infrared Spectroscopy of Low-Mass Kepler Planet-Candidate Host Stars: Effective Temperatures, Metallicities, Masses and Radii,” submitted to Astrophysical Journal Letters (preprint). On early work on circumbinary planets, see Schneider & Chevreton, “The Photometric Search for Earth-sized Extrasolar Planets by Occultation in Binary Systems,” Astronomy & Astrophysics 232, pp. 251-257 (1990). Abstract available.

I’m just back from a long trip and am only now catching up on some of the news stories from late last week. Among these I should mention the discovery of the world with the double sunset, identified through Kepler data and reminiscent of the famous scene from Star Wars, where Luke Skywalker stands on the soil of Tatooine and looks out at twin suns setting. I remember carping about the scene when it first came out because it implied a planet that orbited two stars at once. Now we have confirmation that such a configuration is stable and that planets can exist there.

Kepler-16 isn’t a habitable world by our standard definition, but much more like Saturn, cold and gaseous. One of the stars it circles is a K dwarf of about 69 percent the mass of our Sun, while the other is a red dwarf of about 20 percent solar mass. Some 220 light years away in the direction of the constellation Cygnus, the system is fortuitously edge-on as seen from Earth, allowing Kepler scientists to identify the transits of the planet orbiting both stars. So we have two stars and one planet all eclipsing each other, with the planet’s gravity affecting the precise timing of the stellar eclipses. And each transit occurs at a different orbital phase of the inner star.

Image: This artist’s conception illustrates the Kepler-16 system (white) from an overhead view, showing its planet Kepler-16b and the eccentric orbits of the two stars it circles (labeled A and B). For reference, the orbits of our own solar system’s planets Mercury and Earth are shown in blue. Though the planet orbits its star from a distance comparable to that of Venus in our own solar system, it is actually cold. This is because its two parent stars are both smaller than the sun and don’t give off as much heat. Credit: NASA/Ames/JPL-Caltech.

This is obviously demanding work, but the existence of tertiary and quaternary eclipses at irregular intervals demonstrated the changing position of the two stars as each planetary transit occurred. Researchers were then able to uncover a planetary orbital period of 229 days, while the stellar binary is in a 41-day orbit. It’s interesting to note that the smaller of the two stars is also the smallest low-mass star to have its mass and radius measured at such high precision. The issue of precise stellar measurements is one we’ll return to soon as we look at a paper pointing out how difficult the measurement of stellar radii can be when dealing with M-dwarfs, a fact that has a bearing on what we can deduce about the exoplanets found orbiting them.

Kepler principal investigator William Borucki points out in this NASA news release that most stars in the Milky Way are part of a binary system, so we now see that in addition to planets orbiting around the individual stars in such a system, we also have an option for circumbinary orbits that may offer even more exotic venues for astrobiology. The discovery is also satisfying in a cultural context — I like what John Knoll of Industrial Light & Magic had to say about scientific discoveries and imaginative portrayals like Star Wars:

“Working in film, we often are tasked with creating something never before seen. However, more often than not, scientific discoveries prove to be more spectacular than anything we dare imagine. There is no doubt these discoveries influence and inspire storytellers. Their very existence serves as cause to dream bigger and open our minds to new possibilities beyond what we think we ‘know.'”

That’s certainly the case here, and the push and pull of imaginative depiction interfacing with science is woven through the history of the last hundred years. A science fiction tale may set a young student on a scientific career, which may in turn uncover something that trumps the original story, but in its own way provides fodder for the next generation of writers and filmmakers. The individual scientist may or may not care for science fiction, but from a cultural perspective, science and fiction weave a continuing, enthralling dance, one that can energize non-scientists and build the public case for keeping the discoveries coming.

The paper is Doyle et al., “Kepler-16: A Transiting Circumbinary Planet,” Science Vol. 333 no. 6049 (16 September 2011), pp. 1602-1606 (abstract).

The largest variations in brightness ever seen on a cool brown dwarf have turned up on the brown dwarf 2MASS 2139 (known as 2MASS J21392676+0220226 to its friends). The findings, reported at the Extreme Solar Systems II conference in Jackson Hole, Wyoming, show a remarkable 30 percent change in brightness in a period of just under eight hours. The assumption is that brighter and darker patches of atmosphere are periodically moving into view as the brown dwarf rotates.

In fact, Ray Jayawardhana (University of Toronto), co-author of the paper on this work, thinks one possibility is something similar to what we see in our own Solar System. “We might be looking at a gigantic storm raging on this brown dwarf, perhaps a grander version of the Great Red Spot on Jupiter in our own solar system,” says Jayawardhana, “or we may be seeing the hotter, deeper layers of its atmosphere through big holes in the cloud deck.”

Image: Astronomers have observed extreme brightness changes on a nearby brown dwarf that may indicate a storm grander than any seen yet on a planet. This finding could new shed light on the atmospheres and weather on extra-solar planets. Credit: Art by Jon Lomberg.

Whatever the case, this is helpful stuff. Older brown dwarfs have atmospheres not unlike giant planets, so we may be able to use this brown dwarf work to make inferences about exoplanet atmospheres. That will involve studying the brightness variations on 2MASS 2139 over time as we watch weather patterns evolve. The work should allow us to calculate wind speeds in the atmosphere and help us understand how winds are generated in this extreme environment.

Weather is poorly enough understood even on nearby planets, but brown dwarfs simplify the picture. The paper has this to say on the subject:

BDs represent a simpli?ed case where atmospheric dynamics result primarily as a consequence of rapid rotation and internal heat, without the complication of external forcing due to irradiation from a parent star. In addition, the observation of weather on BDs extends the study of cloud meteorology to a higher gravity regime, never before probed.

L and T-class brown dwarfs offer up temperatures in the range of 2200 to 500 Kelvin, with atmospheres cool enough that we can consider them precursors to the study of giant planet atmospheres. How dust grains of silicates and metals condense to form clouds is an ongoing study:

Our current understanding of ultracool atmospheres, including the formation and sedimentation of condensate clouds has developed based on comparisons of detailed atmosphere models to observations of hundreds of L and T dwarfs identi?ed in the solar neighborhood… Nonetheless, fundamental questions remain concerning the most basic properties of condensates including their vertical and horizontal distributions, and how these evolve as a function of e?ective temperature, as well as the role of secondary parameters such as gravity, metallicity, convection, and rotation.

And the authors go on to point out the need for long-term monitoring both photometrically and spectroscopically over a wide range of wavelengths to reveal the true nature of the brown dwarf’s variability. It’s interesting to see that the researchers looked into the possibility that 2MASS 2139 might be an interacting binary (which is exactly what I wondered when I first read this), but they concluded that the scenario was highly implausible. The study is based on data from the 2.5-meter telescope at Las Campanas Observatory in Chile.

The paper is Radigan et al., “High Amplitude, Periodic Variability of a Cool Brown Dwarf: Evidence for Patchy, High-Contrast Cloud Features,” submitted to the Astrophysical Journal and available online.

This is a big week for exoplanet news with the continuing presentations at the Extreme Solar Systems II conference in Wyoming. But I’m going to have to be sporadic with posts this week because of ongoing commitments. The papers for the upcoming 100 Year Starship Symposium are due within days, which is a major driver, but I’ve also got even more important matters unrelated to my interstellar work to attend to. I’ll probably be able to get another post off this week, and then we can catch up a bit next week. For now, here’s a story I want to get in that involves things we can’t see.

Remember ‘Invisible Invaders’? This 1959 drive-in classic involved aliens you can’t see in spaceships that are likewise transparent, arriving on Earth to take over the bodies of the recently deceased. John Agar and Robert Hutton spent a lot of this movie chasing a comely physicist (Jean Byron) when they weren’t working out a way to foil the aliens’ plans to take over our planet in three days. Knowing my love of old movies, a friend recently burned a copy of this one for me, along with the equally challenging (in terms of suspending disbelief) drive-in classic ‘The Wasp Woman’ (1959).

I will spare you the plot of ‘The Wasp Woman.’ The reason I mention ‘Invisible Invaders’ is that a certain buzz is going around about an ‘invisible planet,’ which sounds so much like the title of one of those 1950s films that I wish someone had actually produced it. But let’s jolt ourselves back to reality. The planet is one in the same system as Kepler-19b, though not the same as Kepler-19b. And of course both planets are invisible to us. If you think about it, all but a handful of the exoplanets we’ve found are worlds we’ve never actually seen, but whose presence we can study through things like lightcurves (transits) and radial velocity wobbles shown by Doppler methods.

Let’s start, then, with Kepler-19b, a world some 650 light years away in the constellation Lyra. We know a bit about this one, discovered by Kepler’s unswerving gaze on its field of view as it transits in front of the primary. The planet is roughly 13,500,000 kilometers from its star, which allows us to calculate a temperature of somewhere around 480 degrees Celsius, and by studying its transits with care, we can determine that its diameter is 29,000 kilometers. This news release from the Harvard-Smithsonian Center for Astrophysics (CfA) notes that it may be a Neptune-class world, but it’s also true that we don’t know its mass and basic composition.

Image: The “invisible” world Kepler-19c, seen in the foreground of this artist’s conception, was discovered solely through its gravitational influence on the companion world Kepler-19b – the dot crossing the star’s face. Kepler-19b is slightly more than twice the diameter of Earth, and is probably a “mini-Neptune.” Nothing is known about Kepler-19c, other than that it exists. Credit: David A. Aguilar (CfA).

The method scientists use to uncover an apparent second planet in this system is transit-timing variations, which David Kipping has used to such good effect in discussing how to detect moons of extrasolar planets. Kepler can tells us how long it takes between transits, and what’s interesting about Kepler-19b is that the transits, instead of being extremely regular, are showing up with variations of about five minutes. That’s a sign that the gravity of another planet is pulling on the planet we can see, although figuring out what the other planet is creates a problem.

Could this in fact be an exomoon we’re picking up through transit timing variations? The authors of the paper doubt it, finding that a moon of the necessary calculated mass would probably be large enough to show up in their data:

…it would probably be big enough to be seen in transit. We examined each transit by eye, to see if any deviated significantly from the single-planet model, as mutual events of the co-orbiting planets would cause shallower transits…, but we found no features of interest. Furthermore, in this scenario, the b-c mutual orbital period would need to be near-resonant with the pair’s orbital period around the star, so that the TTV signal aliases to the long P_ttv = 316 day signal. We find this scenario unlikely.

We have no radial velocity clues that this world exists, and it evidently does not transit its star, which would indicate an orbit tilted in relation to Kepler-19b. So the range of possibilities is wide. “Kepler-19c has multiple personalities consistent with our data. For instance, it could be a rocky planet on a circular 5-day orbit, or a gas-giant planet on an oblong 100-day orbit,” said co-author Daniel Fabrycky of the University of California, Santa Cruz (UCSC). Learning more will involve not just continued Kepler observations but also future work with instruments on the ground.

The paper is “The Kepler-19 System: A Transiting 2.2 R_Earth Planet and a Second Planet Detected via Transit Timing Variations,” accepted for publication in the Astrophysical Journal (preprint).

With the online press conference re new results from the HARPS spectrograph (High Accuracy Radial Velocity Planet Searcher) now being discussed, I want to pause for a moment before getting into them to mention the ongoing Extreme Solar Systems II conference, which runs until the 17th at quite a venue, Jackson Lake Lodge in Wyoming. The tentative program is available online, with the welcome news of new HARPS and Kepler results and any number of intriguing talks on everything from debris disk imaging around nearby stars to core accretion models.

We’ll doubtless be talking about some of these findings in coming weeks. But for now, on to the HARPS discussion at the Wyoming conference. The take-away quote from today’s news was this, from Michel Mayor (University of Geneva):

“The harvest of discoveries from HARPS has exceeded all expectations and includes an exceptionally rich population of super-Earths and Neptune-type planets hosted by stars very similar to our Sun. And even better — the new results show that the pace of discovery is accelerating.”

Mayor is referring to more than 50 new exoplanets orbiting nearby stars, 16 of which are said to be super-Earths. One of them, HD 85512 b, is estimated to be on the edge of, if not within, its star’s habitable zone. Lisa Kaltenegger (Harvard Smithsonian Center for Astrophysics) points out that this world is the lowest-mass planet yet confirmed by radial velocity methods that is potentially in the habitable zone. It’s another triumph for the HARPS spectrograph, which has been shown to make the detection of planets below two Earth masses possible. More on HD 85512 b in HARPS: Hunting for Nearby Earth-like Planets, a Centauri Dreams story from August.

Image: A team of astronomers has shown that the newly discovered exoplanet HD 85512 b lies at the edge of the habitable zone of its star, where liquid water oceans could potentially exist if the atmosphere of the planet has sufficient cloud cover. This diagram shows the distances of the planets in the Solar System (upper row) in the new HD 85512 system (middle) and in the Gliese 581 system (lower row), from their respective stars (left). The habitable zone is indicated as the blue area. Based on an original diagram by Franck Selsis, Univ. of Bordeaux. Credit: ESO.

HD 85512 b is the second HARPS planet potentially inside the habitable zone, the other being the much discussed Gliese 581 d. And in addition to HARPS, which has found about ? of all exoplanets with masses less than Neptune’s, we should put ESPRESSO on our radar. Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations is in its early design stage, but scheduled to begin operations on the ESO Very Large Telescope in 2016. Now we’re really getting down to cases: ESPRESSO will have a radial velocity precision of 0.35 km/h or less. Compare that to the 0.32 km/h that Earth induces on the Sun and you can see why astronomers look forward to using ESPRESSO to discover Earth analogues around nearby stars.

HARPS observations of 376 Sun-like stars are helping us firm up the likelihood of low-mass planets around such stars. The result: About 40 percent of them have at least one planet less massive than Saturn. And the majority of exoplanets of Neptune mass or less appear in systems with multiple planets. The paper is Mayor et al., “The HARPS search for southern extra-solar planets, XXXIV — Occurrence, mass distribution and orbital properties of super-Earths and Neptune-type planets.” Still in preparation, it is to be published in Astronomy & Astrophysics. And see Kaltenegger et al., “A Habitable Planet around HD 85512?” submitted to Astronomy & Astrophysics (preprint).