Weather Patterns on a Brown Dwarf

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.


On Planets and What We Can See

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 Pttv = 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).


New HARPS Planets at Exoplanet Symposium

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).


A Jittery Problem for Kepler

We’ve been assuming all along that it would take the Kepler mission three years-plus to detect true Earth analogues, meaning planets orbiting Sun-like stars at about the Earth’s orbital distance. Now it turns out that figure may have to be extended, as this article in Nature makes clear. Author Ron Cowen points out that a close analysis of approximately 2,500 of the tens of thousands of stars in the Kepler field are flickering more than expected, and that spells trouble.

Image: Kepler’s field of view superimposed on the night sky. Credit: Carter Roberts.

The reason: The dip in starlight signalling the presence of a planet can be masked by the unexpected noise in the Kepler data. As described by Kepler scientist Ron Gilliland (Space Telescope Science Institute), the signal of an Earth analogue — assuming a star much like the Sun — should be a drop of about 85 parts per million when the planet passes in front of its star, lasting a statistical average of 10 hours, and occurring once per year. Moreover, the signal would only be evident after three successive transits, equally spaced in time, had been observed.

But that assumption was made with luminosity fluctuations about the same as the Sun’s. The noise metric the Kepler team uses is called Combined Differential Photometric Precision, or CDPP, and according to Gilliland and colleagues in the paper on this matter, CDPP was intended to be about 20 ppm for dwarf stars at 12th magnitude. But CDPP turns out to be 30 ppm, 50 percent higher than expected. The paper goes on to demonstrate that most of the noise is intrinsic to the stars themselves rather than being the result of instrument problems.

From the paper:

Kepler… is the first mission capable of quantifying the variability of large numbers of stars to the small levels by which the Sun is known to vary. Kepler will continue to provide exciting new insights into the astrophysics of quiet stars, and their galactic distributions. While we are not surprised to have learned new things from this new observational capability, the fact that the stars are more variable than expected has a significant influence on the ability to readily detect Earth-analog planet transits where the expected signal per transit is only a few times the inferred noise level on comparable time scales. Observing for a longer time baseline can compensate for the loss of transit detection sensitivity from the higher than anticipated stellar noise.

A longer time indeed, the upshot being that Kepler will need not three but an average of six transits per planet to adequately verify that the change in starlight is actually a planet. Finding the Earth analogues among the Kepler stars, then, calls for an extended mission, and that will more than double the planned mission life of three and a half years. Cowen quotes Kepler chief data analyst Jon Jenkins as saying that while an overall eight year mission now seems necessary, the extension to the mission is by no means assured. After all, NASA’s budget problems include the costs of the James Webb Space Telescope, and Kepler could be squeezed out of the picture.

Magnetic activity on the Kepler stars may be the reason for the unexpected noise in the data, and if that is the case, it may be because the stars are young, thus spinning faster and feeding a more powerful magnetic field. That runs against predictions that most of the stars in the sample would be older than the Sun, but right now the cause seems less important than the need to get Kepler the extended mission it needs to actually tell us something about the distribution of Earth-like worlds in a large sample of stars. It would be outrageous if Kepler were to be shut down when it was just a few years away from answering such long-held questions, but at the moment NASA isn’t talking about Kepler’s chances. That decision will come next spring.

The paper is Gilliland, “Kepler Mission Stellar and Instrument Noise Properties,” accepted by the Astrophysical Journal (preprint). Thanks to Antonio Tavani for the original link to the Cowen article.

Related: “On Monday, 12 September 2011, astronomers will report significant new results in the field of exoplanets, obtained with the High Accuracy Radial Velocity Planet Searcher, better known as HARPS, the spectrograph on ESO’s 3.6-meter telescope at La Silla Observatory in Chile.” More here.


If You’re Going to the Starship Conference…

Quite a few people involved with Tau Zero and many of the Project Icarus team are planning to be in Orlando at the end of the month for the 100 Year Starship Symposium. I know about most of these, but if you haven’t already told me that you are planning to attend, please leave me a note in the comments to this post. I’m looking forward to meeting many Centauri Dreams readers there.