systemic

We’re now a mere two weeks away from the HD 156846b transit opportunity. As I write, the planet is gathering speed as it plunges toward its steamy periastron encounter with its parent star (or more precisely, given the 49 parsec distance to HD 156846, back in the year 1851, the planet was plunging toward its steamy encounter with the parent star).

With a mass of at least ten Jupiter masses, HD 156846b is pushing the upper limit of the planetary regime. Like Jupiter and Neptune in our own solar system, but unlike all of the other well-characterized transiting extrasolar planets, its energy budget is likely dictated more by its residual heat of formation than by either tidal dissipation or the energy that it receives from its parent star as it circulates on its 360-day orbit.

Remarkably, objects that are very similar in mass and temperature to HD 156846b are starting to be discovered via direct imaging. In an ApJ letter from earlier this year, Luhman, Burgasser and Bochanski reported the discovery of a candidate brown dwarf which, if confirmed, has a positively shirtsleeves ~300K effective temperature and a mass of ~7 Jupiter masses.

This candidate, WD 0806-661 B, is in a ~2500 AU-wide orbit about a nearby white dwarf star that lies 19.2 parsecs away. It can be seen in Spitzer’s 4.5-micron band at two distinct epochs, and was flagged as a result of its common proper motion with its white dwarf primary. As it’s been detected so far only at 4.5 microns, its spectrum is largely unknown. It has a good chance, however, of signing on the dotted line as a first representative of the Y spectral class.

Which underscores the importance that HD 156846b will have it it turns out to transit. At V=6.5, the parent star is very bright, over 2.5 times brighter than either HD 189733 or HD 209458. The transmission spectrum for HD 156846, especially on the cold limb, would thus give an important and detailed clue toward what one might expect from the spectra of field Y dwarfs. And given that one of these guys could be lurking just a light year or two or three away, and given that the WISE preliminary release is on line and available, that’d be a very interesting clue indeed…

Seems like every other year, a good opportunity arises for small-telescope photometric transit observers to participate in a big discovery. In 2007, oklo.org egged everyone on to observe HD 17156 during the transit window of its e=0.69, P=21.2-day planet, and the results were quite satisfactory. In early 2009, there was the exciting detection of the HD 80606b transit. This year, there’s a very interesting opportunity to see whether HD 156846b (RA 17 20 34.31129, DEC -19 20 01.4991, V=6.5) occults its parent star.

HD 156846 b was discovered by the Geneva Team in 2007, and weighs in at a hefty 10+ Jupiter masses. Its orbital period is 359.6 days, just short of a year, and it has a very high eccentricity, e=0.848. The orbital geometry is quite favorable, leading to a ~5% chance that transits will be observable. In addition, the transit window is well constrained as a consequence of the large radial velocity swing that the planet induces in its parent star. Here’s the set-up, with the inner solar system orbits shown for scale:

Observers worldwide should plan to be on the sky this August 23rd, 24th, and 25th, a bit more than three weeks from now. Be sure to check back at oklo.org and to follow twitter.com/transitsearch for updates and interesting details as this opportunity draws near!

Galex Far-UV survey image centered on 40 Eridani B.

Like many kids, I enjoyed reading The Magician’s Nephew by C. S. Lewis. Especially evocative were the descriptions of the dying planet Charn:

The wind that blew in their faces was cold, yet somehow stale. They were looking from a high terrace and there was a great landscape spread out below them.

Low down and near the horizon hung a great red sun, far bigger than our sun. Digory felt at once that it was also older than ours: a sun near the end of its life, weary of looking down upon that world. To the left of the sun, and higher up, there was a single star, big and bright. Those were the only two things to be seen in the dark sky; they made a dismal group. And on the earth in every direction, as far as the eye could reach, there spread a vast city in which there was no living thing to be seen.

The story was written in the early 1950s, just as the future evolution of the Sun was beginning to be understood, but before the Henyey technique for computing full stellar evolutionary sequences had been developed. The scene, while compelling, seems to make little astrophysical sense. It describes a parent star on its ascent of the red giant branch, yet the star’s overall luminosity seems clearly on the wane. In truth, as a red giant swells up, its overall luminosity increases drastically, and the end game for habitable planets consists of fire rather than ice.

Earlier this year, however, there was a very interesting paper by Eric Agol that discusses the possibility of Earth-like planets orbiting white dwarf stars. These planets, if they exist, would be spin-synchronized and would have orbital periods of order 10-20 hours. On such a world, the demise of habitability occurs as the parent white dwarf loses its heat of formation, and grows gradually redder, even as it maintains the same angular size in its fixed position in the sky.

Here’s the relevant summary diagram from Agol’s paper. As the parent white dwarf cools, it travels vertically up the plot.

Now admittedly, this set-up is sailing pretty close to the wind. Indeed, I’ve largely come to adopt the opinion that the whole idea of the “habitable zone” is the modern-day equivalent of Bode’s Law. And furthermore, it’s not exactly clear how one might arrange for habitable planets to be orbiting white dwarfs. The reason I’m enthusiastic is that Agol’s scenario is eminently testable. If white dwarfs harbor Earth-sized planets in quantity, then they can potentially be discovered by backyard astronomers. A one-Earth radius planet on an a=0.013 AU orbit around a typical 0.6 solar-mass white dwarf produces a central transit depth of ~50% during a transit that lasts one or two minutes.

Bruce Gary, who has been a leader in the area of transit detection using small telescopes, has recently organized a pilot photometric project to detect transiting planets orbiting white dwarfs. Here’s his description of the project from an announcement that he sent around last week:

All,

This is a “call for observers” for a 1-month project to evaluate feasibility of amateurs and others to detect white dwarf transits using available hardware.

This should be viewed as a “pilot project” designed to provide a first evaluation of the abundance of exoplanets orbiting white dwarfs in short-period orbits (near the habitable zone). It can play a role in designing a funded project using professional hardware to conduct a long-term and more comprehensive white dwarf (WD) transit search. Professional astronomer guidance is provided by Prof. Eric Agol, who has written several articles on the subject of exoplanets in WD habitable zones. I will archive light curves at a web site in a manner similar to what I did for the Amateur Exoplanet Archive (AXA).

I have tentatively identified September as the observing month. Coordinated observing by partners is encouraged to permit corroboration of any interesting light curve feature. Note that since WDs are very small, comparable to the Earth, a central crossing by an Earth-size exoplanet will produce a very deep transit feature, possibly causing a temporary complete fade. Another consequence of the small size is that transit lengths will be short, typically a couple minutes. In spite of the great depth the search for WD transits is an observational challenge because of the short length. The chance of success in detecting a WD transit may be small but the payoffs for success are great!

Anyone with experience observing exoplanet transits is qualified for this project. However, of the known 20,000 or so WDs only 168 are brighter than V-mag = 14.0. This means that telescope aperture matters, and for most WD targets an aperture of at least 10 inches will be needed.

The project will go by the name Pro-Am White dwarf Monitoring, or PAWM. A description of PAWM can be found at the following web site: http://brucegary.net/WDE/

Please forward this e-mail to anyone who might be interested in participating as an observer or professional adviser. Reply to this e-mail if you would like to receive occasional updates on PAWM.

Bruce L. Gary
Hereford Arizona Observatory

A very exciting project! Once September starts, I’ll be checking the PAWM site to watch how the survey unfolds…

A few weeks ago, I was in West Texas, seeing first-hand places that were familiar only from articles and maps. Marfa, Big Bend, the Glass Mountains.

Image Source.

The landscape west of the Pecos River, dry to begin with, is in the grip of exceptional drought. The temperature was over 100 degrees Fahrenheit, and there was a hot incessant wind. From a ridgeline near the top of an eroded volcanic intrusion in the Chisos Mountains, dry basins and ranges extended into the infinite hazy distance. It was easy to imagine that the Earth had lost its oceans, and had become a desert planet, with isolated pockets of life clinging to retain the veneer of a respectable planetary habitability.

There’s a recent, highly engaging article by Kevin Zahnle and collaborators (Abe et al. 2011) that argues that such a world might be better suited than the present-day Earth at staving off the biosphere-terminating ravages of the runaway greenhouse effect. Desert, or “land” planets keep their stratospheres dry, which allows them to better retain what water they do have, and land planets can more effectively re-radiate infrared radiation into space at given surface atmospheric pressure, allowing a cooler surface temperature at a given stellar flux. It cools down at night in the desert.

Abe et al.’s global climate models indicate that Earth will cease to be habitable in 2.5 Billion years. In the absence of oceans, on the other hand, they find that habitability would be extended by another 2 to 2.5 billion years. And provocatively, if Venus started out as a land planet, it may have been habitable as recently as a billion years ago.

My guess is that nearly everyone who frequents oklo.org has read and liked Frank Herbert’s 1965 science fiction classic Dune, which is folded into the introduction of Abe et al.’s paper:

We can imagine another kind of habitable planet that has only a small amount of water and no oceans; it might be covered by vast dry deserts, but it might also have locally abundant water. We call such a dry planet a ‘‘land planet.’’ The fictional planet known as Arrakis or Dune (Dune, Herbert, 1965) provides an exceptionally well-developed example of a habitable land planet. In its particulars, Dune resembles a bigger, warmer Mars with a breathable oxygen atmosphere. Like Mars, Dune is depicted as a parched desert planet, but there are signs that water flowed in the prehistoric past. Dune has small water ice caps at the poles and more extensive deep polar aquifers. The tropics are exceedingly dry, but the polar regions are cool enough and moist enough to have morning dew.

In Search of Planet Vulcan — The Ghost in Newton’s Clockwork Universe, by Richard Baum and William Sheehan, is one of my favorite astronomy books. It certainly has one of the best overviews of the momentous events and controversies surrounding the discovery of Neptune in September 1846. I’ll take the liberty to quote Baum and Sheehan’s recounting of the exact moment of Neptune’s discovery.

On September 18, Le Verrier wrote to Johann Gottfried Galle, then an obscure astronomer at the Royal Observatory in Berlin. A year earlier, Galle had sent Le Verrier his doctoral dissertation, which concerned observations made by 17th-century Danish astronomer Olaus Roemer. Belatedly, Le Verrier wrote to acknowledge it. Among other things, he queried Galle about Roemer’s Mercury observations, but then came quickly to his point:

“Right now I would like to find a persistent observer, who would be willing to devote some time to an examination of a part of the sky in which there may be a planet to discover… You will see, Sir, that I demonstrate that it is impossible to satisfy the observations of Uranus without introducing the action of a new Planet, thus far unknown; and, remarkably, there is only one single position in the ecliptic where this perturbing Planet can be located… The actual position of this body shows that we are now, and will be for several months, in a favorable situation for the discovery.

Galle indeed proved to be his man. He received Le Verrier’s letter on September 23, and at once sought permission from the observatory’s director, Johann Encke, to carry out the search. Encke was skeptical but nonetheless acquiesced: “Let us oblige the gentleman in Paris.” A young student astronomer, Heinrich Ludwig d’Arrest, begged to be included, and joined Galle as a volunteer observer. That night, they opened the dome to reveal the observatory’s main instrument, a 9-inch Fraunhofer refractor aimed at the spot assigned by Le Verrier. Recalculated for geocentric coordinates, its position was at right ascension 21 h, 46 min, declination -13 deg 24 min, very close to the position occupied by another planet, Saturn.

The question arose: What maps were available? At first they could think of none but “Harding’s very insufficient Atlas.” D’Arrest then suggested “it might be worth looking among the Berliner Akademische Sternkarten to see whether Hora XXI was among those already finished. On looking among a pile of maps in Encke’s hall [Vorzimmer], Dr. Bremiker’s map of Hora XXI [already engraved and printed at the beginning of 1846 but not yet distributed] was soon found.” As d’Arrest later recalled, “We then went back to the dome, where there was a kind of desk, at which I placed myself with the map, while Galle, looking through the refractor, described the configurations of the stars he saw. I followed them on the map one by one, until he [Galle] said: and then there is a star of the 8th magnitude in such and such a position, whereupon I immediately exclaimed, that star is not on the map!”

Neptune’s moment of discovery, at 11 PM Berlin local time on September 23, 1846, corresponded to 22:07 UT, or JD 2395563.4215. The period of Neptune is 60,190.03 days, or 164.79132 years. The first “Neptunian anniversary” of the discovery is therefore CE 2011 July 10 22:49:26.4 UT Sunday, that is, right now.

In 1846, photography was still in its very earliest stages, and it would be nearly two decades until the publication of Jules Verne’s De la Terre à la Lune. The fact that we greet the completion of one orbit in the possession of photographs of a crescent Neptune is a marvelous indeed.

Image Source.

Certainly, an occasion for celebration! On Friday, I got an invitational e-mail from Gaspar Bakos, who is hosting a Neptune-at-One cocktail party in Cambridge, Massachusetts. I briefly perused airfares before sadly having to decline.

A well-known theorem states that there’s no such thing as a free lunch. A corollary is that interesting discoveries tend to be made at the ~3-4 sigma level of confidence, and this is especially true if the supporting data is drawn from the public domain. If a signal is stronger than 4-sigma, then someone else has invariably pointed it out. If it’s weaker than 3-sigma, it’s probably wishful thinking.

With those rules of thumb in mind, I’m very optimistic that Kevin Schlaufman has obtained a genuinely important insight into how the planet formation process works:

The plot shown is above is from a paper that Kevin and I submitted soon after the 1,235 Kepler planet candidates were announced last Spring. After going through review, it was accepted by the ApJ, and it was posted to astro-ph last week.

I wrote about the underlying details of the plot in this post from several months ago. The basic idea is as follows: The 997 Kepler planet candidate host stars are divided up into two groups — (i) the less numerous group of stars that host a candidate with R_pl>5 Earth radii (red), and (ii) the more numerous group of stars that only host a planet (or planets) with R_pl<5 Earth radii (blue). The two groups of stars, along with a control sample of 10,000 non-candidate-bearing dwarf stars from the Kepler field (gray), are plotted in a color-color diagram (and then binned to create the diagram above):

The y-axis corresponds to the magnitude difference between a given star’s green (Sloane g filter) and red (Sloane r filter) colors. The x-axis charts the differences between the 2Mass J and H infrared colors for each star. Metal-rich stars tend to have redder optical colors than metal-poor stars, whereas the J-H index sorts the stars in terms of their overall temperatures (with cool stars to the right and warm stars to the left of the plot). Metal-rich stars thus lie along the upper part of the main-Sequence locus.

The binned version of the plot provides a confirmation of several trends that were already very well known. First, among host stars with masses similar to the Sun that harbor giant planets, there’s a strong preference for metal-rich stars. This is the classic planet-stellar metallicity effect. Second, among low-mass stars, there’s a dearth of giant planet candidates. This is the known giant planet-stellar mass effect. Finally, among the solar mass stars that host low-mass planets, there’s no discernible metallicity correlation.

The new result pertains to low-mass planets orbiting low-mass stars. The diagram shows that for this subset, there’s strong evidence for a metallicity correlation — At masses less than ~0.8 solar masses, higher metallicity stars are more likely to host low-mass planets. We take this as direct evidence regarding the overall bulk efficiency of planet formation for planets that aren’t required to bulk up via rapid gas accretion. Take a 0.7 solar mass with twice the Sun’s metal content and a typical 0.02 solar mass disk. The entire planet-forming disk contains about 150 Earth-masses worth of stuff heavier than hydrogen and helium. Kevin’s result is effectively saying that a good fraction of the time, a good fraction of this total burden of metals winds up in planets.

We had to be careful. There are a lot of systematic “gotchas” that can potentially throw a wrench into the exciting big-picture conclusions, and so much of the paper is devoted to considering potential show stoppers in turn. I think that the result is robust, and that it will hold up as the planet catalog continues to grow.

The French philosopher of science Isidore Auguste Marie François Xavier Comte (1798 – 1857) was the founder of sociology and is widely remembered for the doctrine of positivism, which holds that the scientific method can be used to understand both natural and social phenomena.

He’s well known to astronomers, however, largely because of the spectacularly incorrect statements regarding stars in his Cours de la Philosophie Positive (1830-1842):

On the subject of stars, all investigations which are not ultimately reducible to simple visual observations are … necessarily denied to us. While we can conceive of the possibility of determining their shapes, their sizes, and their motions, we shall never be able by any means to study their chemical composition or their mineralogical structure … Our knowledge concerning their gaseous envelopes is necessarily limited to their existence, size … and refractive power, we shall not at all be able to determine their chemical composition or even their density… I regard any notion concerning the true mean temperature of the various stars as forever denied to us.

Lots of fun to get your introductory lecture on stellar spectroscopy off to a snarky start with that particular zinger.

Comte’s pronouncements on planets are slightly more obscure, but now, given the many and varied successes of the Spitzer Telescope, they provide an equally rich vein for irony-with-20/20-hindsight:

The take away message seems to be “never say never”. Nowadays, an academic with Comte’s flair would certainly have the innate sense to leave some wriggle room in anticipation of unforeseen scientific advances.

In any case, in this evening’s astro-ph mailing, there’s a very interesting article by Brugamyer et al. that touches on the inferred chemical and mineralogical structure of extrasolar planets. The authors of the paper make a detailed examination of the relative oxygen and silicon abundances of stars known to host extrasolar planets.

The context comes from work back in 2006 by co-author Sally Dodson-Robinson which indicated that stars with high silicon abundances relative to iron show increased planet fractions at given metallicity:

The expectation was that stars with high oxygen abundances relative to iron would also show increased planet fractions at given overall metallicity. Oxygen is a key component of the core-building materials for giant planets, and so it stood to reason that the more water available, the more Jupiter-mass planets one ends up with. Remarkably, this turns out not to be true. Here’s the relevant diagram from the Brugamyer et al paper:

Statistically, it appears that an excess of oxygen relative to iron has no influence on the likelihood of a given star hosting a readily detectable planet. The silicon effect, however, is statistically robust and readily detectable in the Brugamyer et al. analysis:

So how does one explain the unexpected result? Brugamyer et al.’s hypothesis is that icy grain nucleation on silicon-rich dust, rather than the subsequent growth of the icy core-forming particles, is the key bottleneck in forming giant planets via core accretion.

An all-time classic of the literature is Alar and Juri Toomre’s 1972 ApJ study of colliding galaxies. With an exceptionally simple physical model — the restricted three body approximation, in which test particles orbit in the joint potential provided by two massive bodies on a conic 2-body trajectory — the Toomre brothers were able to construct startlingly plausible explanations for bizarrely irregular galaxies such as the Antennae.

One is hard-pressed to think of a better example of seeing the essence of a manifestly complicated phenomenon so precisely nailed by a simple model. The take-away lesson seems to be: Keep an eye out for situations in which glorious non-linearity has had of order one Lyapunov time to unfold.

ApJ 178, 623 also presents some of the finest astronomical diagrams ever. They are masterpieces of visual scientific communication. Every single detail conveys information, and nothing is superfluous.

In 1998, when I was a post-doc in Berkeley, my working routine was considerably less hectic than it is now. On the foggy morning of May 29th of that year, I remember buying a copy of the New York Times, and settling in at a Cafe on Telegraph Avenue for a relaxed 11AM coffee. A picture and a slew of familiar names jumped off the front page:

The story, which became a huge media event — even President Clinton made a passing mention of it — stirred up a uniquely unsettled, uniquely urgent feeling of being completely involved and completely left out all at the same time. A runaway planet clearly would have formed via gravitational instability, and I had spent several years studying gravitational instabilities for my PhD thesis. I gulped down my coffee, scooped up the paper and ran to my office in Campbell Hall. The phone was ringing when I got there. Doug Lin was on the line, buzzing with excitement. “It’s a tidal tail! Look at Alar’s ’72 paper!”

There was not a moment to waste… Doug called the editor at Science and informed him that we had an important interpretive result in the works. I stayed up all night putting together SPH simulations. It seemed completely feasible that one could explain the observation with a collision between two protostellar disks, in which the runaway planet formed via gravitational collapse in the tidal tail. We got the paper off to Science in short order, and boy was it exhilarating!

The Toomre brothers’ influence soaks right through the figures that I made for our paper. Thirteen years on, they remind me of listening to a cover of Sympathy for the Devil done by a competent Stones tribute band.

Sadly, a year or so later, it became clear that the TMR-1c runaway “planet” is, in actual fact, an unfortunately placed background star, and the TMR-1c fiasco is commonly used to illustrate the flaws in the publication by press conference model. Our Science paper has languished in obscurity, to the point where one can extract it from behind Science’s formidable pay wall with only a modestly compromising registration agreement to receive e-mail and no money down…

But hope springs eternal. Like everyone else in the community, my eyes lit up upon reading the recent microlensing result that the galaxy is teeming with of order 200 billion rogue planets. Processes like the one that we outlined in our paper may well be operating after all…

It’s Sunday afternoon here in Santa Cruz, meaning that GMT-wise, it’s already June 6th, and the next transit of Venus is exactly one year away. Seems like an appropriate moment to recall a quote by astronomer William Harkness from 1882 (by way of Stephen J. Dick’s Sky and Ocean Joined: The U.S. Naval Observatory 1830-2000).

We are now on the eve of the second transit of a pair, after which there will be no other till the twenty-first century of our era has dawned upon the Earth, and the June flowers are blooming in 2004. When the last transit season occurred the intellectual world was awakening from the slumber of ages, and that wondrous scientific activity which has led to our present advanced knowledge was just beginning. What will be the state of science when the next transit season arrives God only knows. Not even our children’s children will live to take part in the astronomy of that day. As for ourselves, we have to make do with the present.

There’s something oddly appealing about the nonintuitive spacing of Venusian transits, a 243 year repeating pattern, with transits occurring eight years apart, then a gap of 121.5 years, followed by an eight year interval and then a 105.5 year spacing. I’m certainly looking forward to June 6th 2012, when a healthy fraction of the transit will be visible from Lick Observatory on Mt Hamilton. For updates, be sure to bookmark the Transits of Venus Project website, which launched today.

I can’t help feeling uneasy, however, thinking about the state of affairs on Dec. 10-11 2117…

As Mick Jagger famously remarked, you can’t always get what you want. Kepler’s photometric transit observations provide excellent measurements of the planetary orbital periods, the transit epochs and the planet-to-star radius ratios, but they are stingy and tight-lipped when it comes to the planet’s masses, eccentricities, and longitudes of periastron.

Occasionally masses can be inferred from transit timing variations, especially if a system contains more than one transiting planet. Alternately, one can assume a planetary mass-radius relation (keeping in mind, of course, what happens when u assume). For example, M=R^2.06 in units of Earth masses and radii works quite well in our solar system for V-E-S-U-N. Or, dispensing with the trickery, one can pony up and measure radial velocities.

With photometric data alone, information about the orbital eccentricity distribution of the planet census can be deduced by statistically comparing transit durations to orbital periods. The idea is a full elaboration of the simple observation that if a central transit that is substantially shorter than expected, then it’s quite possible that the planet is occulting the parent star near the periastron of an eccentric orbit.

In one of the flurry of Kepler-related papers that accompanied the February data release, Moorhead et al. (2011) implemented just such a program, and generated a statistical analysis of the distribution of transit durations for the Kepler exoplanet candidates. They assumed that the eccentricities conform to a Rayleigh probability distribution function:

where the controlling parameter, sigma, is is related to the mean orbital eccentricity through

To get a sense of what the Rayleigh distributions look like, here are examples for e_av=0.05, e_av=0.21, and e_av=0.50, compared to the distribution of eccentricities in the exoplanet.eu catalog:

Ignoring planets that are likely tidally circularized, the best fit occurs for e_av=0.21. This model, however, underproduces planets at high eccentricity — ’606 wouldn’t have turned up if e_av=0.21 were a hard truth. Moorhead et al.’s analysis of the Kepler data comes up with plausible best-fit values for e_av ranging from 0.1 through 0.25, for cooler stars with effective temperatures less than 5100K. So there is rough agreement, even though the two catalogs have radically different sampling biases.

A significantly non-zero value for average orbital eccentricity has some interesting consequences for transit surveys. At a given semi-major axis, eccentric planets have (on average) a higher chance of transiting. This is easily seen by comparing an e=0.5 orbit with a circular orbit having the same semi-major axis.

For a population of planets having a specific Rayleigh distribution of eccentricities, the average transit probability at a given semi-major axis is increased by a factor

where the normalization factor, N, is given by

For e_av=0.25, this boosts the total population of planets by about 10% over what one would infer from the standard 1/a circular orbit scaling.