systemic

It’s likely that everyone who reads this site has already seen the new Kepler candidates paper. Drawing on 16 months of photometric data, and importantly, on significant improvements to the reduction pipeline, it gives details on 2,323 planet candidates. The cumulative planet candidate table, in particular, makes for interesting reading.

In true Gordon Gekko style, I ran the new candidates table through my valuation formula (see here, here, and here.)

A screenshot of the results, for candidates with valuations greater than ten dollars are shown below. KOI 2650.01 and KOI 2124.01, assuming that they hold up, are both million dollar worlds. The total value of the current catalog is 10.9M.

Hey! Did you see the New York Times article about the discovery of a Jupiter-like planet orbiting Barnard’s Star?

The subtly out-of-date fonts are really the only indication that the above article, which was printed on April 19th, 1963, is nearly a half century old. Certainly, the blandly uninformative expert commentary and the worn-smooth assertion that the new finding adds support to the conviction of astronomers that a great many solar systems exist, some of them possibly supporting life, are both still fully serviceable.

The erstwhile planet(s) orbiting Barnard’s star were the fruit of thousands of astrometric measurements of photographic plates taken from 1938 through 1962 by Dr. Peter van de Kamp and his students from Swarthmore College’s Sproul Observatory. During the 1960s, the existence of van de Kamp’s planets were generally accepted by the astronomical community, and they only began to drift out favor during the 1970s. As explained in this interesting historical review of exoplanet detection, it’s now clear that the apparent astrometric motions of Barnard’s Star over the years can be correlated with telescope adjustments. Modern radial velocity measurements from UVES at the VLT and from the HET telescope show quite definitively that van de Kamp’s planets don’t exist:

Indeed, there must now be enough radial velocity observations of Barnard’s star to put some very interesting limits on any planets that might be lurking in the system… Given that the star is so bright (for a red dwarf) with V=9.5, highly charismatic, and visible from La Silla, I think it’s safe to say that for orbital periods of less than ~100 days, the largest planets that could be hiding there have masses roughly twice that of Earth.

Broadly speaking, the non-hydrogen/helium mass of a planetary system is ~0.02*0.016*Mstar*(10**[Fe/H]). We therefore don’t expect to find bruisers of planets orbiting Barnard’s Star, which has only ~14% of the Sun’s mass, and has a metallicity of order 10-30% that of the Sun. Given what we now know about the galactic planetary census, an educated guess is that Barnard’s star harbors several roughly co-planar planets, none larger than 1.5 Earth masses, with orbital periods less than 100 days. In fact, I think there’s an even chance that within the next four years, we could be reading about just such a system in the New York Times.

Following the 1846 discovery of Neptune by Urbain J. J. LeVerrier of France and Johann Galle of Germany, the British astronomical establishment — the Rev. James Challis, the Astronomer Royal George Biddell Airy, and Sir John Herschel — found themselves in rather hot water. Diffidence, seeming indifference and miscommunications had deprived Britain of a very tangible emblem of national prestige. In the damage-control scramble that ensued, Herschel wrote urgently to William Lassell, a wealthy brewer in Liverpool who owned a 24-inch telescope, exhorting him to search for satellites “with all possible expedition!”. Lassell was on task. A mere 17 days after the announcement of Neptune’s existence, he had discovered Triton, thus handing his countrymen a victory in the losers’ bracket rounds.

The quick discovery of Triton occurred in large part because astronomers were conditioned as to what to expect. Jupiter, Saturn and Uranus all host regular satellite systems in which the orbital periods of the satellites are measured in periods lasting days to weeks, and in which the mass ratio of the satellites to the primary is of order two parts in 10,000. These rules of thumb hold quite nicely, despite the fact that Jupiter has more than 20 times the mass of Uranus.

Much of the bewilderment that has accompanied the discovery of extrasolar planets stems from the fact that planets found orbiting other stars don’t bear much resemblance to configuration of our own planetary system. First, hot Jupiters. Then giant planets on highly eccentric orbits with periods of a few hundred days. And now, the realization that over half of the sun-like stars in the solar neighborhood are accompanied by planets with masses in the superEarth/subNeptune range and orbital periods of less than 100 days. It’s now clear, in fact, that our own solar system is unusual at least at some modest level, and perhaps at quite a significant level.

As hordes of new planets pile into the candidate tables at exoplanet.eu, the correlation diagrams are really beginning to show the true features of the galactic planetary census. The classic log-log mass-period diagram is a good example. Here’s one that’s (already) two months out of date:

The lower-right portion of the above diagram is incomplete, and there are a whole slew of observational biases at work, but nevertheless, the relatively depopulated divisions between the superEarth/subNeptunes, the hot Jupiters and the eccentric giants are real features of the planet distribution. There’s truth in the fact that one can sometimes overlook the forest for the trees. By smearing vaseline on the laptop screen and taking a cell-phone photograph, one obtains a better sense of the outlines of the forest:

It’s interesting to adjust the log-log mass-period diagram so that the y-axis charts the planet-to-star mass ratio rather than planetary mass (an advantage of logarithms is that concerns regarding the difference between M and Msini are effectively academic). With this plotting scheme, Earth and Jupiter are still off the guest list, but remarkably, the regular satellites of the Jovian satellites adhere to the same distribution as the superEarths and subNeptunes:

A comparison that’s made all the more dramatic with the inclusion of the Kepler multiple-transit candidates:

A coincidence? I don’t think so.

In a logarithmic sense, the largest gap in mass among the planets in our solar system lies between the Earth (which has, unsurprisingly, one Earth mass) and Uranus, which is 14.536 times more massive than Earth. One of the most interesting facets of the ongoing detection of extrasolar planets is that we’re now getting real information on planets that fall into this previously unobserved planetary regime.

Indeed, the most startling exoplanet-related revelation of the past few years has been the announcement by the Geneva Planet Search Team that planets in the Earth-Uranus gap are extraordinarily common. Their take-away message has consistently been that 30%-50% of the quiet solar-type stars in the Sun’s neighborhood harbor at least planet with Msin(i)<17 Earth masses, and an orbital period of less than 50 days. Tens of billions of worlds! The Milky Way Galaxy is essentially a Costco full of HD 40307 b’s, c’s, and d’s.

With super-Earths and sub-Neptunes out there in such quantities, it’s not surprising that the Kepler mission has returned a large number of candidates. Rather alarmingly, however, it appears at first glance that Kepler may be seeing significantly fewer planets than the Geneva Team’s predictions might imply. Given a 40% overall occurrence rate of planets with P<50d and mass between Earth and Neptune, and assuming one planet per star, there should be ~60,000 potentially detectable planets orbiting the 150,000 target stars in Kepler’s field of view. For planets in orbits of 50 days or less, the geometric probabilities of transit lie in the 1-15% range. Taking a 5% transit probability (a 10-day orbit) as a benchmark, one ball parks that the number of sub-Neptune-mass planets that Kepler would have been able to detect is ~3,000. If we use a simple mass-radius scaling law, we find that a bit less than 1,000 of Kepler’s planet candidates fall in the sub-Neptune mass range. Naively, it thus seems that the Geneva team’s occurrence rate appears to overestimate the number of planets that Kepler would have detected by a factor of around three.

So what’s up? It seems a-priori highly unlikely that either Kepler or the HARPS analysis pipeline have made a significant error. In collaboration with UCSC Grad student Angie Wolfgang, we’ve made a very detailed attempt to compare the two observational programs, with the goal of seeing whether there’s a sensible way to bring the two surveys into agreement. This task is tricky because Kepler employs transit photometry, where as the Geneva Team’s results are based on radial velocity measurements from HARPS.

To see the details of our work, have a look at the paper that we’ve recently posted to arXiv. The bottom line is that concordance can be obtained, provided that there exist two very different planetary populations in the sub-Neptune mass regime. One population, which is numerically dominant, consists of dense scaled-up terrestrial planets, super-Mercuries if you will. The other population (to which Kepler is selectively sensitive) consists of planets with much lower densities, akin to scaled-down versions of Uranus and Neptune.

Image Source.

Sometimes, you just get these serendipitous moments. Yesterday, in the parking lot of the grocery store, there was a U-haul rental truck sporting a remarkably sophisticated graphic that explains the Manson impact structure in Iowa. When I got home, I went to the U-haul website, and discovered that they have a clear and beautifully self-contained tutorial on giant impacts. The site even explains the terms in the ballistic range equation, which gives the distance from impact that a piece of ejecta lands, given the radius and gravitational acceleration of the Earth, along with the ejection angle and the ejection velocity. And for those wanting more details, U-haul points to Jay Melosh’s Impact Cratering: A Geologic Process (one of the Oxford Monographs on Geology and Geophysics).

Inside the grocery store, at the checkout counter, I noticed that this week’s issue of The Sun is carrying a rather startling astronomically themed story:

Which brings me to the serendipity. Tomorrow afternoon, I’ll be engaging in a joint presentation/discussion with Chris McKay of NASA’s Ames Research Center on the topic of “Real Doomsdays: How Life Could End on Earth”. We’ll be discussing not just the long-term fate of life on Earth, but also the fate of the Earth itself. And indeed, a black hole plunge is one of a handful of fates that Earth might suffer in the ultra-distant future. If our planet isn’t engulfed by the red giant Sun, then it’ll eventually either be ejected into the utter isolation of the exponentially expanding intergalactic medium to slowly evaporate via nucleon decay, or it’ll wind up in the Milky Way-Andromeda central black hole. Presumably, that’s the eventuality that the editors of this week’s Sun are referring to.

Anyway, here are the details. The event is free, and is organized by Tucker Hiatt and the Bay Area Wonderfest organization:

WHO: UC Santa Cruz astrophysicist Greg Laughlin and NASA planetologist Chris McKay

WHAT: “Real Doomsdays: How Life Could End on Earth”

WHERE: Roxie Theater, 3117 – 16th Street, San Francisco

WHEN: 1:00-2:30 PM, Sunday, August 28, 2011

Image: ASTEP Telescope — Yan Fantei-Caujolle (2009)

Ready or not, HD 156846b, is less than a day away from its much-awaited periastron passage and transit opportunity. Let’s have a show of hands: If it’s dark, if the star is up (RA 17 20, Dec -19 20), and if you’re capable of 1% photometry, then you should be out there on the sky!

Mauro Barbieri, who led the HD 17156b transit discovery back in 2007, has been working very hard behind the scenes to orchestrate observing campaigns in various spots around the globe. This morning, he sent me three nights of baseline photometry from Claudio Lopresti, who has been observing from Italy. These baseline observations show how the increasing air-mass will likely lead to a downward drift in the light curve near the end of tonight’s observing session. If the best-fit prediction turns out to be correct (and assuming, of course, that the planet defies the geometric odds and actually occults the star) then it will be tough to convincingly bag the transit from southern Europe. The party, however, could easily start early…

Observatories in South America have a better chance. For example, at La Silla, there are ~6 hours during the 1-sigma transit window when it is both dark and when the star is at an air mass of less than two. Unfortunately, however, at the moment, the weather forecast for La Silla does not look good. The forecast at Cerro Paranal, however, is excellent.

The most exotic photometry is on tap from Dome C at elevation 3233M in Antarctica, where, barring clouds, rain or snow, the ASTEPS telescope is scheduled to observe. According to the Weather Underground, conditions at Dome C are currently overcast, calm and -88F. (“Feels like -88F”)

Just a few more days until the midpoint of the HD 156846b transit opportunity, which is a tough, but in my opinion, highly worthwhile challenge for small-telescope photometric observers. Given the parent star’s -19 degree declination, the best opportunities are south of the border. There is even speculation that an Antarctic time series will be obtained.

As is often the case, observers worldwide will be struggling with high air masses and twilight conditions. Because of this, it’s very important to obtain baseline photometry of HD 156846 on several nights both before and after the main opportunity. This will help inoculate against instances of transit fever.

And when the data come in? Lubos Brat has set up a globally accessible drop at the ETD, which I highly recommend. Quoting Lubos:

Photometry should be uploaded to TRESCA Observer’s log at http://var2.astro.cz/EN/obslog.php. Please use the target name HD156846 and observers project TRESCA while uploading the data. All data will be aggregated, and everybody can see the joined results at the page:

http://var2.astro.cz/EN/obslog.php?obs_id=1&projekt=TRESCA&star=HD156846

HOW TO START TO USE the Observer’s log:
1) Sign in to the var2.astro.cz server.
2) Click to link Observer’s logs
3) Click to Insert new data (Type object name HD156846 and observer project TRESCA)
4) With first data, your observer’s log will be created.
5) All questions can be sent to brat@pod.snezkou.cz

Here’s to clear skies!

The transit discovery opportunity for HD 156846b is fast approaching, and observations, especially for observers at southern latitudes, are very much in demand for the nights of August 23rd, 24th, and 25th. If you are considering observing, please see Lubos Brat’s campaign page at the Exoplanet Transit Database for more details.

And if you have a portable telescope/CCD combination, and a carbon footprint to match, why not consider a last-minute trip to Tahiti for on-the-spot observations? A quick check on Expedia shows that round-trip direct flights departing from Los Angeles this weekend can be had for a mere USD 1537:

HD 156846b clearly owes its current high-eccentricity orbit to ongoing Kozai oscillations driven by BD-19 4605B, a V=14.1 early M-dwarf binary companion to HD 156846 that lies at a projected separation of ~250 AU:

In all likelihood, HD 156846b is currently near the peak eccentricity of its Kozai cycle. During most of the planet’s history, it orbits with a significantly different inclination, and with a significantly less elongated orbit. Konstantin Batygin made some reasonable assumptions regarding the orbital properties of the companion star, and did an integration using the double-averaging method to show that the planet has likely not had sufficient time to lock its spin period to the pseudo-synchronous value. It’s thus quite likely that HD 156846b rotates with a close-to-primordial day of less than 10 hours (like Jupiter) rather than at the much longer pseudo-synchronous spin period that almost certainly characterizes all of the other currently known transiting planets on significantly eccentric orbits.

I’ve written on a number of occasions about the apparent preference for regular satellite and planetary systems in which the total mass contained in satellites is roughly one or two parts in ten thousand as much as that contained in the primary body. This works for the large population of super-Earth/sub-Neptune planets orbiting nearby stars, as well as for the giant planets in our own solar system. Applying this rule of thumb to HD 156846b suggests that it could be accompanied by a satellite with a fair fraction of Earth’s mass. Such a satellite, if located ~0.01 AU from the primary, would cause barycenter-related transit timing shifts of order 6 seconds, and would likely be dynamically stable against both three-body orbital disruption and tidal orbital decay. Veering into an even more speculative mode, such a satellite, like Titan or Ganymede, would likely have a volatile-rich composition. During the current warm, high-eccentricity phase, it might be spewing out a huge cloud of molecules that just might be visible using high-resolution transit spectroscopy…

But first things first! It’s got to be determined that the planet actually transits before one can responsibly engage in such flights of fancy.

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!