oklo

delineating connections

Time for work!

May 23, 2006

I think we’ve finally got the pieces in place. Its time to really push the collaborative aspect of the systemic project. (1) Aaron’s downloadable console has been tested, updated, and is known to work on Mac, Linux, and Windows platforms. (2) Stefano’s systemic back-end collaborative space is tested and working. (3) Eugenio and Paul are standing by and ready to provide technical support. (4) We’ve got nearly 400 unique users visiting oklo.org every day, and (5) with HD 69830, we have an extremely interesting new system to subject to the analytical and computational power of the distributed oklo community.

The questions to be answered are (1) is the published HD 69830 fit unique? and (2) can we get an independent estimation of the errors?

To get an initial analysis of these questions, I’d like to invite (and encourage!) the oklo community to use the console and the back-end environment to obtain a wide variety of fits to a new set of 21 radial velocity datasets. These data have been uploaded onto the web-based console, and they are also packaged into an updated version of the downloadable console. The data sets include the published HD 69830 data, along with 10 bootstrapped datasets, and 10 model-based synthetic data sets. I’ll write much more about bootstrapping and synthetic data sets in upcoming posts. For the time being, we’re simply interested in finding a variety of fits to these data.

The rest of this post will take the form of a brief tutorial to get you going. We really need as many people as possible to participate in this effort.

First, download the console onto your computer. The link to the downloadable console on the right menu bar gives download instructions. If you’re using a non-US English character set on a Windows machine, you will need to switch to the US English set. (We’ll have a fix in for this shortly.) Launch the console on your computer.

Note that the console application, “systemic.jar” is contained in a directory (folder) that contains several subdirectories. These subdirectories are named “datafiles”, “fits”, and “soundClips”:

When the console is running, select one of the HD69830 data sets from the system menu, and obtain a fit. Once you’ve got the fit, use the “save” button (a new feature of the downloadable console) to save the fit in the “fits” directory. Use the suffix “.fit”, as shown below:

Next, point your web-browser to the systemic back-end. The full url is: http://www.oklo.org/php/login.php

You’ll see the login page. Register as a new user. Once you’re logged in, the environment is designed to be as self-explanatory as possible. In particular, you can upload your fit from your computer, and compare it with other users’ fits to the same system. Go ahead and explore! The back-end contains a number of very interesting features, which we’ll look at in the next post.

20 centimeters per second

May 21, 2006

tubeworms

Source: Nicolle Rager Fuller NSF


HD 69830. What a difference a year makes. Last June, HD 69830 languished in the obscure backwaters of the Henry Draper Catalog. Now, however, like its buddy HD 209458, the star is a star. Google “HD 69830”, and the search returns 660 entries (and growing daily). Google “HD 69831” and (until the crawlers manage to find this post) your search does not match any documents.

In Thursday’s post, we gave an overview of the HD 69830 planetary system, which contains Neptune-mass planets in 8.67, 31.6, and 197 day orbits. Perhaps the most astonishing thing about this discovery announcement is the tiny radial velocity amplitude of the 197 day planet in the model. This object induces a radial velocity amplitude of 2.2 meters per second, with a reported error of only twenty centimeters per second. That’s about the speed your finger moves if you trace it quickly across the title of the discovery paper. This detection required a very quiet star and an extraordinary technique. The Swiss seem to have broken through to the next level.

I wonder what that outer planet looks like. Over at transitsearch.org, I have a Fortran cron job that processes all of the known exoplanets every night to produce updated transit ephemeris tables. In order to predict transit depths, the code needs an estimate for the planetary radius, which in turn requires an estimate of the effective surface temperature. The transitsearch model reports 262 K, just below the freezing point (273 K), suggesting a brilliantly reflective orb swathed in white water-based clouds.

In an upcoming post, I’ll delve into some responsible (and also some irresponsible) speculations about the world beneath those clouds. A responsible viewpoint has planet “d” forming at a larger orbital radius than it currently occupies, and then migrating in to position. In this formation-followed-by-migration scenario, there were plenty of ices available during planetary assembly, and the planet will have a structure (and size) very close to those of Neptune.

internal structure of HD 69830 d

An irresponsible, more provocative scanario has planet d forming in-situ, out of refractory silicate and metallic materials. The final product in this case is a super Earth, smack in the sweet spot of the habitable zone, and endowed with ten Hubble times worth of geothermal activity. But more on that in the upcoming post.

A driving goal at oklo.org is to get our readers beneath the headlines and critically examining what the radial velocities themselves have to say. To do this, we need the data. The Lovis et al. discovery article in Nature contains a link to supplementary material, but when you click on the link you get a .pdf article about hydrothermal vent tubeworms:

huh?

Hmmm. Even if we adopt the most optimistic giant-Earth-like structural models for HD 69830 d, this supplementary material seems to be jumping the astrobiological gun. With tubeworms obscuring the radial velocities, we were compelled to resort to Dexter to extract as many velocities as we could from Figure 2 of the .pdf version of the paper. Eugenio managed to scrape 53 data points off the graph. We were busy using the console to work up fits to these dextered velocities when Darin Rogozzine at Caltech managed to guess the correct link and supplied oklo.org with the url.

In the next post, we’ll have a go at the rv’s. If you want an advance crack at them, they’re now on the web-based version of the console. We’ll get them on the downloadable version tomorrow.

XO-1

May 18, 2006

Image source: designonline.se

Today’s astro-ph mailing contains a paper by McCollough et al. detailing the discovery of a new transiting planet orbiting a sun-like star lying in Corona Borealis. Dubbed “XO-1” — easily the coolest name yet for an exoplanet– this world has a year that lasts 3.941534 days, and a mass roughly 90% that of Jupiter. The temperature at the scalding, toxic, vortex-riddled cloudtops should be a torrid 1083 K. It ain’t no habitable world, folks, but as the fifth extrasolar planet found to transit a relatively bright (V=11) parent star, it’s big news nonetheless. Bright parent stars are great for planetary characterization, because they make detailed follow-up a lot easier to carry out.

The preliminary indications are that this planet, which has a measured radius of 1.3 plus or minus 0.1 Jovian radii, is somewhat larger than expected. Our theoretical models predict that XO-1 should have a radius of 1.05 Rjup if there’s a 20-Earth mass core, and 1.11 Rjup if the planet is core-free. If the large radius is confirmed, then we’ll be faced with the same radius problem that we’re facing with HD 209458 b (as explained in this oklo post from Dec. 2005). An interesting clue may be provided by the fact that the XO-1 parent star, like HD 209458 is not particularly metal rich.

The paper lists four amateur observers as co-authors. Three of them, Tonny Vanmunster, Ron Bissinger, and Bruce Gary, are long-time transitsearch.org participants. P.J. Howell is the fourth amateur co-author on the list. As usual, the photometry that these guys are getting is excellent:

amateur lightcurves of XO-1

Figure adapted from the McCollough et al. paper.

The paper describes in detail how the photometry from the amateur observers was able to effectively leverage and speed up the discovery, and how the ready-made network provided by the small-telescope observers eliminated the need for an expensive robotic follow-up observatory.

Sounds to me like it’s time to crack open a bottle of Hennessey XO. Congratulations all around, guys!

Three Neptunes

May 18, 2006

The published census of extrasolar planets grew by 1.57% today, with the Geneva Extrasolar Planet Search Teams’s announcement in the journal Nature that trois Neptunes orbit the nearby sunlike star HD 69830.

There are a number of reasons why the Swiss team’s paper is interesting. The HD 69830 system seems tailor-made for a detailed dissection with the Systemic Console. This dissection can get the oklo user-base up and running with the new beta version of the Systemic Backend. In addition, the configuration of this system has some fascinating consequences for the theory of planetary formation and evolution. It is definitely worth digging into this story for the next few posts.

First, the nuts and bolts of the announcement. The three planets, named — you guessed it — HD 69830 “b”, “c” and “d”, clock in with Msin(i) estimates of 10.2, 11.8, and 18.1 Earth masses respectively. Assuming that the system is co-planar and is being viewed close to edge-on, this places b and c squarely in the mysterious planetary mass range that falls between the ice giants (e.g. Uranus and Neptune) and the familiar terrestrial planets. Planet d is somewhat more massive, with a net bulk almost exactly equal to that of Neptune. [It’s important to remember that the inclination of any given planetary systems is more likely to be viewed edge-on (i=90 deg) than pole-on (i=0 deg) for the same reason that the Earth has more real-estate within a hundred miles of the equator than within a hundred miles of the poles.]

Orbits of HD 69830 b, c and d

The published orbits of HD 69830’s planets are, however, distinctly unreminiscent of Uranus and Neptune. HD 69830 b orbits in 8.667 days, c orbits in 31.6 days, and d circles the star once every 197 days. All three have modest eccentricities.

For kicks, here’s a .wav format sound file which turns the reflex radial velocity waveform from the star into an audio signal. [Note: you can use the downloadable version of the Systemic Console to produce an audio representation of any planetary system, see this post from last week for the details.] In the sample file for HD 69830, I’ve pitched the innermost planet to a rather piercing 3 octaves above A 440. The period difference betwen the inner and outer planets leads to ~4.5 octaves of pitch difference. Planet d’s contribution can be heard as a bass drone about 2 octaves below middle C.

Indeed, the “chord” produced by these three new planets sounds terrible. Badly out of tune, to be precise. This immediately tells us that ther are no strong mean-motion resonances in the published configuration of planets. The human ear-brain system is good at on-the-fly calculation of whether the mean-motion resonance arguments are in circulation or libration. For example, this .wav file corresponds to a (synthetic) planetary system that is participating in several mean-motion resonances. When compared to HD 69830, it sounds awfully good.

As soon as I saw those periods — 8.667 days, 31 days, 197 days — I raced ahead through the paper draft to see if a photometric check for transits was carried out already by the discovery team… Excellent! There’s no mention anywhere in the paper of an attempt at a photometric search for transits of the planets. This will lend a challenging, high-profile opportunity to transitsearch.org. My guess is that the Geneva team was quite eager to get these three worlds out the door, and did not want to hold up the show with an exhaustive photometric check. There’s a real danger that you’ll wind up getting scooped if you cross all your photometric T’s before publishing your radial velocity-detected planets.

In the next post, we’ll look at what the radial velocities have to say.

Negative Heat Capacity

May 16, 2006

hydrodynamic turbulence in a keplerian disk

Imagine leaving the front door open on a cold day, and having the inside of your house grow warmer as a result. Curiously, that’s exactly how self-gravitating systems such as stars, nascent giant planets, accretion disks, and globular clusters behave. Drawing energy from any of these systems causes them to heat up. The negative heat capacity of self-gravitating systems is one of the most central concepts in astrophysics.

The dynamics of the Keplerian orbit can be used to understand how this works. Imagine a particle initially on a circular orbit around a central star. The particle slams into a cloud of dust. As a result, the dust and the particle both heat up and radiate energy. The particle decreases its velocity and drops into an eccentric orbit with a smaller semi-major axis.

particle going through a cloud

Here’s the key point: the smaller semi-major axis means that the average squared speed of the particle (averaged over an orbit) has increased. The fact that the particle is slow near apastron is more than compensated by the high speed near periastron. Since the particle’s kinetic temperature is proportional to its speed squared, the temperature of the system goes up. In effect, the reserve of gravitational potential energy gets double billed: once to provide the radiated energy, and a second time to increase the kinetic energy of the particle.

It’s a lot like taking a cash advance on your credit card and using half to pay late bills and the other half to buy a set of 22 inch rims for your Escalade. It’s a little sad to observe Nature operating on such a dissolute and spendthrift principle.

backend

May 13, 2006

Bertinoro, AGN and galaxy

Hey all! This is Stefano, one of the Systemic team members. I’m an MSc astrophysics student at the University of Bologna, Italy, and will be transferring to the beautiful city of Santa Cruz next year to start working on PhD.

I just came back this evening (18 pm on the West coast) from the National School of Astronomy, Bertinoro, where I’ve been sent to last week. It takes place in an old little city, surrounded by walls and dominated by a castle. The castle has bedrooms and seminar rooms with frescoes and red carpets. I was sleeping IN the castle, when I woke up I could see the green planes of the pianura padana extending for acres and acres, and little rocky houses of farmers. The city is famous for its wine. Galla Placidia, daughter of the Roman emperor Theodosius, drinking a glass of the sweet white wine albana purportedly said to the wine “sei degna di berti in oro” (you deserve to be drank in a golden glass), from which the name of the city “Bertinoro” comes. The city itself is full of little places to drink wine (the amazing Sangiovese) and other kinds of alcoholic beverages, which of course we visited often, more than once a night! Whoever thinks scientists are grey, sad people should have come to one of these crazy nights.
That said, it was my first astrophysics school, and I felt so young and unexperienced! Everyone was working on their PhD, and was brilliant, accomplished, and just plain cool — at least to my eyes. I was feeling really out of place in the midst of these amazing minds talking about galaxies and AGNs citing models and theory with apparent ease.

Thankfully I soon realized that these scientifical “hierarchies” don’t really stop you to have your say and give your, even small, contribution! And anyone, from a last-year student like me to the famous astrophysicist, is collaborating in an amazing community to help develop our knowledge of where we are and what’s been before us.
The astronomer Edwin Hubble
All this to introduce the systemic Backend. The systemic Backend lets you have your say in the field of extrasolar planets!

Thanks to the systemic console, you can fit radial velocity data taken by real astronomers and as easily as possible try to discover the evidence of unseen planets around distant stars. And it doesn’t matter if you’re an astronomer, an high school student or an astrophile out of budget for a telescope: if your findings are consistent with the data and explains the observations better than before, you’ve done it!

The systemic backend lets you share your results with other enthusiastic people, showcase your results and interact with your fellow colleagues, just as you would do on a myspace-like network. You can upload the fits saved from the console online from your account, and have other people enthusiastically comment or bash your findings. You might be doing real astrophysics, while knowing other people.

Try out the beta version of the system now, help us iron the bugs and the improvements to make!

The systemic console and backend will be part of a bigger picture — Greg will be talking about it in a future post.

More soon,
Ste

downloadable console now available

May 11, 2006

chain link fence

The systemic team is pleased to announce the release of an updated systemic console. Thanks to Aaron Wolf for coding it into reality, and to Eugenio Rivera for troubleshooting the platform-specific installation issues.

Downloadable Console: systemic.zip

The new version of the console has been successfully tested on multiple Mac, Windows, and Linux machines. Specific download instructions and Java information for the three different platforms are available on our new downloads page.

We’re very interested in feedback from users. If you are able to download the console, or if you have problems, please register as a user and let us know via the comment space for this post. We need as much specific information as possible regarding your version of Java and your operating system.

Finally, if you are using a Windows-based browser, and you do not see the following links on the sidebar to the right:

screenshot of systemic on safari

You may have to scroll all the way down to the bottom of the window to see the links.

Thanks, and have fun fitting!

— The Systemic Team

Sonified

May 11, 2006

Many systemic readers have not yet experienced the thrill of fitting planetary systems with the systemic console because the console fails to properly launch in their browser. The standard refrain for the last several months has been, “We’re working on it…”

Tomorrow, we’ll be releasing an upgraded version of the console in downloadable form. We’ve tested this version on Mac OSX, Windows, and Linux platforms, and we’ve gotten it to work on all three.

The downloadable version of the console will contain a number of new features, including a sonification button that brings up the following window:

console sonification controller

Sonification takes the N-body initial condition corresponding to the current positions of the console sliders and performs an integration of the equations of motion to produce a self-consistent radial velocity curve for the star. The radial velocity curve is then interpreted as an audio waveform and the resulting audio signal is written to the .wav format. You, the user, choose the duration of the integration and the audio frequency to which the innermost planet’s orbital frequency is mapped (440 Hertz, for example, corresponds to the A below middle C). A simple envelope function is also provided in order to avoid strange-sounding glitches associated with sharp turn-on and turn-off transients.

A single planet in a circular orbit produces a pure sine-wave tone. Very boring. The introduction of orbital eccentricity adds additional frequency content to the single-planet signal, and produces a variety of buzzing hornlike timbres, depending on the chosen values for the eccentricity and longitude of periastron. (For example, here are tones corresponding to keplerian orbits with [1] e=0.5, omega=90 deg; [2] e=0.9, omega=150 deg; and [3] e=0.9, omega=312 deg).

Hewitt, Conceptual Physics, p. 284

I scanned the above photo from my groovy 1974 edition of Conceptual Physics. Author Paul Hewitt is using a pipe to generate what looks to be a 420 Hz tone. The oscilliscope trace indicates that the pipe is producing both a fundamental frequency as well as a first overtone. A similar effect can be had with the console by adding an additional planet and sonifying the resulting radial velocity curve. For example, a quick fit to the 55 Cancri data-set generates a flute-like timbre that arises primarily from the near 3:1 commensurability of the orbits of the 14.65 and 44.3 day planets. Here’s a detail from the waveform:

55 Cancri Waveform

And here’s the .wav format audio file corresponding to the 55 Cancri fit.

Systems in 2:1 mean-motion resonances can generate some very weird audio waveforms. Oklo favorite GJ 876 was the first (and is still by far the best) example of a 2:1 resonant configuration. GJ 876’s audio signal, however, is pretty lackluster (the .wav file is here). This is because the system is so deeply in the resonance that the waveform has a nearly invariant long time-baseline structure. Much more interesting from an audio standpoint, are the newly discovered 2:1 resonant systems HD 128311 and HD 73526. With the console, one can work up a quick fit to the HD 128311 data set which has one 2:1 resonant argument in circulation and the other in libration.

a fit to the 128311 system

The long-term orbital motion is completely bizarre (as shown by this .mpeg animation) and the corresponding audio file [.wav file here] has a certain demented quality. The signal definitely evolves on longer timescales than shown in this snapshot of the fit:

waveform for hd 128311

Results-oriented planet hunters should definitely be asking, “Does sonification have any scientific utility?”

Maybe. I’ll be posting more fairly soon on why we think sonification might be useful, but here’s a straw-man example. Call up the data set for HD 37124 on the console. There are a lot of ways to get an acceptable orbital model for this system, including a panoply of far-out configurations like this one:

hd 37124 alternate orbital configuraton

The corresponding waveform looks like this:

hd 37124 alternate orbital fit

If we sonify the fit, we can literally hear the system going unstable (.wav file here). The question is, can a trained ear “hear” signs of instability well before the actual drama of collisions and ejections occurs?

observations of observations

May 10, 2006

water glass on a placemat

About two weeks ago, I wrote a post about the Dexter application which is available from the ADS website. Dexter extracts digitized data from image files such as .gifs or .jpgs. We’ve been using it to extract radial velocity data sets for planets that have been published without accompanying radial velocity tables. Our goal is to soon have data sets for all of the planets published to date.

That’ll make oklo.org your site for one-stop shopping.

Eugenio will soon be posting a very interesting discussion of the technique and pitfalls of using Dexter to extract radial velocity data sets. In the meantime, I’ve added a sample dextered data set to the systemic console:

dextered selection for hd50499

The data set HD50499d contains velocities digitized from a figure in the California -Carnegie Planet Search Team’s recent ApJ paper, (entitled Five New Multicomponent Systems). This paper also contains the actual radial velocity data for HD 50499 in tabulated form. This actual data is available on the console by clicking HD50499 (i.e. without the “d” for Dexter).

Try using the console to fit to both the actual data and the Dextered data. You should find that for this particular system, the fits are nearly the same. In this case, Dexter did a very good job of extracting the velocities.

fit to the hd50499 radial velocity data set

The HD 50499 system clearly harbors at least two satellites. One of them has a very long period, considerably longer than 10,000 days. The way to get the console to fit this system is to fix the outer planet period at 10,000 days, while minimizing on the other orbital parameters.

Approach

May 8, 2006

Nevada Test Site 1957

Priscilla, Nevada Test Site, 1957 (US National Archives, see Michael Light’s 100 Suns)

Today was a bright spring day in California, and now, as I write, the night air coming through the window is drunken, redolent with the scent of a million flowers.

Spring is also arriving on HD 80606 b, but with devastating ferocity. This morning, HD 80606 b’s parent star, which resembles our Sun in intrinsic size and brightness, subtended more than two degrees as it rose above the horizon. It loomed, angry and white, with more than four times the angular size of a full moon. It grew perceptibly larger as the day wore on. Above the vortical scream of the cloud tops, it was scores of degrees warmer today than yesterday.

Last Friday, HD 80606 b fell through the imaginary boundary given by the size of Mercury’s orbit. Midsummer — HD 80606 b’s periastron passage — will occur on Friday of this week. At this moment, the planet will plunge to within 6 stellar radii, and the furnace of the stellar surface will stretch across 19 degrees of sky.

80606 position today

Five days later, on its way back out to apastron, the planet will perforate the plane containing the line of sight to the Earth. At this moment, there’s a possibility (a 1.7% possibility to be exact) that a transit can be observed.

a selection from the current transit table

Varkaus

May 7, 2006

Varkaus

Image source: NASA Visible Earth

For most people, a mention of Finland brings to mind snow, lakes, conifers, Nokia and Linus Torvalds. Here at oklo.org, however, we hear Finland and we think of top-drawer amateur astronomers. In 2000, the Finn Arto Oksanen was the first amateur to observe the HD 209458 b transit. More recently, both Oksanen and countryman Pertti Paakkonen have contributed a number of observations of TrES-1 and other stars to transitsearch.org. Finnish IP addresses are consistently among the top traffic generators on the Systemic Console.

Now, Veli-Pekka Hentunen and the Warkauden Kassiopeia ry (the Astronomical Association of Varkaus) join the ranks, with a fine observation of last week’s TrES-1 planetary transit. Their lightcurve, shown just below, was obtained with a Meade 12-inch LX200 telescope and a cooled SBIG ST8-XME CCD camera:

April 30, 2006 TrES-1 Transit Photometry

The photometry shows a tantalizing hint of the starspot activity that is known to characterize TrES-1. A more detailed analysis will be needed to see whether the small in-transit bump has statistical significance. As discussed in a previous post, HST has shown that starspot activity on TrES-1 can produce stange-looking features in the light curve:

transit of TrES-1 obtained with HST

Hentunen and his colleagues have constructed a very impressive facility at a dark (and from the look of things cold) site in the Finnish interior. Information in English regarding both their observatory and their scientific work can be found at the Taurus Hill Observatory Website.

varkaus observatory

Taurus Hill Observatory

Hentunen and friends will be able to kick back and take it easy for the next few months because it doesn’t really get dark at their location during the Summer. In the Winter, however, when the weather is clear, they’ll have the opportunity to make long-duration time-series photometric observations of stars near the polar cap. For example, they’ll be in awesome position to snag oklo.org favorite HD 80606 (+50 deg declination) during its Dec. 26, 2006 transit opportunity.

tilt shift

May 5, 2006

When it comes to planetary systems, our own eclectic gathering of eight (or nine, or ten) planets is by far and away the best characterized and best understood. We’ve flung space probes past all of the planets in the solar system, and we’ve directly, physically, probed four of them (in addition to two major satellites). We know their orbits to stunning, uncanny precision. We have actual pieces of Vesta and Mars under minute scrutiny in our laboratories. We have coffee table books overflowing with detailed photographs our our home worlds.

a detailed view of a surface feature on a life-bearing habitable planet

I can step right outside my door and photograph the surface details of a habitable terrestrial planet.

Sadly, we don’t have anything rembling this wealth of detail when it comes to extrasolar planets. Most of our information is encapsulated in the tables of radial velocity measurements accessible to the Systemic Console. Much of what I write about in these posts, and indeed, most of what we can infer about these distant worlds, must be squeezed from sparse columns of times, velocities, and velocity uncertainty estimates.

(more…)

Where we’re at

May 4, 2006

banana leaf

The systemic collaboration website has now been on the air for six months. Traffic has been increasingly steadily. By the end of April, oklo.org has been averaging 250 visitors a day, with a total of 1661 unique “real” visitors for the month. (This brings to mind a philosophical question: if a tree falls in a forest, and only robots, worms, or replies with special HTTP status codes comment, did it make a sound?)

april showers

The Systemic Team is enthusiastic about a number of improvements that will be coming on line very soon. Here’s a rundown of what to look for during May:

1. Aaron Wolf is putting the finishing touches on the next release of the systemic console. The updated version will have a number of subtle improvements to the existing controls, and will have several completely new features, including a sonification utility and a folding window. Sonification allows the user to create a .wav format audio file of the radial velocity waveform produced by a given configuration of planets orbiting a star:

console sonification controller

As we’ll discuss in future posts, the ability to “listen” to dynamical systems provides a startlingly effective and completely novel way to evaluate the long-term orbital stability of a hypothesized system of planets. For example, when a configuration of planets is stable, one generally gets a sound with a steady timbre: [example 1.5 MB .wav file corresponding to a stable planetary system].

On the other hand, when a configuration of planets is unstable, the radial velocity waveform of the star can get pretty crazy, which can lead to an inifinite variety of very weird sounds: [example 0.5 MB .wav file corresponding to a dynamically unstable planetary system].

2. Stefano Meschiari, who will be transferring as a graduate student to the UCSC graduate program this Fall (yes!), has developed a PHP-based collaborative environment for the systemic project. Think flickr, think myspace, think the Extrasolar Planets Encyclopedia, think seti@home, and think effective scientific collaboration all rolled into one. I’m not kidding, folks, it’s amazing.

G.I. No

May 2, 2006

planet formation is not yet in focus

In a comment on yesterday’s core-accretion post, a reader anticipated that all is not hunky-dory with the core-accretion scenario for the formation of the gas giant planets in our solar system, and asked if is there any support for Alan Boss’ disk instability model. In the Boss model (described here by Alan, see also the buff 137-strong citation list) gas giant planets condense directly out of the protostellar disk as the result of gravitational instability in the disk.

The handy thing about an extrasolar planet web log is that you can express your opinions on the formation of extrasolar planets. In my opinion, there are a number of very serious difficulties with the hypothesis that gravitational instability is the dominant mechanism for giant planet formation. Here are three:

(1) In order to have gravitational instability work in the manner shown in the fragmentation simulations, you need to start with an axisymmetric disk that has a sufficiently low value for the Toomre Q parameter. That is, in order for the initial conditions in the successful Boss simulations to be valid, a growing protostellar disk needs to remain completely stable with respect to low-level non-axisymmetric disturbances until BOOM, it reaches a threshold Q value where it is prone to spiral instabilities that exponentiate on a near-orbital timescale.

In reality, I think that a growing (or alternately, a cooling) protostellar disk will be prone to low-level spiral disturbances that steadily transport mass inward and angular momentum outward, allowing the disk to avoid ever reaching the state where instabitilies can grow on an orbital timescale. (For a bulked-up version of this argument, see the papers (one and two) that I wrote with Vladimir Korchagin and Fred Adams on this issue).

(2) The core accretion model provides a very natural explanation for both the planet-metallicity correlation, as well as the paucity of Jovian-mass planets found in orbit around low-mass M type stars. The gravitational instability model predicts that the incidence of Jovian-mass planets should be independant of both the stellar metallicity and the parent star mass.

(3) There’s simply no way that the gravitational instability model can produce the 72 Earth Masses of heavy elements in HD 149026 b. (See this paper for a thorough discussion).

To be fair, there are also some thorny problems associated with core-accretion. In the next few posts of the giant planet formation series [1, 2, 3, 4 and 5] that we’ve been running, I’ll describe these in more detail.

Planet Orbiting a Brown Dwarf

Photo credit: ESO (VLT/NACO)

Another important point to stress is that Alan’s simulations certainly aren’t in error in the sense of being computationally wrong. It’s just that I don’t agree with the generic validity of the initial conditions. Indeed, I do think that gravitational instability sometimes plays a role in giant planet formation. The best example is probably the 5 Jupiter-mass companion to the brown dwarf 2M1207 discovered by Chauvin et al. last year. (The ESO press release on this system is here.) I see no way in which the core-accretion process could have made any headway at 55 AU in this particular system.

Finally, GJ 876, which is by far the best RV-characterized extrasolar planetary system, provides a tough challenge to both the gravitational instability and the core-accretion theories. The inner 7.5 Earth Mass planet in the GJ 876 system is almost certainly an accreted protoplanetary core (regardless of whether it formed in-situ, or migrated from a larger radius). It would be nearly impossible to form lil’ D via gravitational instability. The outer two planets, on the other hand, contain more than three Jupiters worth of mass, and stand in embarrassing conflict with the notion that core-accretion process is difficult to carry through to Jovian-mass completion in red-dwarf protostellar disks.

I would very much like the 411 on what went down in GJ 876’s protostellar disk.

Oligarchic Growth

April 30, 2006

[A continuation of posts 1, 2, 3, 4 and 5 on the formation of Jovian planets.]

nucleus of a comet

Phoebe (photographed by Cassini). The cores of the giant planets were built from millions of these objects.

In the primitive solar system, ice formed at the expense of water vapor wherever the temperature was lower than 150 degrees above absolute zero. The 150 K isotherm in the disk was located roughly at Jupiter’s current distance from the Sun, and is known colloquially as the “snowline”. Just beyond the snowline, the planetesimals achieved their greatest ability to rapidly build themselves into larger bodies. It was cold enough for ice to be stable, yet close enough for the overall density of the disk to be high, and the planetesimals were prone to frequent collisions.

Low-speed planetesimals collisions were sticky events that can be simulated to wonderful effect with fast desktop computers. Imagine two Michelin Men, each loosely glued together, heading toward each other in a headlong embrace. A spare tire or two is lost in the collision, but one remains with a jumbled, combined mess. The first planetesimal collisions that seeded Jupiter’s core looked something like that.

Thousands of years passed, punctuated by these (initially) slow-motion catastrophes. The planetesimals gradually become fewer in number and individually larger. Those that experienced a few extra collisions in the beginning were able to take advantage of their burgeoning self-gravity to collide more often, and were thus able to grow faster (sound familiar?) Inevitably, a few big winners, called oligarchs, began to emerge. These oligarchs, with radii thousands of miles across, were massive enough to simply haul in their neighboring small-fry kilometer-sized brethren. The more an oligarch gets, the more it wants, and the farther its reach. A runaway occurs. Somewhere in the current vicinity of Jupiter, 4.54 billion years ago, an oligarch reached an Earth mass.

three stages of core accretion

What would this oligarch have been like? Certainly, it would be something that we would have little difficulty calling a planet in distress. All riled up. A five hundred mile wide core of molten iron, surrounded by perhaps a thousand miles of pressurized plastic rock, not unlike the mantle of the Earth. Above that, thousands upon thousands of miles of hot, pressurized, water ocean. Floating atop the ocean, a tarry layer of hydrocarbons, perhaps with a smell like hot asphalt, and with an indistinct surface merging into a choking thick noxious atmosphere.

The atmosphere bulks up fast. Liberated hydrogen and helium gas bubbles up from the layers of denser materials in the interior. The oligarch passes several Earth masses in size, and grows massive enough to grab gas directly from the disk. Meanwhile, new planetesimals are arriving all the time. Kilometer-sized projectiles streak through the exosphere, exploding as they slam into the atmosphere. The unsettled skies are continuously ablaze with meteors. The temperature rises, becoming so warm that the atmosphere glows a dull coal-red in the darkness of the nebula.

When the growing oligarch, now a full-fledged protoplanet, reaches seven or ten times the mass of the Earth, it is pulling in gas as fast as it can. The atmosphere has swelled and bloated to a thickness of literally hundreds of thousands of kilometers. The gas glows fire-engine orange, and pours infrared light out into space. This radiation is accompanied by slow settling of the lower layers, providing room at the top for more gas to flow in.

Finally, the growth experiences its first taste of a slowdown. The consumption of rocky icy planetesimals has been so rapid that the total reservoir of these objects in the annular region of the nebula occupied by the planet is depleted. The planet has managed to effectively clear out the solid material from a vast ring-like region of the nebula. The region around the protoplanet still contains vast quantities of gas, but this gas is prevented from accreting onto the bloated planet. The planet can only add new gas as fast as the older gas is able to settle, and the gas can only settle by radiating and cooling.

For the next million years, the planet grows slowly. Gas flows in from the disk as fast as the cooling of the planet will allow, and as the mass of the planet increases, the annular ring of the disk from which planetesimals can be drawn also slowly increases in size…