Monthly Archives: August 2009

Lobbying for Alpha Cen

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Philippe Thebault sent me a link to an article on the Alpha Centauri planet search published earlier this month in the Frankfurter Allgemeine Zeitung. The text is in German, but the Google translator does a passable job of getting the gist across.

I got my first inkling of the Geneva Planet Search’s Alpha Centauri campaign through Lee Billings’ article in Seed Magazine. (See this post). In the Frankfurter Allgemeine article, Francesco Pepe gives further details — Alpha Cen B is one out of ten stars that are receiving special scrutiny for terrestrial planets at HARPS. They are getting one observation every two weeks, meaning that the star is being hit roughly one out of every two of their planet search nights:

“Allerdings müssen wir uns Harps mit anderen Gruppen teilen”, sagt er. Zudem ist Alpha Centauri B nur einer von zehn Sternen, die sie auf erdähnliche Planeten absuchen wollen. “Aber alle zwei Wochen schauen wir damit auf Alpha Centauri, und das Gerät ist sehr effizient.”

This quote implies that my speculations regarding the Geneva team’s data collection rate on Alpha Cen B were somewhat overheated. Instead of getting 100 ultra-high-precision HARPS velocities per year, it looks like a more realistic estimate of their current rate is 25 velocities per year. Since signal-to-noise increases as the root of the number of observations, this means that the minimum mass threshold for Alpha Cen Bb at any given time is approximately doubled relative to my estimates at the beginning of the Summer. Instead of arriving at 2.5 Earth masses in the habitable zone a bit more than a year from now, they’ll be at roughly 5 Earth masses.

Now nobody likes backseat drivers. As the saying goes, “theorists know the way, but they can’t drive”, and theorists have had a particularly dismal record in predicting nearly everything exoplanetary.

But nevertheless, I’m urging a factor-of-four increase to that data rate on Alpha Cen B. I would advocate two fully p-mode averaged velocities per night, 50 nights per year. I know that because Alpha Cen B is so bright, the duty cycle isn’t great. I know that there are a whole panoply of other interesting systems calling for time. It is indeed a gamble, but from the big-picture point of view, there’s a hugely nonlinear payoff in finding a potentially habitable planet around Alpha Centauri in comparison to any other star.

During the next few months, it’s inevitable that one of the numerous Super-Earths that have been turning up in the radial velocity surveys will be announced to be observable in transit (see, e.g. this post). When that occurs, we’ll effectively have had our last first look at a truly new category of planet — the logarithmic mass interval between Earth an Uranus is currently by far the largest among the 70-odd planets that have accurately determined radii. My own guess is that the emerging population of super-Earths will be better described as a population of sub-Neptunes. That is, the transit depths will indicate compositions that are largely water.

So if 5-Earth mass planets turn out to be primarily water-based rather than rock-based, it’s (in my mind) an argument in favor of cranking up the data rate on Alpha Cen B. There were no structurally substantial quantities of water in the Alpha Cen planet-forming environment. If we’re seeing sub-Neptunes rather than super-Earths in the HD 40307, Gliese 581, et al. systems, then the odds are heightened that any planets orbiting Alpha Cen B are less than 2 Earth masses. There’s no payoff in tuning your Alpha Cen B strategy for sub-Neptunes. Finding truly terrestrial-mass planets will require paying full freight.

In the early nineteenth century, the detection of stellar parallax was a problem fully equivalent in both scientific excitement and prestige to the modern-day detection of the first potentially habitable extrasolar planet. I think it’s worth noting that the prize of discovery of the first stellar parallax went not to the eminently capable (but overly cautious and slow-moving) observer who accumulated data on the best star in the sky, but rather to an observer who focused on a rather obscure star in the constellation Cygnus.

Here’s a link to the article, “Thomas Henderson and Alpha Centauri” by Brian Warner of the University of Cape Town.

Keep hope alive

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At first glance, through a telescope, Venus looks like it just might be habitable. Earth-like mass. Earth-like size. Close to the Sun, yes, but the white clouds reflect most of the incident sunlight.

A lifetime ago, it was perfectly reasonable to imagine that swampy Devonian-era conditions prevail on Venus. In his remarkable book, Venus Revealed, David Grinspoon recounts an expert opinion voiced by the Nobel-prize winning chemist Svante Arrhenius in 1918:

The humidity is probably about six times the average of that on Earth. We must conclude that everything on Venus is dripping wet. The vegetative processes are greatly accelerated by the high temperature, therefore, the lifetime of organisms is probably short.

There’s definite allure to the watery Venus meme. C.S. Lewis does an interesting treatment in Perelandra. I’ve always liked Ray Bradbury’s vision of Venus in The Long Rain:

The rain continued. It was a hard rain, a perpetual rain, a sweating and steaming rain; it was a mizzle, a downpour, a fountain, a whipping at the eyes, an undertow at the ankles; it was a rain to drown all rains and the memory of rains. It came by the pound and the ton, it hacked at the jungle and cut the trees like scissors and shaved the grass and tunneled the soil and molted the bushes. It shrank men’s hands into the hands of wrinkled apes; it rained a solid glassy rain, and it never stopped.

Frustratingly, just as the prospect of interplanetary travel was evolving into a concrete engineering problem, Venus’ spoilsport nature was revealed. In the late 1950s, Venus was observed to be glowing brightly in the microwave region of the spectrum (see, e.g. this article). The immediate — and ultimately correct — interpretation is that the microwaves are the long-wavelength tail of blackbody emission from a lead-melting surface, but at that time, the situation was not entirely clear. Even as the first astronauts were orbiting the Earth, one could optimistically chalk up the Venusian microwaves to phenomena in its ionosphere. (See, for example, this 1963 review). The space race, the cold war, the whole twentieth century would have unfolded very differently had Venus been Earth-like beneath its inscrutable clouds.

August 27, 1962 — Launch of an Atlas Agena B with Mariner 2: Destination Venus.

The microwave radiometer on Mariner 2 brought a quick end to fading hopes of a habitable Venus. Here’s the link to the baleful 1964 summary of the mission results. With the equally bleak assessment of Mars courtesy of Mariner 4, genuinely habitable extraterrestrial worlds in the solar system were a no-go. The space race fizzled out. Now we’re looking at retro-futuristic voyages to the Moon in the 2020s and dreaming of Alpha Centauri.

Speaking of which, two recent theoretical papers have come down on the pro-planet side of the ongoing terrestrial-planets-orbiting-Alpha-Centauri debate. In an article that’ll be on astro-ph within the next day or so, Payne, Wyatt and Thebault suggest that outward migration of planetary embryos in the Alpha Cen B protoplanetary disk can provide a mechanism for circumventing the problems associated with habitable planet formation in the binary environment. In the second paper (posted to astro-ph earlier this year) Xie and Zhou argue that a modest inclination between Alpha Cen A’s proptoplanetary disk and Alpha Cen B’s orbit can also tip the balance quite significantly in favor of terrestrial planet accretion around A (and with similar logic applying to planet formation around B).

Last November, in the comments section to the Alpha Cen Bb post, I was asked:

What do you think the odds now are of there being a planet somewhere in the Alpha Centauri system?

I answered:

Hazarding a guess, I’d say 60%. A better answer might be, “High enough to warrant mounting an inexpensive (in comparison to most other planet-search efforts in operation or contemplation) ground-based search.”

I’d like to raise those odds to 68.3%.

Retrograde

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Turn your world upside-down and you’re looking at a very different planet. Antarctica, ringed by the vast exapse of the Southern Ocean, draws all the attention. Viewed from beneath, I think Earth might better resemble the habitable planets that are out there in the local galactic neighborhood, waiting to be found.

Speaking of upside-down planets, last week brought a curious back-to-back development. Three separate papers (one, two, three), posted to astro-ph on two successive days, presented strong Rossiter-McLaughlin-based evidence that both WASP-17b and HAT-P-7b are on severely misaligned, potentially retrograde orbits around their parent stars. Winn et al.’s data for HAT-P-7 are a near-exact inversion of the familiar sawtooth produced by well-behaved hot Jupiters such as HD 209458b or HD 189733b. It would appear that Dr. Kozai exerted a heavy hand during HAT-P-7b’s early days:

The HAT-P-7 system is alarmingly compact. The star is roughly 80% larger than the Sun, and the orbit of the transiting planet is only about four times larger than the star itself. It looks, in fact, when drawn to scale and tilted to the proper inclination, like a schematic cartoon of a transiting system.

Remarkably, HAT-P-7 lies in the Kepler field, and was the subject of a teaser-like “brevia” published in Science a few weeks ago. In the folded Kepler light curve for HAT-P-7b it’s easy to see the phase function of the orbiting planet, along with the primary transit and the secondary eclipse. The well-resolved depth of the secondary eclipse indicates that the spacecraft is performing up to spec and will be able to detect the transits of Earth-sized planets orbiting Sun-sized stars.

Interestingly, a near-perfectly inverted Rossiter-McLaughlin waveform doesn’t necessarily mean that the planetary orbit is retrograde, but rather only that the angle between the planet’s orbital angular momentum vector and the sky-projected spin axis of the star is close to 180 degrees. If the star’s polar axis is pointing nearly in our direction, then the planetary orbit is close to polar. The small vsin(i) for HAT-P-7 provides a piece of evidence that HAT-P-7b’s orbit might in fact be close to polar.

HD209458set on HD 209458b

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During my visit to the Paris Observatory earlier this summer, Alain Lecavelier showed me the work that he and David Sing and their collaborators have been doing to get a better handle on the atmospheric conditions on HD 209458b. Using the STIS spectrograph on HST, they’ve obtained both medium-resolution and low-resolution visible-wavelength absorption spectra of starlight shining through the atmosphere of the planet as it transits the parent star.

HST is sensitive enough to allow startlingly detailed portraits of “sunsets” that took place back in the mid-1850s. Here’s a reworking of Figure 1 from Sing et al. (2008):

Illustrator-editable .pdf of above with title and source.

Sing et al. manage to do a good job of matching the features in the spectrum. The big absorption spike in the orange is due to the presence of atomic sodium. Their atmospheric models also include Raleigh scattering by hydrogen molecules, a temperature inversion in the atmosphere, condensation of sodium sulfate on the planet’s night side, and the presence of titanium and vanadium oxide in the atmosphere. (Titanium oxide can be invoked to play a big role in modulating the visual appearance of hot Jupiters for much the same reason that it’s used as an opacifier in ordinary paint.)

With a detailed atmospheric model in hand, it’s possible to calculate both the color of the sky and the color of HD 209458b at various sight lines through the air column. David and Alain did exactly that, and have made an animation from the perspective of an observer in an asbestos-coated balloon drifting nightward across the terminator. The effect is reminiscent of a Turrell skyspace:



Here’s a link to their French-language press release. According to the inimitable google translator, “star at bedtime absorption is cyan”

Inside Jupiter

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Image Source: ESO

Two weeks ago, I spent a day with a team from Flight 33 productions working on an episode for the ongoing Universe series on the History Channel. Over the past several seasons I’ve appeared on occasional episodes of this show, either in connection with extrasolar planets or with regards to the ultra-distant future. The topic of the latest episode was extraterrestrial liquids, running the gamut from the (relatively) familiar and accessible — azure oceans on TPF dream planets — to the bizarre: vast expanses of liquid metallic hydrogen in the interiors of giant planets and hypothesized superfluids miles beneath the surfaces of neutron stars.

How can one get liquid metallic hydrogen’s essence across during a brief segment of commercial television? By comparison, conveying the atmosphere of a Jovian planet is quite easy. Towering sunlit clouds. The chilly deluge of the Jovian rainstorms. The awful smell. Liquid metallic hydrogen, on the other hand, couldn’t be any more alien. It exists at typical pressures of ten million atmospheres. In Jupiter, there are hundreds of Earth masses of the stuff, all at temperatures several times hotter than the surface of the Sun. A handful of the deep Jovian interior, materialized somehow on the surface of the Earth for the sake of demonstration, would instantly explode with fully counterproductive newsworthy effect.

The analogy I came up with is provided at a heavily congested bumper car rink in which the bumper car drivers are free to jump between cars. In this model, the cars represent the heavy protons and the drivers represent the much lighter electrons. Arrangements were made to utilize the Santa Cruz Beach Boardwalk for the filming of this mock-up of the Jovian interior. The logistics of the event drew together a rather diverse range of participants, and the event snowballed to make the front page of the Santa Cruz Sentinel (link to the article).

It’ll be interesting to see how things turned out when the episode airs.

the pause that refreshes

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The systemic backend will be offline for a period of time starting on Monday Aug. 03. We’re pulling our server from its current rack space. When it comes back on line, it be on the UCSC network. The database has been fully backed up, so despite the temporary unavailability, there’ll be no loss of data. The oklo.org web log will continue uninterrupted.

When we return, we have several goals in mind for the backend. First, there will be support. Several UCSC physics and computer engineering undergrads will be joining the systemic team, and will be focused on improving the backend and keeping it running smoothly. Due to time constraints, and despite best efforts, we just weren’t able to keep up with this ourselves. Second, the backend will maintain improved integration with the console as the console develops, and will be more focused on scientific tools rather than the web 2.0 social network aspect. Third, we’ll be introducing features geared toward the use of the console as an instructional tool in astronomy, physics and astrobiology classes.