by Paul Gilster | Apr 12, 2007 | Exoplanetary Science |
Another discovery thanks to transits. The atmosphere of the exoplanet HD 209458b has been found to contain water vapor. And while that’s not unexpected, the effectiveness of the transit method in making the find underlines how significant are the occasions when a planet passes in front of its star as seen from Earth. Studying the infrared spectrum, as Travis Barman did at Lowell Observatory, shows the apparent signature of water vapor absorption when compared to the visible spectrum.
But don’t expect an ocean world here. The planet involved orbits its star every three and a half days; HD 209458b is, in fact, a ‘hot Jupiter,’ its upper atmosphere heated to temperatures as high as 10,000 degrees K. The planet is doubtless losing thousands of tons of material every second as it vents gases into the incendiary environment so near its primary.
Nonetheless, finding water vapor does provide confirmation of theories that suggest almost all extrasolar planets have water vapor in their atmospheres. “We know that water vapor exists in the atmospheres of one extrasolar planet,” says Barman, “and there is good reason to believe that other extrasolar planets contain water vapor.”
Barman compared his theoretical models with visible and infrared Hubble Space Telescope data on the star collected by Harvard graduate student Heather Knutson. The paper on the work is Barman, “Identification of Absorption Features in an Extrasolar Planet Atmosphere,” accepted for publication in The Astrophysical Journal (preprint available). Also see Dennis Overbye’s story in the New York Times, which introduces questions regarding some of Barman’s data in connection with earlier observations.
by Paul Gilster | Apr 11, 2007 | Exoplanetary Science |
Years ago I wrote a story called ‘Rembrandt’s Eye,’ using as background a planet whose foliage was predominantly red. The story, which ran in a short-lived semi-pro magazine called Just Pulp, came back to mind when the news from Caltech arrived. Researchers at the Virtual Planetary Laboratory there now believe that Earth-type worlds may have foliage that is largely yellow, orange or, as in the case of my planet, red. The green of Earth’s plant life is anything but a universal standard.
This interesting conclusion emerges from computer models designed to provide pointers for the future search for plant life on exoplanets. After all, astronomers will need to know what they might see in the spectra we’ll one day be able to harvest from space-borne observatories. Ponder everything that’s involved, from the color of the main sequence primary star to the aquatic habitats of aqueous plants. The search involves the way photosynthesis might occur under varying conditions, with the filtering effect of planetary atmospheres as a major player.
Image (click to enlarge): This graph shows the intensity of light by color (wavelength) that reaches the surface of Earth-like planets orbiting different types of stars. From hotter to cooler, the star types are F, G, K, and M. Our Sun is a G2 star (yellow line). A planet orbiting an F2 star (red line) has more blue light at the surface, whereas Earth and the K2 star planet receive more red light. Planets around M stars receive much less visible light but much more infrared light. Atmospheric gases such as ozone (O3), oxygen (O2), water vapor (H2O), and carbon dioxide (CO2) absorb light at specific wavelengths, producing the pronounced dips that astronomers might someday detect. Then in the diagram’s horizontal axis, mark the wavelengths from 0 to 0.4 microns as UV, 0.4 to 0.7 as visible, and longer than 0.7 as infrared. Credit: NASA.
Also affecting foliage color are factors like stellar flare activity, the chemical reactions stellar radiation causes in the atmosphere, the role of ozone, carbon dioxide and water vapor, the amount of water available and the quantity of light that reaches the surface. As the simulations ran, a wide variety of habitable scenarios came into play, including one that removed most of the ozone that shields against surface radiation.
Surprisingly, survivable habitats may occur even in places like this in a ‘sweet spot’ below the surface of the water, says Victoria Meadows (Caltech VPL):
“We found that the sweet spot could be up to nine meters underwater for a planet orbiting a star significantly cooler than our sun, and photosynthesis could still take place. Something with a floatation capability could be protected from solar flares and still get enough photons to carry on.”
On Earth, plants absorb blue and red light while reflecting away large amounts of green. But the dominant color on other planets depends on so many different factors in the atmosphere and the light emitted by the planet’s star that not even infrared can be ruled out for photosynthesis. Some of the more exotic landscapes of science fiction authors may yet be realized, though doubtless in ways that will continue to surprise us.
The papers are Kiang et al., “Spectral signatures of photosynthesis I: Review of Earth organisms” (abstract here) and “Spectral signatures of photosynthesis II: coevolution with other stars and the atmosphere on extrasolar worlds” (abstract), both slated to appear in a forthcoming issue of Astrobiology. Also see this NASA backgrounder.
by Paul Gilster | Apr 10, 2007 | Exotic Physics |
Traveling to the planets takes big money and we’ve been part of the squabbing over where NASA money in particular ought to be allocated. But what about projects that take small money? The term is relative, of course, but John Cramer (University of Washington) thinks $20,000 should suffice to run his experiment in time travel, and with NASA’s Institute for Advanced Concepts now shutting down, he’s having a hard time raising it. This Seattle Post-Intelligencer story has more.
We’ve looked at Cramer’s work before, but a brief summary is in order. It involves Einstein’s ‘spooky action at a distance,’ the so-called Einstein-Podolsky-Rosen effect. Quantum entanglement seems to mean that two entangled particles influence each other no matter how far distant in space. That action appears to be instantaneous, which introduces the paradoxical outcome of suggesting that something can communicate faster than the speed of light.
Einstein, of course, would say that’s flat out impossible. Quantum theorists, for their part, have come up with ways of explaining entanglement that don’t involve communication, but Cramer disagrees. He believes that communication does occur but involves movement both forwards and backwards in time. To test the proposition, he would send entangled photons along fiber-optic cables of different lengths, causing the one taking the longer path to be delayed.
Because the photons are engangled, a measurement of one as a particle or a wave determines what happens to the other. Cramer wonders whether he can’t use this effect to make a signal arrive before it was sent. Here’s a description of his idea that the San Francisco Chronicle reprinted from New Scientist:
[Cramer’s] extra twist is to run the photons you choose how to measure through several kilometers of coiled-up fiber-optic cable, thereby delaying them by microseconds. This delay means that the other beam will arrive at its detector before you make your choice. However, since the rules of quantum mechanics are indifferent to the timing of measurements, the state of the other beam should correspond to how you choose to measure the delayed beam. The effect of your choice can be seen, in principle, before you have even made it.
Note the ingenuity of the experiment, its elegant simplicity, and its modest budget. This test of what Cramer calls the transactional interpretation of quantum mechanics could tell us whether particle interactions do indeed move both backward and forward in time, a phenomenon known as retrocausality. If they do, we’ve taken a step forward in working out what could one day become the unification of quantum mechanics and relativity. “In 20 years, nobody has been able to tell me why this can’t work,” says Cramer.
With government funding unlikely, the case for cutting-edge experiments funded by philanthropy is stronger than ever. The key here is that the kind of money the Tau Zero Foundation and others will need to raise for a given project isn’t necessarily vast. We’ve already seen that players like Elon Musk and Paul Allen are committed to using part of their personal fortunes for the advancement of space exploration. As the Foundation begins to explore the philanthropic terrain, keep your fingers crossed that less expensive projects like Cramer’s will soon find a receptive audience.
by Paul Gilster | Apr 9, 2007 | Exoplanetary Science |
Since we’ve recently been discussing astrometry, the discipline that measures star distances and movements, now would be a good time to look at two significant projects that go beyond optical methods to use radio astrometry in planet hunting. The Radio Interferometric Planet Search (RIPL) will draw on the Very Long Baseline Array, ten dish antennae spanning more than 5000 miles, and the 100-meter Green Bank telescope in West Virginia. The target: 29 active low-mass stars to be examined in a three-year planet hunt.
The targets are significant because they’re a kind of star that’s currently out of reach for radial velocity techniques. All are M dwarfs that are active, meaning they display ‘starspots’ (analogous to sunspots), flares or other activity in their chromospheres. The more active a faint star like this, the more likely that radial velocity measurements will be distorted with a ‘jitter’ that disturbs the precision of the measurement. RIPL ought to be able to sort out the situation and bring planet hunting to this niche.
The new observing project ought to be useful, then, in fleshing out our slowly growing knowledge of M dwarf systems. The authors of a recent paper outlining the project go so far as to say this:
Radio astrometric searches can determine whether or not M dwarfs, the largest stellar constituent of the Galaxy, are surrounded by planetary systems as frequently as FGK stars and if the planet mass-period relation varies with stellar type. The population of gas giants at a few AU around low mass stars is an important discriminant between planet formation models.
We’ve seen that issue in play recently with the planet discovered around GJ 674, which seems to reinforce the core accretion model of planet formation. What RIPL brings to the table is the ability to find lower mass planets in long-period orbits, and to define precise astrometric positions for these stars and their planets. Interestingly, radio astrometry is quite useful in making measurements of distant objects, including the proper motion of some pulsars and even the motion of Sagittarius A*, the apparent black hole at the center of the Milky Way. So employing radio techniques in the planet hunt is complementary to existing searches.
Beyond RIPL, the proposed radio telescope called the Square Kilometer Array (SKA) would have a collecting area roughly 200 times that of the VLBA. Here we achieve the possibility of detecting Earth mass planets around such stars, although the instrument will be sensitive enough to extend the search from M dwarfs to stars much like the Sun. For this one, though, we wait. Construction is not planned for more than a decade.
The overview paper is Bower et al., “Radio Astrometric Detection and Characterization of Extra-Solar Planets: A White Paper Submitted to the NSF ExoPlanet Task Force,” available here.
by Paul Gilster | Apr 7, 2007 | Deep Sky Astronomy & Telescopes |
Cepheid variables are simply indispensable. It was Harvard’s Henrietta Leavitt who, in 1912, discovered a relationship between the cycle of variable brightness in these stars and their luminosity. With a classic Cepheid, the longer the period of the star, the greater its intrinsic brightness. That sets up the method: Determine the period of the variable, check its apparent magnitude with the absolute magnitude corresponding to that period, and you can measure the distance. The relevant term is ‘standard candle.’
But put telescopes into space and you can refine these measurements, as studies of Cepheid variables with the Hubble Space Telescope have now shown. That’s helpful because we’d like to know the Hubble constant — the universe’s rate of expansion — as accurately as possible, and Cepheids are one of our best tools. To fine-tune the Cepheid method, a team from the University of Texas at Austin has directly measured the distance to ten Cepheid variables, using Hubble to trace their apparent motion in the sky, called parallax.
Parallax has a distinguished history. Looking at a star from opposite sides of Earth’s orbit around the Sun, astronomers made early distance measurements by seeing how far the star was displaced — the star seems to make a small circle on the sky. That apparent motion is helpful for calculating the distance to nearby stars, but far trickier when measuring more distant ones. The Texas’ team’s Milky Way targets were tough — the circle they drew was the equivalent of a quarter seen at 1500 miles — demanding the use of Hubble’s Fine Guidance Sensors.
But once achieved, the parallax findings give us a precise distance that can be weighed against the intrinsic brightness measurements for the Cepheid, and that helps us tune the period-luminosity relationship. Having made the calibration, astronomers can more accurately deduce the distance to Cepheids in distant galaxies. Ultimately, using such data should improve the accuracy of the Hubble constant, giving us precise measurements to any galaxy whose redshift can be measured.
There are other standard candles besides Cepheid variables, and I want to look at some of these in a future post. For now, though, the paper is Benedict et al., “Hubble Space Telescope Fine Guidance Sensor Parallaxes of Galactic Cepheid Variable Stars: Period-Luminosity Relations,” Astronomical Journal 1816 (April 2007), pp. 1810-1827. Abstract available.
