Pale Blue Dot?
Water is high on the ingredients list of astrobiologists hunting for signs of prebiotic conditions elsewhere in the universe. Evidence for an ocean-covered world, or pale blue dot, would provide some of the most compelling evidence that at least simple microbial life might find safe harbor outside of the Earth. Indeed, in looking back at our own planet from space–the "Pale Blue Dot"-Carl Sagan used the Viking photograph of Earth from over 3 billion miles to highlight both the vastness of space and the relative importance of water for life on Earth. Water detection on extrasolar planets has long been considered a daunting challenge for astronomers. The 1996 NASA-Ames Workshop, called the Pale Blue Dot, summarized the prospects: "The presence of H2O bands in a planet’s atmosphere, combined with knowledge of the amount of stellar radiation it absorbs and its effective radiating temperature, should provide a good indication of whether liquid water exists at the planet’s surface. Liquid water is considered essential for life as we know it. This direct information about the presence of water would augment purely theoretical estimates of whether the planet orbited within the star’s liquid water habitable zone."
|The view back toward Earth from 3 billion+ miles, Viking photo of Pale Blue Dot underscores the difficult task of imaging planets at all, much less water presence using anything but inferred evidence like spectral data
Credit: Viking NASA
New tantalising results for a star called Upsilon Andromedae may suggest that such water detection is possible in exoplanetary searches. The findings derived from observations using the 32-metre Medicina radio telescope near Bologna, Italy. The reception on the large telescope was narrowed to a microwave frequency (22 gigahertz, GHz) known to signal a sharp spike specific to vibrations in water molecules. The Italian team of scientists, headed by Cristiano Cosmovici, from the Institute for Cosmic and Planetary Sciences in Rome, recently reported their preliminary research this week at the Second European Workshop on Exo/Astrobiology in Graz, Austria. "This would be a historic discovery – the first detection of a prebiotic molecule in an extrasolar planet," reported Cosmovici.
Water searches are done by a global enterprise of radio telescopes. For instance, largely because of the presence of frozen water in many comets (often characterized as "dirty snowballs"), comet clouds do present opportunities for water detection in the neighborhood of distant stars. In July of last year, a team of astronomers from Harvard, NASA/Ames and Johns Hopkins reported in Nature on circumstellar water around an aging carbon star (IRC+10216) and inferred the presence of icy bodies and cometary debris. So while the Italian team is seeking confirmation of their most recent conference publication from other researchers around the world, whether the water detection is a viable signal or not will hinge greatly on what is already known about their primary target, a triple-planet system orbiting Ups Andromedae.
A Triple-Planet System Orbiting Ups Andromedae
|Cristiano Cosmovici Institute for Cosmic and Planetary Sciences in Rome|
Of the planets catalogued so far, a number of surprising patterns have been revealed. About half have small orbits and reside very close to their parent star (about 1/5th the distance from the Earth to the Sun, or 0.2 AU). Such proximities would suggest extreme temperatures. The majority also have highly elliptical or eccentric orbits, not circular ones. Prior to the flurry of post-1996 planet discoveries, most theories suggested that as a parent star’s disk began to evaporate during planet formation, circular orbits would dominate mainly as in our own solar system. One explanation for the large number of elliptical orbits posits that planet’s interact gravitationally with each other strongly to generate stable, non-circular orbits. In the case of Ups Andromedae, a 3-planet system has been proposed by Marcy, et al. with small and eccentric orbits. Thus the discovery of this multiple planet system suggested a new paradigm for planet formation where many small seed planets known as planetesimals might develop in the disk of matter surrounding a star. "The Upsilon Andromedae system suggests that gravitational interactions between Jupiter-mass planets can play a powerful role in sculpting solar systems," said Butler, staff astronomer at the Anglo-Australian Observatory.
|The trio of planets and an overlay orbital scale of our own solar system
Located 44 light-years from Earth, the parent star itself (HD 9826) is probably a single star, although the Bright Star catalog lists it as a possible binary, but the star’s motion is more consistent without including a companion. The star is approximately 1.2 times as massive as our Sun and around 2.6 billion years old (2/3 the age of our Sun). With its trio of planets, Upsilon Andromedae harbors the first planetary system that is reminiscent of our own Solar System.
"The usual picture is that gas giant planets form at least four AU away from a star, where temperatures are low enough for ice to condense and start the process of planet formation," said Timothy Brown of the High Altitude Observatory (HAO). "But all three giant planets around Upsilon Andromedae now reside inside this theoretical ice boundary." The planets may have formed close to the host star, or, like balls on a billiard table, the planets may have scattered off of each other, migrating into their current orbits from a more distant place of origin.
The inner planet, or "hot Jupiter" candidate, was first announced by Butler et al. in Jan 1997. This innermost of the three planets contains at least three-quarters of the mass of Jupiter and orbits only 0.06 AU from the star. It traverses a circular orbit every 4.6 days. The middle planet contains at least twice the mass of Jupiter and it takes 242 days to orbit the star once. It resides approximately 0.83 AU from the star, similar to the orbital distance of Venus. The outermost planet has a mass of at least four Jupiters and completes one orbit every 3.5 to 4 years, placing it 2.5 AU from the star. The two outer planets have elliptical (oval) orbits which is characteristic of other extrasolar planets in distant orbits around their stars.
All 3 planets are predicted roughly to have very high temperatures, based on standard reflection estimates (albedo like Jupiter, or 0.35) and their theorized orbits. From inner to outer planets found, the trio register temperatures of 1097 C (1947 F), 92 C (198 F), and -63 C (-81 F). Previous publications (Butler, et al. Astrophys. J) have noted that "Internal heating probably raises the temperature of the middle companion above the boiling point of water, while the outer companion is probably pushed up to near the water freezing point at a pressure of 1 atmosphere".
|The trio of planets as they might appear around Ups Andromedae: artist rendering
No current theory predicted that so many giant worlds would form around a star. "I am mystified at how such a system of Jupiter-like planets might have been created," said Geoffrey Marcy, San Francisco State University’s (SFSU’s) Distinguished Professor of Science. "This will shake up the theory of planet formation."
Robert Noyes, a professor of astronomy at Harvard-Smithsonian CfA and a member of the CfA-HAO team, said, "A nagging question was whether the massive bodies orbiting in apparent isolation around stars really are planets, but now that we see three around the same star, it is hard to imagine anything else."
If these Jupiter-mass planets are like our own Jupiter, they would not be expected to have solid Earth-like surfaces. But, Peter Nisenson of the Harvard-Smithsonian Center for Astrophysics (CfA) noted, "our observations can’t rule out Earth-sized planets as well in this planetary system, because their gravity would be too weak for them to be detectable with present instruments."
Water Maser Emissions: Shock Physics
|The 32 meter radio telescope in Bologna, Italy
Credit: Italian Astronomy
For radio astronomers, the 22 GHz water maser line is the brightest spectral line in the radio universe. Water production is a very common natural result from shockwaves when stars are formed, and thus are of keen interest for studying stellar evolution in radio bands. The birth of a massive star generates strong stellar winds (with velocities up to a few thousand km/s), which shock the ambient cloud material. Shocks with velocities exceeding some 20 km/s running into high-density magnetized material successfully explain water maser emission around some stars.
But for the more aged stars like Ups Andromedae, shockwaves are long subsided and another water source must be proposed. Detection of water registering as microwaves (or maser) signals at 22 gigahertz (Gz), Cosmovici wrote, yields "a powerful diagnostic tool for planetary search outside the Solar system".
In deep space, as the tiny wishbone-shaped water molecule (H2O) rotates around its central oxygen atom, a very high frequency resonance reaches microwave detectability. To record (or generate) these molecular rotations, a "maser" is the microwave equivalent of a laser, operating on (higher frequency) rotational energy states instead of vibrational states or electron shell jumps as in an ordinary light laser. (The name MASER stands for "Microwave Amplification by Stimulated Emission of Radiation", while laser is the "Light Amplification…" equivalent. )
|Comparison of Mars, Venus and Earth in water bands, showing the clear presence of water on Earth uniquely
Credit: NASA Workshop, Pale Blue Dot
Other than water detection, a number of other signatures are of interest for probing the characteristics of extrasolar planets. In the near future, medium-resolution spectroscopy will be obtained of about 75 of the most interesting planetary systems detected, enabling the measurement of broad water-vapour and carbon dioxide spectral features – the signatures of an Earth-like oxidising atmosphere – if present. Finally, even higher resolution spectra will be obtainable for up to 30 of the most promising planets, and will be capable of detecting the infrared 10-micron ozone band, a life-signature on Earth.
Other possible targets for water detection around exoplanets included in Cosmovici’s survey yield less certain probabilities of true planetary systems. Epsilon Eridani, a Sun-like star 10 light years away, and Lalande 21185, a red dwarf about 8 light years away, are both closer than the 44 light-year distance to Ups Andromedae but offer a less complete picture of what might be going on orbitally around those parent stars.
The star, Ups Andromedae should make an ideal target for NASA’s upcoming Space Interferometry Mission (SIM). Expected to launch in 2005, SIM will spend five years probing nearby stars for Earth-sized planets and will test technology slated for future planet-searching telescopes. The ongoing ground-based planet search will enable SIM to home in on those stars most likely to harbor small planets.
References C.B.Cosmovici, S.Montebugnoli, A.Orfei, S.Pogrebenko and P.Colom, Planet.Space.Sci.,44,735,1996
Second European Workshop on Exo/Astrobiology, Graz, Austria, September 16 – 19, 2002