by Paul Gilster | Jun 5, 2008 | Astrobiology and SETI |
Imagine a team of astronomers from a distant extraterrestrial civilization. Anxious to find blue and green living planets like their own, they study various methods of planetary detection and put them to work on small, relatively nearby stars. Detecting planetary transits, they refine their techniques until they trace the signature of a planet much like home.
Now assume that, despite the presence of their own version of skeptics like myself (some of us think that sending deliberate signals to the stars is premature without further, wider discussion), they decide to encode information about themselves into a message to be sent by a repeating beacon. Naturally, they turn to those stars around which they’ve found planets that look to be not only the right size, but in the right position, within the habitable zone where water could exist on the surface.
Fanciful? You bet, especially in the idea that a nearby extraterrestrial civilization would be more or less at the same state of technological development as ourselves. But maybe they’re not. Maybe they made this detection aeons ago, and have been signaling ever since in the hope that a civilization would arise. Without knowing more about what’s out there, we can’t rule scenarios like this out. And if you take a step in the other direction, you can see that there are SETI applications here on Earth. Hunting for other civilizations might depend upon their being a bit like us.
After all, we’ve always tried to figure out where to search, which is why so much attention has historically been paid to factors like the most obvious frequency for transmissions. Now Richard Conn Henry (Johns Hopkins) looks in at our Solar System from the outside, telling the recent American Astronomical Society meeting that our SETI search should focus on the swath of sky within which an alien civilization might be able to detect the Earth as it makes its own transit across the face of our Sun.
Henry is talking about the ecliptic, which is the plane of the Earth’s orbit around the Sun. As we see things from Earth, the Sun seems to move along this circle each year. Taking up just three percent of the sky, the ecliptic usefully constrains the area that observatories like the Allen Telescope Array (ATA) would have to examine. Henry puts the matter concisely:
“If those civilizations are out there — and we don’t know that they are — those that inhabit star systems that lie close to the plane of the Earth’s orbit around the sun will be the most motivated to send communications signals toward Earth, because those civilizations will surely have detected our annual transit across the face of the sun, telling them that Earth lies in a habitable zone, where liquid water is stable. Through spectroscopic analysis of our atmosphere, they will know that Earth likely bears life.”
Image: The plane of the ecliptic is illustrated in this Clementine star tracker camera image which reveals (from right to left) the Moon lit by Earthshine, the Sun’s corona rising over the Moon’s dark limb, and the planets Saturn, Mars, and Mercury. The ecliptic plane is defined as the imaginary plane containing the Earth’s orbit around the Sun. In the course of a year, the Sun’s apparent path through the sky lies in this plane. The planetary bodies of our solar system all tend to lie near this plane, since they were formed from the Sun’s spinning, flattened, proto-planetary disk. The snapshot above nicely captures a momentary line-up looking out along this fundamental plane of our solar system. Credit: NASA/The Clementine Project.
Yes, and making smart decisions about how you allocate your observing time could spell the difference between a hit and a miss. If you look at the ecliptic in relation to the galactic plane, the two circles intersect in the region of Taurus and again in Sagittarius, the implication being that these are the most likely regions for finding an extraterrestrial civilization. Assuming long life to civilizations, a factor Henry goes on to investigate:
“These models are nothing but pure speculation. But hey … it is educational to explore possibilities. We have no idea how many — if any — other civilizations there are in our galaxy. One critical factor is how long a civilization — for example, our own — remains in existence. If, as we dearly hope, the answer is many millions of years, then even if civilizations are fairly rare, those in our ecliptic plane will have learned of our existence. They will know that life exists on Earth and they will have the patience to beam easily detectable radio (or optical) signals in our direction, if necessary, for millions of years in the hope, now realized, that a technological civilization will appear on Earth.”
Henry joins Seth Shostak (SETI Institute) and Steve Kilston (Henry Foundation) in using the Allen Telescope Array to perform this search. Ultimately, the entire ecliptic will be investigated through the ATA, but the intersection of ecliptic and galactic planes does present an area of unusual interest. We’ve long known that this area could be interesting, but the ATA’s hundreds of dishes and computer processing capabilities offer a significant upgrade to our search methods. Count me a SETI skeptic, at least in terms of finding intelligent life via radio signals, but count me, too, as one who would rejoice at being proven wrong.
by Paul Gilster | Jun 4, 2008 | Deep Sky Astronomy & Telescopes |
The feeling I have when deciding what to discuss next about this year’s American Astronomical Society meeting is like what I get in a good used bookstore. Where to turn next? We’ve already looked at several stories with exoplanetary significance, but the arrival of a new type of star entirely seems to vault past even these in significance. If, of course, the so-called ‘quark star’ is real, a question sure to remain controversial as the study of extremely bright supernovae continues.
When I say bright, I’m talking about three events in particular, each of which produced one hundred times more light energy than normal supernovae. The events, designated SN2006gy, SN2005gj and SN2005ap, have been under intense scrutiny, among the researchers a team from the University of Calgary, who point to the lack of a satisfactory explanation of these events. The hypothesis they defended at AAS is that neutron stars are not the most compact solid objects known to exist. That honor belongs to still denser quark stars.
Take an average neutron star, maybe sixteen miles across but 1.5 times as massive as the Sun. Produced by the catastrophic collapse of a massive star (and thus associated with the accompanying supernova explosion), neutron stars could theoretically be packed tighter still, the same mass being squeezed to an object just twelve miles across. At this point, the neutrons dissolve into quarks and vast amounts of energy are unlocked, causing the aforementioned super-luminous events. The researchers — Denis Leahy and Rachid Ouyed — are quick to point out that competing explanations of these supernovae cannot be ruled out without further observations of these exotic phenomena.
All of which is highly speculative but a stunning possibility just the same. What’s happening to the Milky Way itself is also a bit of a surprise, for at the same AAS meeting, a team led by Robert Benjamin (University of Wisconsin, Whitewater) used new imagery from the Spitzer Space Telescope to re-examine the galaxy’s structure. The result: There appear to be not four but just two major arms to our galaxy, a possibility neatly captured in the image below. Benjamin notes how tricky studying a galaxy from within can be:
“For years, people created maps of the whole galaxy based on studying just one section of it, or using only one method. Unfortunately, when the models from various groups were compared, they didn’t always agree. It’s a bit like studying an elephant blind-folded.”
Image (click to enlarge): Like early explorers mapping the continents of our globe, astronomers are busy charting the spiral structure of our galaxy, the Milky Way. Using infrared images from NASA’s Spitzer Space Telescope, scientists have discovered that the Milky Way’s elegant spiral structure is dominated by just two arms wrapping off the ends of a central bar of stars. Previously, our galaxy was thought to possess four major arms. Credit: NASA/JPL-Caltech.
Earlier radio surveys and the infrared surveys that followed them had revised the initial model of a spiral with four major star-forming arms, but Benjamin’s software has gone to work counting stars and measuring stellar densities, employing a vast Spitzer mosaic that takes in some 110 million stars. The Milky Way now appears to be like other galaxies we have observed with a central bar of stars (the latter a discovery made in the 1990s). The two major arms are now seen to be the Scutum-Centaurus and Perseus arms (although the Perseus arm is not visible in the field of view covered by the new Spitzer images).
The Sagittarius and Norma arms are now considered to be minor, with the Perseus and Scutum-Centaurus arms showing the greatest density of both young, bright stars and older red giants. Bear in mind that our own small star is currently found near the partial arm known as the Orion Spur, located between the Sagittarius and Perseus arms. But as this JPL news release points out, stars tend to move in and out of arms as they orbit the galaxy’s center. In fact, our Sun would have made sixteen circuits of the Milky Way since its formation four billion years ago.
by Paul Gilster | Jun 3, 2008 | Exoplanetary Science |
Yesterday’s story on the smallest exoplanet yet discovered somewhat obscured work on brown dwarfs released at the same conference. But this year’s meeting of the American Astronomical Society has been filled with interesting items, and I don’t want to neglect the latest news about a type of star that may be as plentiful as any in the cosmos. We don’t know that that is the case, but we have much to learn about brown dwarfs as we compile a census of those in the Sun’s neighborhood, including the question of what kind of planets might circle them.
New observations studied by Michael Liu (University of Hawaii) and team have now been able to determine the masses of a number of brown dwarfs, with findings that suggest the shape of future research. Says Liu:
“Mass is the fundamental parameter that governs the life-history of a free-floating object, and thus after many years of patient measurements, we are delighted to report the first masses of the very faintest, coldest brown dwarfs. After weighing these tiny, dim, cold objects, we have confirmed that the theoretical predictions are mostly correct, but not entirely so.”
It’s understandable that these objects should be tricky to observe. They can be 300,000 times less energetic than the Sun, with temperatures at the surface below 500 degrees Celsius. The image at left shows two such brown dwarfs, orbiting a star not terribly different from Sol. How tempting it would be to a civilization just moving out into its solar system to have a nearby stellar target like this with, perhaps, a planet or two around each brown dwarf. The incentive to develop deep space technologies might well be accelerated with the prospect of exploring such exotic targets. Call it the ‘brown dwarf incentive.’
Image: Infrared image of the dusty brown dwarf binary HD 130948BC. The binary is seen in the upper left and has a total mass about 11 percent the mass of the sun. The binary is in orbit around a young sun-like star, seen to the lower right. This image was obtained with the adaptive optics system on the Keck II Telescope, located on Mauna Kea, Hawaii. The image is 3.75 arc seconds on a side (about 1/500 the size of the moon), and the binary’s separation is about 0.1 arc seconds. Credit: Mr. Trent Dupuy and Dr. Michael Liu (Institute for Astronomy, University of Hawaii).
About fifteen percent of the brown dwarfs within 100 light years of Earth occur in binary systems, and it is these that Dr. Liu’s team has focused on, for the study of their orbits (in size and duration) can help determine the total mass of the system. These pairs are between 45 and 60 light years from Earth, with the two components of each separated by about two AU, a distance somewhat larger than the distance of Mars from the Sun.
Using the Keck II telescope (Mauna Kea) and previous data from the Hubble instrument, precise measurements of the orbits in question became possible. Each brown dwarf in the binary HD 130948BC shows an individual mass of about 5.5 percent the mass of the Sun. The brown dwarfs in the other binary, 2MASS 1534-2952AB, each weigh in with a mass about three percent of that of the Sun. That would make each of the objects the equivalent of about thirty Jupiter masses.
As to that disagreement in theory that Liu refers to above, it emerges when you compare these mass measurements with what you would expect when studying the energy output and temperature of these objects. One binary pair was cooler than theory would predict, the other warmer. The implication is that the model for temperature determination or energy output is off, but it will take more brown dwarf measurements to pin down precisely what is happening. We’ll follow that investigation with interest, and I’ll post links to the two papers, slated for the Astrophysical Journal, as soon as they become available online.
by Paul Gilster | Jun 2, 2008 | Exoplanetary Science |
Smaller and smaller planets keep coming into view. A prime goal, of course, is to find something around the size of the Earth, implying as it would the existence of a world that might be like ours in other ways. My suspicion is that one day soon a transit study is going to come up with an exoplanet that’s closer to the size of Mars (definitely possible with today’s technologies), and we’ll skip right past the ‘Earth twin’ point before finding a planet that really is close to the same diameter.
But so far we’re still looking at worlds larger than Earth, like the tongue-twisting MOA-2007-BLG-192Lb, now thought to be the lowest mass planet ever found around another star. Announced today at the American Astronomical Society’s meeting in St. Louis, the new planet orbits a brown dwarf. At six percent of the mass of the Sun (and thus unable to sustain nuclear reactions in its core), the host is the lowest mass star to have a companion with a planetary mass ratio. But the fudge factor in the data does allow for the possibility of it being an extremely low mass hydrogen-burning star rather than a brown dwarf.
The news conference making these announcements is just winding down, a major point being that before this discovery, no planets had been found around stars less than twenty percent as massive as the Sun. We’re finding out that low mass stars can indeed host planets in the Earth-mass range, making nearby red dwarfs more and more interesting as systems with astrobiological possibilities. And with hundreds of brown dwarfs within 100 light years of Earth (fifteen percent of them in binary systems), we now must factor in the kind of planets we can expect to find around them.
Image: Artist’s conception of the newly discovered planet MOA-2007-BLG-192Lb orbiting a brown dwarf “star” with a mass of only 6% of that of the Sun. Theory suggests that the 3-earth-mass planet is made primarily of rock and ice. Observational and theoretical studies of brown dwarfs reveal that they have a magenta color due to absorption by elements such as sodium and potassium in their atmospheres. Credit: David Bennett.
This particular world has an orbital radius similar to that of Venus, but in a system dominated by such a faint central star, temperatures at the top of the atmosphere would be frigid. Even so, the possibility is strong that the planet has a thick atmosphere, one that would allow warmer temperatures on the surface, and there is some speculation that interior heating by radioactive decay could keep that surface warmer still. We may be dealing with a vast, deep ocean under all that atmosphere. But let me quote the paper on this:
…it is possible that MOA-2007-BLG-192Lb could have a habitable surface temperature itself, despite the fact that its host star or brown dwarf provides extremely feeble radiative heating. Stevenson (1999) has speculated that even a free ?oating Earth-mass planet could have a surface temperature that would allow liquid water even though the heating from internal radioactive decays provides a factor of ? 104 times less energy than the Earth receives from the Sun. The key point of Stevenson’s argument was that such a free ?oating planet might retain a molecular Hydrogen atmosphere that could provide very strong insulation that would allow the surface temperature to remain above the melting point of water ice. If it was possible to detect nearby analogs to MOA-2007-BLG-192Lb, it would be worthwhile to attempt to study their spectra to see if they do have H2 atmospheres that might allow warm surface temperatures.
Congratulations to two microlensing teams — the Microlensing Observations in Astrophysics (MOA) and Optical Gravitational Lensing Experiment (OGLE) collaborations — which used lensing to detect the planet. In this method, light from a background star is magnified by an intervening star (in this case, the planetary host), with the planet being spotted by additional perturbations to that light. I’m now looking at a press release on this work and noting a statement by David Bennett (University of Notre Dame), who led the team: “I’ll hazard a prediction that the first extra-solar Earth-mass planet will be found by microlensing. But we’ll have to be very quick to beat the radial velocity programs and NASA’s Kepler mission, which will be launched in early 2009.”
Yes, better hurry, for each planet detection method is improving rapidly, but my money is still on the transit method to make that first Earth-mass discovery. The paper is Bennett et al. (46 co-authors!), “A Low-Mass Planet with a Possible Sub-Stellar-Mass Host in Microlensing Event MOA-2007-BLG-192,” accepted by the Astrophysical Journal. I’ll have the arXiv link up as soon as it becomes available.
