by Paul Gilster | Sep 18, 2008 | Astrobiology and SETI |
The idea of a galactic habitable zone (GHZ) has a certain inevitability. After all, we talk about habitable zones around stars, so why not galaxies? A stellar habitable zone is usually considered to refer to those areas around the star where liquid water can exist on a planetary surface. Those who believe that confining habitable zones to regions like these carries an implicit bias — limiting them to life much like our own — miss the point. The habitable zone concept simply tells us where it makes the most sense to search for the kind of life we can most readily recognize, and as such, it hardly rules out other, more exotic forms of life.
But while liquid water takes precedence in a stellar habitable zone, a galactic HZ is still being defined. Charles Lineweaver and team have examined it, among other things, in terms of stellar metallicity (the elements heavier than hydrogen and helium found in the body of a star), concluding that there is a ring several kiloparsecs wide surrounding galactic center in which life would be most likely to be found. But the ring evolves, spreads outwards with time, leaving us to recognize that galactic habitable zones can vary over the eons.
Image: A computer simulation showing the development and evolution of the disk of a galaxy such as the Milky Way. Credit: Rok Roškar/University of Washington.
That evolution now gets a much closer look from a team at the University of Washington, which ran 100,000 hours of computer simulations to study how galactic disks evolve, beginning with conditions nine billion years ago. The resultant data show that the average star can migrate through the galaxy, thus skewing the results of any habitable zone based partly on the abundance of certain chemical elements necessary for life. UW graduate student Rok Roškar puts the case this way:
“Our view of the extent of the habitable zone is based in part on the idea that certain chemical elements necessary for life are available in some parts of a galaxy’s disk but not others. If stars migrate, then that zone can’t be a stationary place.”
Stellar migration comes in handy because our understanding of the relationship between age and metallicity across star populations is changing. The paper on this work explains that relationship in terms of galactic chemical evolution:
Stars of the same age in the same general region of the galaxy are… expected to have similar metallicities. Indeed, early determinations of the AMR [age-metallicity relationship] confirmed that the mean trend of stars in the solar neighborhood is toward lower metallicity with increasing age. Models, which assume that stars remain where they are born and return their nucleosynthetic yields to their local ISM [interstellar medium], typically successfully reproduce this trend.
All of which gets interesting when you consider that in the part of the galaxy in which the Sun resides, field stars and open clusters show a wide range of metallicities. The ‘scatter’ in these data implies that stars near the Sun may well have come from completely different parts of the galaxy. The findings from the simulated galaxy are clear:
Roughly 50% of all “solar neighborhood” stars have come from elsewhere, primarily from the disk interior. Interestingly, some metal poor stars have been scattered into the solar neighborhood from the outer part of the disk. Such migration has recently been inferred from observational data… Metal-rich stars, like our Sun, could have originated almost anywhere in the Galaxy.
We just looked at the question of the Sun’s siblings, and whether or not systems forming in the same early cluster as the Sun might have exchanged life-bearing materials. Tracing stellar movements back in time to reveal which stars comprise those siblings may be a tricky matter. Stars encountering a galactic spiral arm seem to retain the circularity of their orbits after such an encounter, but their orbits may change considerably in size, meaning the Sun could have been in a much different position in relation to galactic center when it formed than it is today.
Bear in mind that galactic habitable zones also factor in supernovae explosions, which could cause exterminations on nearby worlds. We have much to learn in the study of supernovae, but the UW simulations suggest that the GHZ may be a more flexible area than we have previously considered. The paper is Roškar et al., “Riding the Spiral Waves: Implications of Stellar Migration for the Properties of Galactic Disks,” accepted for publication in Astrophysical Journal Letters (abstract). The earlier Charles Lineweaver paper is “The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way,” Science Vol. 303, No. 5654 (2 January 2004), pp. 59-62, with abstract here.
by Paul Gilster | Sep 17, 2008 | Exoplanetary Science |
As I taper back on my post-surgical medications (see yesterday’s post), a coherent universe is gradually coalescing around me once again. Still, I think I’ll take today relatively easy, looking at just one of the two stories I’ve been pondering during my brief convalescence. The first is intriguing not so much because of what it appears to be — a planet around another star, as imaged by the Gemini North Telescope on Mauna Kea — but rather because of where that planet seems to have formed. Have a look.
The image at right shows the torturously named 1RXS J160929.1-210524, a star some 500 light years from Earth, along with the apparent companion of that star. The team behind this work has been surveying a group of stars in the so-called Upper Scorpius association, a group of relatively young stars that formed some five million years ago. Gemini is equipped with adaptive optics capabilities that make finding different types of companions around such stars feasible. This one seems to be some eight times the size of Jupiter although much hotter than that planet (about 1500 C compared to Jupiter’s -110 C), orbiting a K7 star with a mass about 85 percent that of the Sun.
Image: Gemini adaptive optics image of 1RSX J160929.1-210524 and its likely ~8 Jupiter-mass companion (within red circle). This image is a composite of J-, H- and K-band near-infrared images. All images obtained with the Gemini Altair adaptive optics system and the Near-Infrared Imager (NIRI) on the Gemini North telescope. Photo Credit: Gemini Observatory.
Of course, the crucial thing is to find out whether or not this really is an orbiting object or a chance alignment, and a good deal of work will have to go into the proof. Also in need of serious study is the object’s distance from the star, which is 330 AU. That’s about ten times the distance between the Sun and Neptune. A planet at this distance is a bit of a surprise in terms of formation theories, but it’s not inconsistent with the separations of the few low-mass companions found by this method.
On the other hand, consider the limitations of our methods. Radial velocity and transit searches are effective primarily on planets orbiting within a few AU of their host stars. The question we’ve always had to ask is, just how representative are the systems we’re discovering? Direct imaging is in its infancy, but now it has delivered up a candidate planet at a seriously wide separation from a normal star (as opposed to a brown dwarf). It will be interesting to see how this plays out, bearing in mind direct imaging’s own limitation (at least at this point) of working best with nearby young stars and bright, hot planets.
Chances are that such wide separations will remain a rarity. Here’s a note on this from the paper:
Previous direct imaging surveys for planets around nearby solar-type stars have put upper limits of ?6% on the fraction of stars with planets more massive than 5 MJupiter at separations over ?50 AU… Our single detection of such a planet out of a sample of ?80 stars is consistent with these results, and con?rms that planets on wide separations are rare also at ages of a few Myr.
As to confirming that this is indeed a planet bound to the star, the authors peg the likelihood of a free-floating planet falling within the observed parameters of this one to be only 3 X 10-4, making the scenario unlikely. Further study of other possible companions or the star’s debris disk may help nail down the identification, but the authors also note this interesting point: “For now, the very existence of the 1RXSJ1609-2105 system poses a challenge to theories of planet and star formation, and may well suggest that there is more than one mechanism in nature for producing planetary mass companions around normal stars.”
The paper is Lafrenière at al., “Direct Imaging and Spectroscopy of a Planetary Mass Candidate Companion to a Young Solar Analog,” submitted to Astrophysical Journal Letters and available here. You may also want to look at the Gemini Observatory news release.
by Paul Gilster | Sep 16, 2008 | Administrative |
Several interesting items in the news today but I won’t be able to get to them, try as I might. I’m just coming off surgery yesterday (minor), and although I’m otherwise fine, the pain medication I’m taking makes me so groggy that I hesitate to post. So bear with me until tomorrow, when I should have a new item up some time in the afternoon.
by Paul Gilster | Sep 15, 2008 | Exoplanetary Science |
Using a near-infrared spectrograph attached to ESO’s Very Large Telescope, astronomers have been able to examine the inner protoplanetary disks around three interesting stars, with results showing the sheer diversity of the apparently emerging systems. Only a few million years old, all three stars could be considered analogs of our own Sun, going through processes like those that produced the Solar System some 4.6 billion years ago. The disks under study show regions where the dust has been cleared out, the possible signature of planetary influence.
The new work, which offers higher resolution than was earlier available, demonstrates that the previously known gaps in the dust still contain molecular gas, an indication that the dust has begun to form planetary embryos or that a planet has already formed and is clearing the disk gas as it orbits. The likely planets include a massive gas giant orbiting the star SR 21 at a distance of something less than 3.5 AU, and a possible planet around HD 135344B between 10 and 20 AU. The third star, TW Hydrae, may also show the development of one and possibly two planets. In the words of Klaus Pontoppidan (Caltech):
“Our observations with the CRIRES instrument on ESO’s Very Large Telescope clearly reveal that the disks around these three young, Sun-like stars are all very different and will most likely result in very different planetary systems.”
Image: Astronomers have been able to study planet-forming disks around young Sun-like stars in unsurpassed detail, using ESO’s Very Large Telescope. The studied disks were known to have gaps (represented by the brownish color in the image) but the astronomers found that gas is still present inside these gaps (represented by the white color in the image). This can either mean that the dust has clumped together to form planetary embryos, or that a planet has already formed and is in the process of clearing the gas in the disk. Credit: European Southern Observatory.
The techniques on display here, collectively called ‘spectro-astrometric imaging,’ are dazzling, allowing the researchers to see into the inner disk regions around stars that are more than 200 light years away, measuring distances down to one-tenth of an AU while simultaneously measuring the velocity of the gas. The disks themselves are about 100 AU across. Chalk up yet another win for adaptive optics, as the CRIRES spectrograph is fed by an AO module that corrects for atmospheric blurring, allowing high resolution. As good as these results are, they’ll be surpassed not many years from now by the ALMA (Atacama Large Millimeter/submillimeter) Array, whose operations commence in 2012.
Addendum: andy has passed along the link to the paper on this work, which is Pontoppidan et al., “Spectro-astrometric imaging of molecular gas within protoplanetary disk gaps,” accepted for publication in the Astrophysical Journal. The original ESO news release is here.
by Paul Gilster | Sep 12, 2008 | Astrobiology and SETI |
The idea that life on Earth might have originated elsewhere, on Mars, for example, has gained currency in recent times as we’ve learned more about the transfer of materials between planets. Mars cooled before the Earth and may well have become habitable at a time when our planet was not. There seems nothing particularly outrageous in the idea that dormant bacteria inside chunks of the Martian surface, blasted into space by comet or asteroid impacts, might have crossed the interplanetary gulf and given rise to life here.
But what of an interstellar origin for life on Earth? The odds on meteoroids from a system around the average galactic field star not only striking the early Earth but delivering viable microbes are long indeed. But if we consider the Sun’s probable origin in a cluster of young stars, all emerging from the same collapsing cloud, the picture changes significantly.
Now we’re dealing with much smaller distances between stars and slow relative motion as well, conditions that could make such a transfer possible. A new paper makes the case that this process might well have been two-way:
…there is a definite possibility that bacteria carrying meteoroids of extrasolar origin have landed on the Earth. In reverse, it is possible that at least one other planetary system in our birth star cluster received a life-carrying asteroid from the Earth; and it is not excluded that the whole birth star cluster was ‘fertilized’ in this way by live bacteria from the Earth.
So write Mauri Valtonen (Turku University, Finland) and team in an upcoming paper. The authors believe bacterial exchanges between planets of different solar systems could only have occurred during the birth cluster stage of these systems, but given this constraint, it is possible that the Earth received large numbers of life-bearing bodies early on. The broad process of panspermia in which life spreads through an entire galaxy from a single source seems unlikely, but “…life-carrying bodies originating from our solar system may have found their way to our original neighbours, and …all conditions being optimal, life seeded by our system could have spread to many other solar systems.”
And now it gets interesting for future space missions. Because we may be getting into position to find our long lost relatives, the stars from the original cluster that gave birth to our Sun. They’ve long since moved away from us, but missions like Gaia may be able to track them down. Gaia will study a billion stars in the Milky Way, monitoring each some seventy times over the course of a five year period. The mission is expected to discover extrasolar planets, brown dwarfs and numerous other interesting objects, and will help us extend our picture of galactic structure three-dimensionally, perhaps pin-pointing our siblings.
Gaia is expected to be launched in 2011, a much improved version of the Hipparcos mission that did so much to catalog the more than 100,000 stars it looked at. It’s a significant upgrade, and other missions to be launched within the next two decades could go on to provide us with a look at planetary systems around the stars Gaia identifies as members of our original stellar family. If we ever confirm the existence of life on these planets, we may be looking at worlds that either gave birth to life here or received life’s impetus from Earth.
The paper is Valtonen et al., “Natural Transfer of Viable Microbes in Space from Planets in the Extra-Solar Systems to a Planet in our Solar System and Vice-Versa,” accepted by the Astrophysical Journal and available online.
