Small Rocky Worlds by the Billions?

My local paper is running a story on page 11A entitled “Astronomers Report Earth-like Planet.” It’s a tantalizing headline, but obviously one that bears further investigation. For what’s being reported here is background information on one of the 45 planets — I should say ‘candidate’ planets — recently discussed at the Boston meeting of the IAU. These have been extracted from the HARPS planet survey, but we’ll probably have to wait until mid-June for further confirmation, which may well occur at the upcoming Extrasolar Super-Earths workshop in Nantes.

This would be an interesting world if things do play out, a rocky ‘super Earth’ just over four times as massive as Earth, and hence the smallest world yet in our attempt to find planets not so different from our own. If the press continues to generate a buzz about this, we should look at the contrast with the Gliese 581 story. There we wound up with two planets of astrobiological interest, one apparently on the inner edge of the habitable zone and probably across it, too hot for life, with another on the outer edge. The jury is out on both in terms of habitability, but the odds went down considerably as various teams ran the numbers.

But while Gliese 581 is an M-class dwarf, the HARPS survey has been looking at F, G and K-class stars, the latter two classes not much different from our Sun. If we were to find a rocky world in the habitable zone of one of these, we would be a step closer to an ‘Earth-like’ world than a hot, tidally locked planet in tight orbit around a red dwarf. No wonder the press is interested. But again, we’ll have to await confirmation and the inevitable follow-up studies to the Geneva team’s work, nor have I seen any verification of the McClatchy news story’s further claim that the potential new world orbits in the habitable zone of its star (see below).

Somewhat misleading headlines aside, what really came out of the Boston IAU session was the growing understanding of how frequently rocky worlds occur. Based on the recent findings, they could outnumber Jupiter-class planets by three to one. Sara Seager (MIT) is being widely quoted on this, including this from the McClatchy story:

“The mass of the planets and the sheer number of them represents a huge step toward finding planets of the Earth’s mass and ones that might be suitable for life as we know it. What amazes me is that these planets may be very, very common.”

No wonder Seager sees the HARPS windfall as “…the beginning of the detailed exploration of super-Earths.” Excitingly, we’re also looking at the growing possibility of finding such a world in transit. To my knowledge, the 45 HARPS planet candidates all orbit in less than fifty days (making the habitability question seemingly moot around F, G and K-class stars), with some in orbits as short as ten days. Bagging a transit to follow up the HARPS radial velocity studies becomes easier when orbits are close and frequent, and such a transit would provide information about the planet’s diameter, density and composition, not to mention allowing potential studies of its atmosphere. But unless HARPS has other planets up its sleeve, ‘habitability’ may not be a factor in the next headline.

Exoplanet Update and GLAST News

Following up on yesterday’s post on EPOCh, the extended exoplanet mission of the Deep Impact spacecraft, I want to mention that principal investigator Drake Deming (NASA GSFC) will be in my old home town of St. Louis on June 2 as part of the 212th meeting of the American Astronomical Society. Deming will be giving an update on the search for ‘super Earths’ of the sort that EPOCh may be able to spot during its investigations, while David Bennett (Notre Dame) as well as Michael Liu and Trent Dupuy (University of Hawaii) will be discussing other developments related to the exoplanet hunt and the study of brown dwarfs. We’ll keep an eye out for EPOCh results, particularly re GJ 436.

Also of relevance to future exoplanet as well as other astronomical studies is an upcoming report by Paul Chen (Catholic University) on work at NASA Goddard on inexpensive ways to make giant telescope mirrors on the Moon. That session will take place at the AAS on June 4 under the heading ‘Speculative Astrophysics,’ and speculative as it may be, the physics behind creating such mirrors seems feasible. What’s problematic is the engineering and, of course, the commitment to create and expand a serious scientific base on the Moon. Still, the mind turns to Claudio Maccone’s studies of dark side observatories free of Earthly interference and the possibilities become dazzling.

And finally, the GLAST (Gamma-Ray Large Area Space Telescope) mission, so potentially useful in the study of gamma-ray bursts (GRBs), is now closing on a June 5 launch, the window being from 1545 to 1740 UTC (remaining open through August 7). NASA TV will have launch commentary beginning at 1345. Among GLAST’s exciting possibilities (recently discussed here) is detecting the signature of WIMPs (weakly interacting massive particles), the leading candidate for dark matter. The latter, if indeed composed of WIMPs, may release a continuing stream of gamma rays and secondary particles that would contrast sharply with the abrupt GRBs that are under such active scrutiny.

Image: The first half of the payload fairing is moved into place around NASA’s Gamma-Ray Large Area Space Telescope within the mobile service tower on Launch Pad 17-B at Cape Canaveral Air Force Station. The fairing is a molded structure that fits flush with the outside surface of the Delta II upper stage booster and forms an aerodynamically smooth nose cone, protecting the spacecraft during launch and ascent. Credit: NASA/Jim Grossmann.

This, of course, is how science works. You study natural phenomena and create hypotheses to explain what you see (i.e., the apparent effect of ‘missing’ mass in galaxy formation and the gravitational lensing that seems to be produced by that mass). You test your models in hopes of finding the most reasonable explanation. Your predictions may agree with your hypothesis, but if they don’t, you go back to work on the original model. Gravitational lensing involving galactic clusters is widely observable (over a hundred galactic arcs have been found), but detecting the signature of WIMPs would add highly useful background data to the dark matter hunt underway in Earth-based detectors right now.

EPOCh Mission Now Focusing On GJ 436

I recently wrote about EPOXI, the dual-purpose extended mission being flown by the Deep Impact spacecraft. Yes, this is the same spacecraft that delivered an impactor to comet Tempel 1 with such spectacular results back in 2005. The vehicle now proceeds to a flyby of comet Hartley 2, but along the way a second extended mission has been coaxed out of it, this one targeting several known transiting planets in a search for signs of undiscovered worlds in those same systems. The mission will also look for possible moons or rings around the giant planets already discovered.

Another goal: To study the Earth, by way of calibrating the kind of ‘pale blue dot’ imagery a future terrestrial planet finder might see. In fact, observations taking place this very day should be helpful because the Moon will ‘transit’ the Earth from the spacecraft’s perspective.

And yes, the nomenclature is confusing, but acronyms are the name of the game in space operations. EPOXI is actually a conflation of two other acronyms: DIXI is the Hartley 2 mission ((Deep Impact Extended Investigation), while the extrasolar observations operate under the name EPOCh (Extrasolar Planet Observation and Characterization).

A planetary transit

Image: During the EPOCh phase, the spacecraft will observe known transiting planets. This graphic shows approximately what we expect to see as the planet begins to cross in front of its parent star and how the light coming from that star will slightly lessen because the planet is blocking a little bit of the light. The details of the light curve — how deep the dip is, how wide, how steep the drop off — reveal subtle clues about the planet. Credit: NASA/JPL-Caltech/UMD/GSFC

The Hartley 2 encounter is scheduled for October, 2010, but the EPOCh work has been underway since late January. According to principal investigator Drake Deming, the most recent observations have focused on the red dwarf GJ 436, known to be orbited by a Neptune-class planet whose eccentric orbit may be caused by gravitational effects from a second planet, possibly with a mass comparable to that of Earth, and in an orbital period ranging from twenty to thirty days. It is conceivable that such a planet would fall within this small star’s habitable zone (but see below), making any transit measurements quite helpful, since EPOCh can detect transiting planets as small as half the size of Earth.

Absent a transit of a new planet, though, EPOCh can still study the known planet to collect further data about its orbital perturbations. Transits are useful not only for discovery purposes, but because they can help us peg the length of the planet’s orbital period and its diameter. Moreover, EPOCh will have a serious edge over ground-based observatories, being able to watch each target for weeks at a time and working without an atmosphere that could distort received data. You can find a target list for EPOCh here. Thus far, observations of the giant planet HAT-P-4 seem to have been a particularly successful, with confirmed transits in the voluminous data awaiting further investigation.

I mentioned how tricky some of our acronyms can be, but how about star designations? In a recent newsletter covering the EPOCh work, Deming noted that HAT-P-4 is also known, depending on the database you use, as SAO 64638 and TYC 2569-1599-1. We’re dealing with an 11th magnitude G-class star located in the constellation Boötes. Of the 293 planets thus far found around other stars, 51 are known to be transiting.

Because such transits are detectable with properly configured amateur equipment, do be aware of TransitSearch, which works with observers worldwide to observe candidate stars when transits might occur. Interestingly, GJ 436, the EPOCh target discussed above, is in need of all the observations it can get, recent work suggesting the proposed second planet may not in fact exist. EPOCh should be able to tell us more, but interested amateurs can help by getting involved with TransitSearch.

Milky Way Re-Sized

If you want to understand the size of the Milky Way, you have to know something about how fast stars move. Measuring the velocities of stars in the galaxy’s stellar halo — a spherical halo of old stars and globular clusters surrounding the disk — you can figure out the mass of the whole by examining the gravity needed to keep these stars in their orbits. The Milky Way’s stars are a part of that mass, of course, but so is the extended distribution of dark matter, about which we know all too little.

This is where the so-called ‘blue horizontal branch’ stars (BHB) come into play. These ancient objects have evolved past their red giant phase and now burn helium. Because they tend to be both distant and bright (BHB stars are generally of spectral class B or A), they make useful markers for measuring stellar velocities out to a distance of 180,000 light years from the Sun, far beyond the confines of the primary galaxy. The huge star survey called SEGUE (a part of the Sloan Digital Sky Survey) has been using 2400 BHB stars to take such readings.

The results suggested by the observed stellar velocities: The Milky Way is not as massive as we believed. So says team leader Xiangxiang Xue (National Astronomical Observatories of China):

“The Galaxy is slimmer than we thought. That means it has less dark matter than previously believed, but also that it was more efficient in converting its original supply of hydrogen and helium into stars.”

Milky Way and dark matter

Image: Our sun lies about 25,000 light years from the center of the Galaxy, roughly halfway out in the Galactic disk. The new mass determination is based on the measured motions of 2,400 “blue horizontal branch” stars in the extended stellar halo that surrounds the disk. These measurements reach distances of nearly 200,000 light years from Galactic center, roughly the edge of the region illustrated above. The visible part of our Milky Way is embedded into its much more massive and more extended dark matter halo, indicated in dim red. The ‘blue horizontal branch stars’ that were found and measured in the SDSS-II study are orbiting the galaxy at large distances. Credit: Axel Quetz, Max Planck Institute for Astrophysics (Heidelberg), SDSS-II Collaboration.

The findings are useful because SEGUE’s large sample allows the method to be calibrated against existing computer simulations, giving us a better understanding of the Milky Way’s total mass. How the galaxy compares to distant galaxies that we see from without rather than within is a study that can help us in the quest to understand the broader principles of galactic formation. Just as significant, such work offers a valuable perspective on how the visible galaxy interacts with its dark matter halo.

More in this Sloan Digital Sky Survey news release. The paper, due to run in the Astrophysical Journal this fall, is Hue et al., “The Milky Way’s Circular Velocity Curve to 60 kpc and an Estimate of the Dark Matter Halo Mass from Kinematics of ~2500 SDSS Blue Horizontal Branch Stars,” available online.

Astrobiology: Finding a Place Like Ours

It’s not my usual practice to begin a post with a quotation, but Lee Billings, writing in a recent essay for SEED Magazine, so precisely captures an essential truth about our future in space that I want to give it pride of place. Looking at the ways we search for life on planets around other stars, Billings says this:

Throughout history, our knowledge has grown through human ambition and curiosity, only to regress beneath human apathy and caprice. The greatest obstacle to efforts to find another Earth, to discover life elsewhere in the universe, isn’t some flaw in our methodology or our technology, but in our will. Most of us alive today are unlikely to see these efforts bear their fullest fruit. Even optimistic young astronomers are uncertain that they will see the light from other living worlds in their careers, or even their lifetimes. But they work as though they will. Whether they see it personally doesn’t matter; what matters is that these other planets be seen someday. In preparation for that day, they continue to send modern-day robotic voyagers, like NASA’s EPOXI spacecraft, to gaze at our own planet from deep space, while still searching, as EPOXI also does, for planets elsewhere.

EPOXI is one of two extended missions for NASA’s Deep Impact spacecraft, the one that made such a splash when its impactor crashed into comet Tempel 1 back in 2005. The mission target is five nearby stars with transiting exoplanets, known ‘hot Jupiters,’ and the idea is that there may be other worlds in the systems around these stars. In fact, says EPOXI deputy principal investigator Drake Deming (NASA GSFC), “We’re on the hunt for planets down to the size of Earth, orbiting some of our closest neighboring stars.”

You heard what the man said: Planets down to the size of Earth.

A Long-Haul Science

A second Earth

Long-term thinking is always a tough sell, particularly in an age as frenetic as ours, but missions like EPOXI fit into the gradual accretion of exoplanetary knowledge that will build the foundation for far more remarkable discoveries. And Billings is right on target here: The stars demand better of us than a fixation on quick results. Wonderful if we do get them, but budgetary and technological realities may push the definitive discovery of life around another star into an ambiguous future.

Image: A terrestrial planet around another star as seen from one of its moons. Artist impression by David Hardy.

Nonetheless, in whatever time frame, it’s coming. Billing’s essay considers the staggering possibilities of analyzing distant points of light, the planetary specks revealed by future space observatories that can block out the light from a central star to reveal the system around it. Recent work by Darren Williams (Penn State, Erie) and Peter McCullough (Space Telescope Science Institute) suggests that patterns of reflected light might reveal planetary seas and the continents that interrupt their signature. Sara Seager (MIT) and Eric Ford (University of Florida) are working on the analysis of color, studying fluctuations in brightness as indicators of days and seasons.

Fold this work in with spectroscopy, and you’re starting to characterize a terrestrial-class world to a hitherto impossible degree of precision. Ozone is one kind of marker, revelatory of oxygen, the latter being so chemically reactive that it needs sources of renewal. Couple it with methane and you’re looking at two gases that shouldn’t co-exist for long periods. The Galileo spacecraft saw such a signature when it made an Earth flyby in 1990 on its way to Jupiter. You can read about the ‘astrobiological’ investigation of Earth in Carl Sagan’s Pale Blue Dot (1994), the point being that our planet offers an initial sample to submit to analysis like that we’ll one day run on a real exoplanet.

The myriad ways we can find life’s signature are growing as our instrumentation improves in sophistication, and it may well be that we eventually have methods — doubtless using some kind of interferometry, so that widely spaced observatories become the equivalent of a single instrument — that can actually pull up imagery from a distant world. One of Webster Cash’s most advanced designs for a ‘starshade’ mission is theoretically capable of showing us a planetary surface from the equivalent of a few hundred miles out, but that particular New Worlds off-shoot would be an expensive culmination of decades of previous, less powerful missions. Step by step, stone by stone, we build the edifice.

Handing Off the Gift

Edwin Hubble at Mt. WilsonSo that when Billings talks about researchers not knowing whether they will see the results of their work, he’s noting that no matter what we live to see (and we will see wonders), the push into the universe will reveal far more to ensuing generations. We build the things we build not only for what they show us today, but because they are stepping stones in the journey to build the next class of tools. The Hubble Space Telescope is a grand instrument, but you don’t get Hubble before building Galileo’s simple telescope first, or Mt. Wilson, or the Yerkes Observatory refractor. That means recognizing that we’re part of a larger scientific enterprise, one before which individual ego (sometimes potent!) is dwarfed by the need to defer to a vision centered on our entire species.

Image: Building up the edifice: Edwin Hubble at the Mt. Wilson 100-inch instrument. The photographic plates the astronomer created here helped him demonstrate the nature of galaxies in the Local Group.

I’m sometimes asked why I’m so provincial in talking about biosignatures for life like our own, when around other stars the possibilities may be far beyond our imagining. And that’s just the point, I think. The Galileo mission’s Earth fly-by drove home the idea that finding life is hard even when you’re dealing with a known entity like our green world. The best shot we have in the early going is to look for life that we already understand, carbon-based, dependent upon water and the chemical signatures we would expect to find where such life flourishes. Are other life-forms out there? Possibly, and we may just find them — in the long-term, after building the edifice higher still, and handing off our work to generations that will recognize and improve the gift yet again.