by Paul Gilster | Jul 26, 2007 | Exoplanetary Science, Missions |
The European Space Agency’s DARWIN mission proposal is now available online, well worth a look if you’re hoping to keep up with planet-hunter spacecraft technologies. With a launch date dependent upon the evolution of its technology, DARWIN probably won’t get off for another decade, but with a primary goal of detecting and studying terrestrial planets around other stars, it is sure to be a high-visibility mission as it continues development.
According to the proposal, the baseline DARWIN mission is to last five years and will target approximately 200 individual stars at mid-infrared wavelengths. The focus is on stellar types F, G, K and some M stars (about ten percent of the total). Of these, between twenty-five and fifty planets will be studied spectroscopically for evidence of gases such as CO2, O3 and H20. The mission planners are currently assuming the number of terrestrial planets in the habitable zone is one per system, adding that data from NASA’s Kepler mission will be useful in evaluating this conclusion.
And note this from the report:
…nearby K and M dwarfs are the easiest targets in terms of Earth-like planet detection for a given integration time. This is because the thermal infrared luminosity of a planet in the Habitable Zone depends only on its size. On the other hand, the stellar luminosity is a strong function of its spectral type. This means that the star/planet contrast varies with spectral type. Compared to the case of the Sun and Earth, this contrast is two times higher for F stars, a factor of three lower for K stars, and more than an order of magnitude lower for M dwarfs. Nevertheless, Darwin will focus on solar-type G type stars (50% of the observing time), and a significant number of them can be screened and any discovered terrestrial planets studied.
To retrieve the faint emission of a terrestrial planet from the nearby starshine, ESA estimates a 100-meter telescope would be required, a vast undertaking not currently within our technological limits. So DARWIN turns to four collector spacecraft feeding light to a ‘beam combiner’ craft. The chosen interferometer array is called Emma, and it allows the formation-flying telescopes to perform like a single, larger instrument.
The diameter of the collector telescopes is a key issue: With 1-meter telescopes, the number of targets screened is reduced to about 75; with 3-meter mirrors, 2/3 of the catalog of stars compiled as possible targets can be examined. The report notes the potential for spurious signals from residual starlight (stellar leakage), the local zodiacal background produced by dust particles around our own Sun, the exo-zodiacal light from the target star’s dust disk, and the instrumental background produced by thermal emissions within the instruments. Details are in the report, but this is enough to suggest the magnitude of the detection task.
Image: Mass ejections and winds from low mass M and K-type stars can erode the atmosphere of planets in the habitable zone. Darwin will study how such activity influences the magnetic dynamo, atmosphere, and biology of these planets. Credit: DARWIN Mission Proposal.
Keep your eye on missions that have a direct impact on DARWIN:
- COROT, already operational, should be able to detect planets down to about two Earth radii that orbit close to their stars (orbital period in the range of fifty days). COROT data should help firm up our ideas about the abundance of hot, rocky worlds around the stellar types under study.
- NASA’s Kepler mission, scheduled for launch within the next two years, is designed to detect terrestrial planets near the habitable zones of the stars it studies. 100,000 main sequence stars will ultimately be monitored, with the spacecraft continually pointing its telescope at a single field looking for Earth-sized planetary transits. As Kepler proceeds, the statistical data on terrestrial worlds should become more and more reliable, allowing more fine-tuning to the DARWIN catalog.
- Prisma (Prototype Research Instruments and Space Mission technology Advancement), a Swedish project with joint European funding, will be launched in the fall of 2008, with the objective of studying guidance, navigation, control and sensor techniques as the two spacecraft involved demonstrate formation flying, so necessary to the accuracy of the DARWIN attempt.
- PROBA3, not yet fully funded, but intended as a follow-up to PRISMA and an attempt to further advance formation flying techniques.
- Pegase/PERSEE, designed to study extrasolar giant planets at near-infrared wavelengths, a mission that could be extended to brown dwarfs and other targets of astrophysical interest. Not yet funded, this mission may well wind up merged with PROBA3.
How close are we to making DARWIN a reality? Here’s the report’s conclusion, referring to the TRL (Technology Readiness Level) scale from 1 to 9, in which a 6 indicates readiness for a technology demonstration and a 9 is readiness for launch and operations:
The message from the last decade of Darwin technology development is clear: if the Research and Technology effort that has been pursued in both Europe and the United States continues vigorously, Darwin’s technology will reach TRL 5/6 by 2010, allowing it to be selected as ESA’s first L mission for launch in 2018-2020.
So that’s the time frame. With as much play in the numbers as this suggests, and given the amount of work needed in tuning up both interferometry and formation flying, this may be an optimistic target. ESA is upfront about viewing DARWIN as one of the most ambitious missions it has ever undertaken. The potential scientific reward, however, is huge. With spectroscopy of individual Earth-like planets, we’ll have a good chance at discovering whether or not we’re looking at living worlds. No possibility could be more tantalizing.
The DARWIN mission proposal to ESA can be downloaded here in basic form, with a high-resolution version also available.
by Paul Gilster | Jul 25, 2007 | Exoplanetary Science |
HD 98800 is an unusual system indeed. About 150 light years away in the constellation TW Hydrae, the four stars that make it up consist of two binary pairs that circle each other. The distance between the two pairs is about 50 AU, which is roughly the average distance between Pluto and the Sun. Imagine having, instead of icy Kuiper Belt objects, a binary star system at the edge of our Solar System. Note: The reference above should probably be to the TW Hydrae association, not ‘constellation,’ as noted in the comments below.
Image: This artist concept depicts the quadruple-star system HD 98800. The system is approximately 10 million years old, and is located 150 light-years away in the constellation TW Hydrae. HD 98800 contains four stars, which are paired off into doublets, or binaries. The stars in the binary pairs orbit around each other, and the two pairs also circle each other like choreographed ballerinas. Credit: NASA/JPL-Caltech/T. Pyle (SSC-Caltech).
The idea of planet formation here may not be completely far fetched, because HD 98800B, one of the binary pairs, has a dusty disk around it. The evidence coming in from the Spitzer Space Telescope shows that the disk is actually made up of two belts, one at about the distance from the Sun to Jupiter, the other at around 1.5 to 2 AU (think of Mars and the asteroid belt in our own system). If planets did form here, we could certainly wind up talking about some interesting orbital mechanics. But the evidence is hardly complete. Says Elise Furlan UC-Los Angeles):
“Typically, when astronomers see gaps like this in a debris disk, they suspect that a planet has cleared the path. However, given the presence of the diskless pair of stars sitting 50 AU away, the inward-migrating dust particles are likely subject to complex, time-varying forces, so at this point the existence of a planet is just speculation.”
Speculation indeed. The paper on this work zeroes in on the factors involved, noting that the object under study appears to be a debris disk rather than a protoplanetary disk in the process of formation. As to the gravitational perturbations of the second binary pair:
This type of perturbation can pump up eccentricities and inclinations of particles, and cause particles to be trapped in mean motion resonances, as was likely the case for Kuiper Belt objects under the in?uence of the giant planets and possibly a close encounter by a passing, nearby star. Periodic stirring of planetesimals in the outer disk around HD 98800 B by the A pair could be responsible for generating copious amounts of dust. HD 98800 B is thus a unique type of debris disk, whose infrared excess is elevated to levels comparable to that of protoplanetary disks due to the particular con?guration of the four components in this system, resulting in gravitational perturbations that prevent the dust from settling into a ?at disk.
So that second binary pair 50 AU away complicates the picture considerably. Dust particles migrating to the inner disk following outer system collisions between rocky objects should produce a relatively continuous disk unless perturbed. We’re either seeing the beginnings of a planetary system, with the new planets clearing out debris in their orbital path, or we’re finding gaps created by the complex gravitational pull of the four stars in this curious system. Or perhaps both? Spitzer’s infrared spectrometer can give us that much, but to this point not much more.
As we find more binary and multiple systems with planets, we’re learning how complicated the process of disk formation in these environments can be. The paper is Furlan et al., “HD 98800: A 10-Myr-Old Transition Disk,” accepted by the Astrophysical Journal and available online.
by Paul Gilster | Jul 24, 2007 | Interstellar Medium |
Scientists studying the chemistry of interstellar space have identified around 130 neutral molecules along with perhaps a dozen positively charged molecules, but it was only late last year that the first negatively charged molecule — anion — was found, consisting of six carbon atoms and one hydrogen atom. It was a significant find because logic seemed to suggest that molecules would have a hard time retaining extra electrons, and thus a negative charge, in a star-rich environment.
Now we have a new anion, found using data from the Green Bank Telescope in West Virginia. The molecule is negatively-charged octatetraynyl, consisting of eight carbon atoms and one hydrogen atom, and it’s been located in the envelope of gas around an old, evolved star known as IRC +10 216, about 550 light-years from Earth. That makes three anions found in less than a year and in a range of environments.
Image: Astronomers using the Robert C. Byrd Green Bank Telescope found the negatively-charged form of octatetraynyl (C8H-) in a cold interstellar cloud (middle left) and in the gaseous envelope surrounding an old, evolved star (middle right). This is the largest negatively-charged molecule yet found in space. The scientists believe it probably is formed in steps, illustrated here, proceeding downward.
1. A molecule of C2H attaches to a molecule of C6H2, producing a molecule of C8H2 and a hydrogen atom.
2. Radiation (squiggly line) breaks one hydrogen atom from the C8H2, leaving C8H and a hydrogen atom.
3. Finally, an electron attaches itself to the C8H molecule, freeing a burst of radiation (overall glow seen around the molecule) and leaving the negatively-charged ion C8H-.
Credit: Bill Saxton, NRAO/AUI/NSF.
NASA’s Jan Hollis (GSFC) has this to say about this significance of the findings:
“Until recently, many theoretical models of how chemical reactions evolve in interstellar space have largely neglected the presence of anions. This can no longer be the case, and this means that there are many more ways to build large organic molecules in cosmic environments than have been explored.”
This is interesting work. What you do to identify such molecules is to figure out what radio frequencies are characteristic of the charged molecule, a task accomplished in a laboratory to nail down the frequency range the astronomers need to look for. A team from the Harvard-Smithsonian Center for Astrophysics (CfA) used such methods (and data from the GBT) to find the same anion in a cloud of molecular gas called TMC-1, which is in the constellation Taurus. It’s likely, based on this work, that many more negatively charged molecules exist in space.
Thus we keep learning more about the different ways complex organic and other molecular types can form, some of which may be precursors to life. The interstellar medium continues to give us clues about how widespread living things may be. The relevant papers are Remijan et al., “Detection of C8H– and Comparison with C8H toward IRC +10 216,” Astrophysical Journal Letters 664 (July 20, 2007), pp. L47-L50 (abstract) and Brünken et al., “Detection of the Carbon Chain Negative Ion C8H– in TMC-1,” in the same issue, pp. L43-L46 (abstract).
Related: University of Arizona astronomers have been examining the supergiant VY Canis Majoris, about 5000 light years from Earth and 25 times as massive as the Sun. The star is huge, but losing mass so quickly that it will be gone in a scant million years. The team have already detected a score of molecules never before found in interstellar space, including table salt (NaCl); a compound called phosphorus nitride (PN), which contains two of the five most necessary ingredients for life; molecules of HNC, which is a variant form of the organic molecule, hydrogen cyanide; and an ion molecule form of carbon monoxide that comes with a proton attached (HCO+). More in this news release.
Do life’s origins go back to the chemistry found in exotic places like the space around VY Canis Majoris? The paper is Ziurys et al., “Chemical complexity in the winds of the oxygen-rich supergiant star VY Canis Majoris,” Nature 447 (28 June 2007), pp. 1094-1097 (abstract).
by Paul Gilster | Jul 24, 2007 | Culture and Society |
Just a note that the History Channel’s series The Universe continues with a look at Saturn that is scheduled to run tonight at 9 PM EST here in the States, with a re-showing at 1 AM Wednesday morning. You can get a full schedule of repeat showings here — I notice the Saturn show pops up several more times in early August. I’ve enjoyed the series so far, and as you’ll see from its site, the History Channel is supporting it with various interactive features. You’ll see some names familiar from Centauri Dreams stories popping up among the researchers interviewed each week.
by Paul Gilster | Jul 23, 2007 | Exoplanetary Science |
What are the two most fundamental properties of the stars we study? If you said mass and chemical composition, you get the prize, at least as determined by the California & Carnegie Planet Search team. Their new paper lays out the discovery of a gas giant orbiting the M-class red dwarf GJ 317. And they first discuss the discovery in the context of the core accretion model for planetary formation, and the correlation between the metallicity of a star and the chances of its harboring detectable planets.
The notion seems sound: The host star inherits its characteristics from the same disk out of which the planets around it form. If you increase the amount of metals in the system (metals being defined as elements higher than hydrogen and helium), you increase the surface density of solid particulates, and that ought to bump up the growth rate for the core materials that become planets. In a gas giant, such a core then becomes massive enough to capture a gas envelope.
But the case around M dwarfs, those dim red stars that may comprise as much as 75 percent of the galaxy, needs a special look. If the mass of a protoplanetary disk scales with the mass of its central star, then larger mass stars should be more likely to produce planets. Greg Laughlin (UC-Santa Cruz) has studied this relationship in low mass stars like red dwarfs. His finding: Because their disks have lower surface densities (and longer orbital time scales), such stars should have problems producing Jupiter-mass planets. That leads to Neptune mass worlds that have exhausted their supply of gases in the disk through which they move.
So far, the analysis squares with observation. Most of the planets detected around M dwarfs are a good deal smaller than Jupiter. Until the California & Carnegie team’s recent work, only two nearby M dwarfs were known to have Jupiter-mass companions: GJ 876 and GJ 849. The upshot is that the frequency of giant planets is two to three times higher among stars of the Sun’s mass as compared to M dwarfs. All this, of course, has to be kept in the context of the relatively small sample within whose confines we work.
But GJ 317 now adds to the total, with the team finding a gas giant in an 1.897 year orbit around the star. We now have six M dwarfs known to harbor at least one Doppler-detected planet, with GJ 317 being just the third out of 300 surveyed to show a Jupiter-class world. A second possible Jovian planet is also being monitored in a 2700 day orbit, but no firm detection is yet being claimed.
But if the second planet is borne out, the result shouldn’t surprise us. Note this from the discovery paper (internal references deleted for brevity):
Multi-planet systems appear to be relatively common among M dwarfs compared to Sun-like stars. All M stars with one Jovian planet show evidence of a second companion. GJ 876 has a pair of Jupiter-mass planets in a 2:1 mean motion resonance, along with an inner “super Earth” GJ 849 has a long-period Jovian planet with a linear trend. Of the 3 M dwarfs with Neptune-mass planets, two have multiple planets or evidence of an additional companion: GJ 581 harbors 3 low-mass planets, and GJ 436 has a linear trend. Only GJ 674 appears to be in a single-planet system. From the first 6 planet detections around low-mass stars, it appears as though M dwarfs have an 80% occurrence rate of multi-planet systems, compared to the 30% rate measured for FGK stars.
Interesting, no? One possible cause for the difference is that fact that planets around low mass stars are more readily detectible by radial velocity methods. On the other hand, the team notes that all the Jovian planets detected around M dwarfs so far would have been detectable around Solar-mass stars. So we seem to be looking at a real effect here, one that will demand the accumulation of a lot more data through future surveys before its implications are fully understood.
Another key finding of this work is that A-type stars in the broader stellar sample studied are fully five times more likely than M dwarfs to harbor a gas giant. The team’s conclusion:
This important result establishes stellar mass as an additional sign post for exoplanets, along with metallicity. Just as metallicity informs the target selection of searches for short-period planets, stellar mass will be an important factor in the target selection of future high-contrast direct imaging surveys. While the lower luminosities of M dwarfs provide favorable contrast ratios that facilitate the detection of thermal emission from young giant planets, our results show that A-type stars are far more likely to harbor such planets.
Thus the correlation between stellar mass and the likelihood of finding giant planets seems to be firming up, but as the authors note, we need a larger star sample to really understand the relationship. The California & Carnegie team have added a number of higher mass stars to their earlier samples in hopes of tightening the focus. As we range between the smallest and some of the largest planet host stars, we’ll not only find more and more planets, but also uncover facts that should help in the target choice for future planet-hunter spacecraft.
The paper is Johnson et al., “A New Planet Around an M Dwarf: Revealing a Correlation Between Exoplanets and Stellar Mass,” accepted by the Astrophysical Journal and available online.
