by Paul Gilster | Jan 7, 2015 | Exoplanetary Science |
The eight ‘habitable zone’ planets we discussed yesterday appear today in a much broader context. The Kepler mission has verified its 1000th planet, and with the detection of 554 more planet candidates, the total candidate count has now reached 4175. According to this NASA news release, six of the new planet candidates are near-Earth size and orbit in the habitable zone of stars similar to the Sun. These all require follow-up observation to confirm their status as planets, but with confirmed planets like Kepler-438b and Kepler-442b, along with these further candidates in the habitable zone, the numbers keep inching us closer to an Earth 2.0.
“Kepler collected data for four years — long enough that we can now tease out the Earth-size candidates in one Earth-year orbits,” says Fergal Mullally, a SETI Institute Kepler scientist at Ames who led the analysis of a new candidate catalog. “We’re closer than we’ve ever been to finding Earth twins around other sun-like stars. These are the planets we’re looking for.”
The next catalog release of the Kepler data set is now in the works, an analysis that will include the final month of mission data put through a new and more sensitive software iteration, one that should be able to tease out still further signatures of Earth-size planets.
Image: NASA Kepler’s Hall of Fame: Of the more than 1,000 verified planets found by NASA’s Kepler Space Telescope, eight are less than twice Earth-size and in their stars’ habitable zone. All eight orbit stars cooler and smaller than our Sun. The search continues for Earth-size habitable zone worlds around Sun-like stars. Credit: NASA/Kepler team.
The paper on the recently validated ‘habzone’ planets has now reached arXiv, where you can find the preprint of Torres et al., “Validation of Twelve Small Kepler Transiting Planets in the Habitable Zone.” I want to pull a paragraph out of this paper with regard to the BLENDER analysis I referred to yesterday. BLENDER is a powerful tool using software analysis of false-positive scenarios that is particularly useful when planets are so far away that radial velocity confirmation is difficult. A planetary transit can be mimicked by other phenomena, such as background or foreground eclipsing binary stars, whose light can create a ‘blend’ of signals that has to be untangled. The paper describes how BLENDER can rule false positives out:
BLENDER makes full use of the detailed shape of the transits to limit the pool of viable blends. It does this by simulating large numbers of blend scenarios and comparing each of them with the Kepler photometry in a ?2 sense. Fits that give the wrong shape for the transit are considered to be ruled out. This enables us to place useful constraints on the properties of the objects that make up the blend, including their sizes and masses, overall color and brightness, the linear distance between the background/foreground eclipsing pair and the target, and even the eccentricities of the orbits. Those constraints are then used to estimate the frequencies of blends of different kinds.
Image: False positive scenarios of the kind BLENDER can help resolve. Credit: Calar Alto Observatory/J. Lillo-Box.
BLENDER’s simulated lightcurves, along with other tools like high-resolution spectroscopy, allow tricky catches like Kepler-438b and Kepler-442b to be validated. The process is exhaustive and highlights just how far we’re progressing in our ability to find these small worlds. The paper presents the current state of the art in action, a fascinating and encouraging account that has been accepted for publication in The Astrophysical Journal.
Also apropos of this week of planetary announcements keyed to the American Astronomical Society meeting in Seattle is work that Courtney Dressing (Harvard-Smithsonian Center for Astrophysics) presented on Monday. Using the HARPS-North instrument on the 3.6-meter Telescopio Nazionale Galileo in the Canary Islands, Dressing and Harvard astronomer David Charbonneau have been examining the ten known exoplanets with diameter less than 2.7 times that of Earth that have accurately measured masses. The result: The five planets with diameters smaller than 1.6 that of Earth showed a tight relationship between mass and size.
“To find a truly Earth-like world,” says Dressing, “we should focus on planets less than 1.6 times the size of Earth, because those are the rocky worlds.”
In this study, larger and more massive exoplanets showed significantly lower densities, an indication that they include a large amount of water or other volatiles like hydrogen or helium. This CfA news release notes that Dressing and Charbonneau do not believe that all planets less than six times the mass of Earth are necessarily rocky — planets like those in the Kepler-11 system show both low mass and low density. But the assumption here is that the average low-mass planet orbiting near its star has a high chance of having a rocky composition like the Earth.
The paper on this work, which has been accepted for publication at The Astrophysical Journal, isn’t yet up on arXiv, but I’ll publish the reference when it becomes available. Meanwhile, the paper by Laura Schaefer and Dimitar Sasselov on ‘super-Earth’ oceans, examined here on Monday, has now appeared. It’s “The persistence of oceans on Earth-like planets: insights from the deep-water cycle,” likewise accepted for publication at The Astrophysical Journal (preprint).
by Paul Gilster | Jan 6, 2015 | Exoplanetary Science |
One way to confirm the existence of a transiting planet is to run a radial velocity check to see if it shows up there as a gravitationally induced ‘wobble’ in the host star. But in many cases, the parent stars are too far away to allow accurate measurements of the planet’s mass. What Guillermo Torres (Harvard-Smithsonian Center for Astrophysics) did in the case of eight new candidates possibly in their stars’ habitable zones was to use BLENDER, a software program he and Francois Fressin developed that runs at NASA Ames on the Pleiades supercomputer.
A BLENDER analysis can determine whether candidates are statistically likely to be planets. Torres and Fressin have applied it before in the case of small worlds like Kepler 20e and Kepler 20f, important finds because both were exoplanets near the size of the Earth. Using the software allowed the researchers to create a range of false-positive scenarios to see which could reproduce the observed signal. A nearby binary star system, for example, could cause a dimming of the star’s light that might be mistaken for a planet. The Pleiades supercomputer allowed the team to work through almost a billion different scenarios, which in the case of Kepler 20e showed that it was 3,400 times more likely to be a planet than a false positive.
Applying the same techniques to the eight new planet candidates, Torres and team went on to spend a year doing follow-up work in adaptive optics imaging, high-resolution spectroscopy and speckle interferometry to characterize the new systems. We learn from all this that all eight of these worlds meet the team’s standards for verifiability. All orbit at a distance where liquid water could occur on their surfaces, while two are, as researchers told a meeting of the American Astronomical Society today, more similar to Earth than any exoplanets we’ve yet found.
The similarity in question refers to the size and composition of the two planets rather than other broad characteristics like the star they orbit. Unlike our G-class star, the primary star for Kepler-438b is a red dwarf, while Kepler-442b orbits a K-class star. Kepler-438b receives about 40 percent more light than Earth (Venus receives twice the solar flux of Earth), while Kepler 442b gets about two-thirds the light of Earth. The team gives the latter a 97 percent chance of being in the habitable zone, while the former’s chances are calculated at 70 percent.
Image: This artist’s conception depicts an Earth-like planet orbiting an evolved star that has formed a stunning “planetary nebula.” Earlier in its life, this planet may have been like one of the eight newly discovered worlds orbiting in the habitable zones of their stars. Credit: David A. Aguilar (CfA).
Kepler-438b, 470 light years from Earth, is in a 35-day orbit, while Kepler 442b (1100 light years away) completes an orbit around its star every 112 days. Four of the eight newly found planets are in multiple-star systems, although in each case, according to this CfA news release, the companion stars are far enough away not to exert a significant influence on the observed planets.
A key question is whether these really are rocky worlds — without a measurement of planetary mass, their composition is unknown. Torres and colleagues think that Kepler-438b, with a diameter about 12 percent larger than Earth, has a 70 percent chance of being rocky. Kepler 442b is about a third larger than Earth, but by the team’s reckoning has a 60 percent chance of being rocky. So these are intriguing possibilities, but it has to be said that habitability remains no more than an inference. “We don’t know for sure whether any of the planets in our sample are truly habitable,” says second author David Kipping (CfA). “All we can say is that they’re promising candidates.”
by Paul Gilster | Jan 5, 2015 | Exoplanetary Science |
We often think about how thin Earth’s atmosphere is, imagining our planet as an apple, with the atmosphere no thicker than the skin of the fruit. That vast blue sky can seem all but infinite, but the great bulk of it is within sixteen kilometers of the surface, always thinning as we climb toward space. Now a presentation by graduate student Laura Schaefer (Harvard-Smithsonian Center for Astrophysics) at the 225th meeting of the American Astronomical Society in Seattle points out that, like the atmosphere, water is also a tiny fraction of what makes up our planet.
A small enough fraction, in fact, that although water does cover seventy percent of the Earth’s surface, it makes up only about a tenth of one percent of the overall bulk of a world that is predominantly rock and iron. Dimitar Sasselov (CfA), co-author of the paper on this work, thinks of Earth’s oceans as a film as thin as fog on a bathroom mirror. But we’ve seen recently that water isn’t strictly a surface phenomenon. The Earth’s mantle, in fact, holds several oceans of water pulled underground by plate tectonics and subduction of the ocean seafloor.
What Schaefer presented at the AAS is a report on her computer simulations of the planet-wide recycling that keeps Earth’s oceans from disappearing. Volcanic outgassing from the mantle, primarily at the mid-ocean ridges, keeps water returning to the surface even as subduction returns water to the mantle. The cycle maintains the oceans over aeons. The question for the researchers was whether similar cycles occur on super-Earths, and how long it would take an ocean to form after the cooling of a planet’s crust during its formation period.
The results are encouraging for those hoping to find stable oceans on super-Earths. Planets two to four times Earth’s mass turn out to be better at maintaining their oceans than Earth itself. Super-Earth oceans can persist for ten billion years unless destroyed by a red giant primary star as it nears the end of its life. The largest planet in these simulations — five times Earth’s mass — took a billion years to develop its ocean in the first place, however, the result of a thicker crust and lithosphere and the resultant delay in volcanic outgassing.
Image: This artist’s depiction shows a gas giant planet rising over the horizon of an alien waterworld. New research shows that oceans on super-Earths, once established, can last for billions of years. Credit: David A. Aguilar (CfA).
We have nothing to compare the timeframe of life’s development on Earth with, having no data on life elsewhere. But if we took our model as the norm, says this CfA news release, we would be wise to look for life on older super-Earths, those perhaps a billion years older than the Earth, given the lag time in getting those oceans into play. Sasselov notes:
“It takes time to develop the chemical processes for life on a global scale, and time for life to change a planet’s atmosphere. So, it takes time for life to become detectable.”
My own guess is that once we do develop the ability to study exoplanet atmospheres on the level of Earth-sized worlds, we’ll run into surprises on this front as well, depending on how typical the experience of getting life started on Earth really was. In any case, screening for older planets as the best targets for complex life seems like a rational procedure, but especially with super-Earths for whom surface water may be a slow-developing resource.
The paper is Schaefer and Sasselov, “Persistence of oceans on Earth-like planets,” American Astronomical Society, AAS Meeting #225, #406.04 (abstract).
by Paul Gilster | Jan 2, 2015 | Deep Sky Astronomy & Telescopes |
Every time I mention stellar distances I’m forced to remind myself that the cosmos is anything but static. Barnard’s Star, for instance, is roughly six light years away, a red dwarf that was the target of the original Daedalus starship designers back in the 1970s. But that distance is changing. If we were a species with a longer lifetime, we could wait about eight thousand years, at which time Barnard’s Star would close to less than four light years. No star shows a larger proper motion relative to the Solar System than this one, which is approaching at about 140 kilometers per second.
The Alpha Centauri stars are the touchstone for close mission targets, but here again we could make our journey shorter with a little patience. In 28,000 years, having moved into the constellation Hydra, these stars will have closed to less than 3 light years from the Sun. Some time back, Erik Anderson discussed star motion in his highly readable Vistas of Many Worlds (Ashland Astronomy Studio, 2012), where I learned that the star Gliese 710, currently 64 light years out in the constellation Serpens, is headed squarely in our direction. Wait around for 1.3 million years or so and Gl 710 will push right through the Oort Cloud, with who knows what results in the inner system. A new paper considers these matters and tunes up the numbers on stellar encounters.
Image: Could a passing star dislodge comets from otherwise stable orbits so that they enter the inner system? Credit: NASA/JPL-Caltech).
A close pass from a star is bound to cause effects elsewhere in the Solar System, as Coryn Bailer-Jones (Max Planck Institute for Astronomy, Heidelberg) notes in his latest paper. Such an encounter can disrupt cometary orbits in the Oort, sending them into the inner system. Earth’s catalog of impact craters, which contains almost 200 known craters and doubtless should include many awaiting discovery, some of them beneath the oceans, is a reminder of what can happen. Nor should we forget that if we really drew the wild card, a close star turning supernova could have disastrous effects on surface life. So how many stars are problematic?
Bailer-Jones identifies the key candidates in this paper, assuming an Oort Cloud that extends to about 0.5 parsecs (1.6 light years), but he notes that a star passing even as close as several parsecs could produce significant cometary disruptions if the star were massive and slow enough. The author worked with 50,000 stars from the Hipparcos astrometric catalog in hopes of fine-tuning earlier studies of passing stars, but he notes that the search can’t be considered complete because radial velocities are not available for all stars and many are fainter than the Hipparcos work could detect. Further analysis will be needed using upcoming Gaia data.
But studying stars within a few tens of light years from the Solar System, Bailer-Jones finds forty that at some point were or will be within 6.4 light years of the Sun — the timeframe here extends from 20 million years in the past to 20 million years in the future. Fourteen stars, in fact, come within 3 light years of the Sun, with the closest encounter being with HIP 85605, which is currently about 16 light years away in the constellation of Hercules. The paper cites “…a 90% probability of [the star] coming between 0.04 and 0.20 pc” somewhere between 240,000 and 470,000 years from now, but Bailer Jones notes that this encounter has to be treated with caution because the astrometry may be incorrect. Future Gaia data should resolve this.
If HIP 85605 were to close to 0.04 parsecs of the Sun, it would be .13 light years out, or roughly 8200 AU, a close pass indeed. But one thing to keep in mind: Oort Cloud perturbation is not an unusual phenomenon, and the situation we are dealing with today is partially the result of encounters with stars that have occurred in the past. We have no data on the time between stellar encounters like these and the subsequent entry of comets into the inner system, making it all but impossible to link a specific passing star with a rise in the rate of Earth impacts. Bailer-Jones discusses all this on his website at the MPIA, where he notes the following:
A close encountering star is likely to perturb the Oort cloud sufficiently to increase the flux of comets entering the inner solar system. Let’s not forget, however, that this kind of perturbation is happening all the time due to the gravitational effect of the Galaxy as whole, and due to stars which [were] encountered even earlier. That is, there is a “background” of comets entering the inner solar system which we cannot necessarily associate with a particular stellar encounter. This is also because the time between an encounter and the time that comets enter the inner solar system could be many or even many tens of millions of years, much longer that than the typical time between close encounters.
Gl 710 is generally cited as the star making the closest encounter in previous studies, and Bailer-Jones sees a 90 percent probability that it passes within 0.10 to 0.44 parsecs, meaning an Oort Cloud passage in 1.3 million years. Looking into the past, the star gamma Microscopii, a G6 giant, encountered the Sun 3.8 million years ago, probably the most massive encounter within one parsec or less. Some encounters are recent: Tiny Van Maanen’s star, a white dwarf, passed near our Sun as recently as 15,000 years ago. While data from the Gaia mission will help us improve the parameters of this catalog of passing stars, Bailer-Jones believes the Gaia results will also make it possible to investigate the link between stellar encounters and impacts in a broad, statistical sense, helping us better understand the history of Earth impacts.
The paper is Bailer-Jones, “Close Encounters of the Stellar Kind,” accepted at Astronomy & Astrophysics (preprint).
