by Paul Gilster | Jan 12, 2012 | Exoplanetary Science |
It’s always gratifying to note the contributions of amateur astronomers to front-line science. In the case of three small planets discovered around the Kepler star KOI-961, the kudos go to Kevin Apps, now a co-author of a paper on the new work. It was Apps who put postdoc Philip Muirhead (Caltech) on to the idea that KOI-961, a red dwarf, was quite similar to another red dwarf, the well-characterized Barnard’s Star, some six light years away in the constellation Ophiuchus. It was a useful idea, because we do have accurate estimates of Barnard Star’s size, and the size of the star becomes a key factor in exoplanet detections.
For the depth of a light curve — the dimming of the star over time due to the passage of planets across its surface as seen from Kepler — reveals the size of the respective planets. Researchers from Vanderbilt University aided the Caltech team in determining KOI-961’s size, a difficult call because while Kepler offers data about a star’s diameter, that data is considered unreliable for red dwarfs. Detailed spectra from both Palomar and Keck showed that Kevin Apps was right. “When we compared its fingerprint with those of the best known M dwarfs we found that Barnard’s Star was the best match,” says Vanderbilt astronomer Keivan Stassun.
Having established the size, mass and luminosity of KOI-961, the team could calculate the size and characteristics of the planets around it. Confirming three planets that are actually smaller than Earth took intensive follow-up, using photographs taken with the Palomar Observatory’s Samuel Oschin Telescope in 1951. KOI-961 is in Cygnus about 130 light years away, close enough to show motion in the time period involved, and it was clear from studying the photographs that no background stars could have accounted for the light curves.
We wind up with three small exoplanets that range in size from 0.57 to 0.78 times the radius of Earth. The primary is only about 70 percent bigger than Jupiter, but all three of the planets are so close to it that their temperatures are expected to range from 200 degrees Celsius for the outermost planet up to 500 degrees Celsius for the innermost. The entire system is small enough that Caltech astronomer John Johnson compares it to Jupiter and its moons, adding “This is causing me to have to fully recalibrate my notion of planetary and stellar systems.”
Image: This artist’s conception compares the KOI-961 planetary system to Jupiter and the largest four of its moons. The KOI-961 system hosts the three smallest planets known to orbit a star beyond our sun (called KOI-961.01, KOI-961.02 and KOI-961.03). The planet and moon orbits are drawn to the same scale. The sizes of the stars, planets and moons have been increased for visibility. Credit: Caltech.
All three planets are thought to be rocky, so small that only such a composition would allow them to hold together. And while the three are obviously not habitable, the fact that we’re now finding small worlds around red dwarfs has undeniable implications. Johnson again:
“Red dwarfs make up eight out of every ten stars in the galaxy. That boosts the chances of other life being in the universe—that’s the ultimate result here. If these planets are as common as they appear—and because red dwarfs themselves are so common—then the whole galaxy must be just swarming with little habitable planets around faint red dwarfs.”
After all, while Kepler reported 900 exoplanet candidates in February, only 85 or so were in red dwarf systems, a small sample but one that is already producing planets. The vast numbers of red dwarfs in the Milky Way thus gain further interest as possible sites for life, although as we’ve discussed many times in these pages, huge issues remain, such as tidal locking and the flare activity often found in younger red dwarfs. We can expect continuing study of the question of whether a rocky world in the habitable zone of such a star could offer life a viable foothold.
Ongoing red dwarf studies will add to our picture of the prevalence of such planets, as the paper on this work notes:
Combined with the low probability of a planetary system being geometrically aligned such that a transit is observed (only 13% in the case of KOI 961.01), Kepler’s discovery of planets around KOI 961 could be an indication that planets are common around mid-to-late M dwarfs, or at least not rare. This would be consistent with the results of Howard et al. (2011), who used the Kepler detections to show that the frequency of sub-Neptune-size planets (RP = 2-4 R?) increases with decreasing stellar e?ective temperature for stars earlier than M0. Results from on-going exoplanet surveys of M dwarfs such as MEarth (e.g. Irwin et al. 2011b), and future programs such as the Habitable Zone Planet Finder (e.g. Mahadevan et al. 2010; Ramsey et al. 2008) and CARMENES (Quirrenbach et al. 2010), will shed light on the statistics of low-mass planets around mid and later M dwarfs.
And what of Barnard’s Star itself? This Vanderbilt news release discusses the star’s appearance in televised science fiction but misses the wonderful use Robert Forward made of it in Rocheworld, a novel he wrote to illustrate his concept of ‘staged’ laser sails carrying a manned mission to the stars. In any case, the idea of planets around Barnard’s Star is not new. It was in the late 1960s that Peter van de Kamp announced what he thought were two gas giants in orbit around the star, but neither planet could be confirmed and the detection is now thought to have been the result of a systematic error produced by van de Kamp’s equipment.
Nonetheless, while we’ve been able to exclude planets larger than about five Earth masses within 1.8 AU of the star, Barnard’s Star could still have smaller undetected planets. Doubtless the steadily growing interest in red dwarfs will result in further studies of this nearby target.
The paper is Muirhead et al., “Characterizing the Cool KOIs III. KOI-961: A Small Star with Large Proper Motion and Three Small Planets,” accepted at the Astrophysical Journal (abstract).
by Paul Gilster | Aug 18, 2011 | Exoplanetary Science |
One of the problems with building a backlog of stories is that items occasionally get pushed farther back in the rotation than I had intended. Such is the case with an article in Astrobiology Magazine that talks about how much of a factor a large moon may be in making a planet habitable (thanks to Mark Wakely for passing the link along). It’s an interesting question because some have argued that without our own Moon, the tilt of the Earth’s axis, its ‘obliquity,’ would move over time from zero degrees to 85 degrees, a massive swing that would take the Sun from a position over the equator to one where it would shine almost directly over one of the poles.
The resulting climate changes would be severe, potentially affecting the development of life. The thinking is that just as the direction of the tilt of a planet varies with time — astronomers say that it ‘precesses’ — so does the orbital plane of the planet. The gravity of a large moon like ours affords a stabilizing effect by speeding up the Earth’s rotational precession and keeping it out of synch with the precession of the planet’s orbit. When rotational and orbital plane precession are synchronized, the obliquity begins to change chaotically. The Moon’s job, then, is to keep the two out of synch, minimizing the kind of fluctuations that would play havoc with life on the planet.
Image: The image shows Earth’s axial tilt (or obliquity), rotation axis, plane of orbit, celestial equator and ecliptic. Earth is shown as viewed from the Sun; the orbit direction is counter-clockwise (to the left). Credit: Wikimedia Commons/Astrobiology Magazine.
Jason Barnes (University of Idaho) and colleagues are behind the latest work (presented at the most recent American Astronomical Society meeting) which suggests another interpretation, arguing that the effect of the Moon on the Earth’s obliquity has been overstated — the Moon is not in fact crucial for the development of life. We can hope this bears out, because estimates of how many terrestrial planets will have a substantial moon get as low as one percent. Most of these worlds, then, under previous thinking, would experience huge changes in their obliquity, pointing toward a ‘rare Earth’ conclusion.
But that thinking is under challenge. Recent work by Sebastian Elser (University of Zurich) argues that the chances of large moons for such planets are as high as 10 percent. And Barnes’ team contrasts the gravitational effects of the Moon with those of other planets orbiting the Sun. The conclusion: The Moon does provide some stability to our planet, but the pull of Jupiter and, to a lesser extent, other planets orbiting the Sun would tend to keep the Earth’s obliquity swings in check. In fact, Barnes has determined that the Earth’s obliquity without a moon would vary only ten to twenty degrees over half a billion years. That’s enough to cause major climate changes, but it would “…not preclude the development of large scale, intelligent life,” Barnes adds.
I was intrigued by the team’s findings about planets with retrograde motion in their orbits. With or without a moon, the obliquity variations of a planet in this configuration — spinning in the opposite direction from their star — should be smaller than those orbiting in the same direction as the star. Barnes believes that the initial rotation direction of a planet should be random, voicing his suspicion that “whatever smacks the planet last establishes its rotation rate.” If this is true, the odds on retrograde precession are 50/50, lengthening the odds for relatively modest obliquity.
So we get help from the spin of a planet where it is retrograde, and also the combined gravitational effects of other planets in the system that help to reduce the planet’s tilt. If Barnes and team are right, then worlds lacking a large moon are still very much in the running for the development of life, a stability that he reckons may account for 75 percent of the rocky planets in the habitable zone. We’re a long way from confirming that idea, but it’s refreshing to hear this assertion that a large moon may not be a sine qua non after all, given how little we know about exoplanetary moons and the likelihood of their emergence in the right size range.
by Paul Gilster | Apr 22, 2011 | Exoplanetary Science |
Although we’re finding more and more exoplanets, we can always use another technique besides radial velocity, transit searches, direct imaging and microlensing. And now Jonathan Nichols (University of Leicester) has proposed one at the Royal Astronomical Society’s meeting in Llandudno, Wales, which concluded its proceedings yesterday. Nichols has the notion of looking for the radio emissions generated by the aurorae of planets like Jupiter, believing that these could be detected by radar telescopes like the soon to be completed LOFAR.
Now LOFAR (Low Frequency Array) is quite a story in itself, being the largest radio telescope ever constructed. The idea here is to create a vast array of some 7000 small antennae, distributed among some 77 larger stations across the Netherlands, Germany, Great Britain, France and Sweden. You wind up with a total collecting area whose interferometric data can be processed by a supercomputer at the University of Groningen in the Netherlands. The key here is to bring huge new sensitivity to radio frequencies below 250 MHz. In fact, LOFAR will reach down to about 10 MHz.
And that takes me right back to my childhood, working with an old Hallicrafters shortwave receiver. I had read that Jupiter could outshine the Sun at the wavelengths my receiver could detect, and today we know that interactions between Jupiter and Io can provide a power source for the radio emissions that move away from the planet’s magnetic poles in cone-shaped beams. It was possible to pick up oddball noises — people always describe them as staccato sounds like woodpeckers banging on the side of a house, and sometimes slow, swelling sounds that evoke waves rushing in to a shore — and pulling them in on a desktop receiver was a thrill. The article Detecting Jupiter’s Radio Emissions is a good introduction to Jovian radio possibilities.
Image: The Hubble telescope’s view of the rapid, spectacular dance of luminescent gases high in Jupiter’s atmosphere is allowing astronomers to map Jupiter’s immense magnetic field and better understand how it generates such phenomena. The ultraviolet-light images [bottom frames] show how the auroral emissions change in brightness and shape as Jupiter rotates. The aurorae are the bright, circular features at the top and bottom of the planet. The top panel illustrates the effects of emissions from Io, one of Jupiter’s moons. Io ejects an invisible electrical current of charged particles that flow along the planet’s magnetic field lines. Credit: ESA.
But back to Nichols, who told the RAS meeting in Llandudno that the LOFAR antennae will be sufficiently sensitive to detect the kind of emissions Jupiter makes in our own Solar System, even when they occur in systems many light years away:
“This is the first study to predict the radio emissions by exoplanetary systems similar to those we find at Jupiter or Saturn. At both planets, we see radio waves associated with auroras generated by interactions with ionized gas escaping from the volcanic moons, Io and Enceladus. Our study shows that we could detect emissions from radio auroras from Jupiter-like systems orbiting at distances as far out as Pluto,” said Nichols.
This is useful stuff, for we’d like to find more solar systems something like our own, thinking that these might be prime candidates for terrestrial-class worlds in inner orbits. But finding a Jupiter or a Saturn using transit or radial velocity methods would be a long process, given their distance from the central star. Nichols believes that planets orbiting UV-bright stars at distances between 1 and 50 astronomical units would generate enough radio power to be detectable from Earth. In the best case scenario, we should be able to detect such planets 150 light years away.
The paper is Nichols, “Magnetosphere-ionosphere coupling at Jupiter-like exoplanets with internal plasma sources: implications for detectability of auroral radio emissions,” accepted by Monthly Notices of the Royal Astronomical Society (preprint).
by Paul Gilster | Nov 4, 2010 | Exoplanetary Science |
I had hoped to be able to cover the Hartley 2 flyby today, but I’m traveling on Tau Zero business and have to write this entry early. Instead, I’ll at least keep the EPOXI mission focus by talking about the other half of this unique venture, an investigation of exoplanet systems. We can always talk about what the Hartley 2 encounter produced next week, but not before, as the schedule is crowded and I doubt I’ll be able to get an entry posted here at all on Friday.
Remember that the Deep Impact spacecraft that visited Tempel 1 in 2005 is now on an adventuresome extended mission called EPOXI (although the spacecraft, confusingly enough, still retains the original ‘Deep Impact’ name). The spacecraft was reawakened in the fall of 2007 and directed to a flyby of the Earth for a gravitational assist that would put it into a heliocentric orbit for the Hartley 2 encounter.
On the cruise portion of that journey, the extrasolar component of the mission kicked in. EPOCh (Extrasolar Planet Observation and Characterization) is the name of that investigation, the observing phase of which began in January of 2008 and ran until August of that year. A deep search of a nearby red dwarf star looking for a planet comparable in size to the Earth has been part of the larger EPOCh study.
33 light years from Earth, GJ 436 is known to host a Neptune-sized planet that transits every 2.6 days. The fact that GJ 436b’s orbit is slightly eccentric has drawn the attention of researchers interested in whether an unseen planet is behind the eccentricity, perhaps one as small as the Earth. Three weeks of continuous observations produced an excellent lightcurve of the Neptune-class world, but despite the fact that the potential planet was expected to orbit nearly edge-on to our line of sight, no transit or other data signature for it was found.
Image: EPOCh data for the transit of the Neptune-sized planet orbiting the red dwarf star GJ436. Using data of this quality, EPOCh was able to attain sensitivity to planets as small as 1.5 Earth radii. Credit: Ballard et al. 2010, Astrophysical Journal, Vol. 716, p. 1047.
The team is able to rule out transiting planets over 1.5 Earth radii to a high degree of confidence. Even in the absence of a transit, variations in the transits of the known planet could signal the existence of further planets. So is there a GJ 436c, or is there some other explanation for the eccentricity of GJ 436b’s orbit? Drake Deming, deputy principal investigator for EPOCh), has this to say in a post on the EPOXI site:
[The] negative result has provided motivation for theorists to consider that GJ436c may not exist. Theorists are re-examining their original conclusion that the orbital eccentricity of GJ436b requires the presence of a second planet. It is possible that tidal forces from the star are not sufficient — even in the absence of a second planet — to quickly drive the orbit of GJ436b to a circular state, as had been previously believed. That would obviate the need for GJ436c, but would require the interior structure of GJ436b to be unlike the gaseous planets of our solar system.
GJ 436 is one of eight stars chosen for the original EPOCh survey, all known to have other transiting planets. The other transiting planet systems included HAT-P-4, TrES-2, TrES-3, XO-2, GJ436, WASP-3, and HAT-P-7. In addition to finding new worlds, intense study of the known transiting planets could reveal the existence of moons or rings around the planet. The EPOCh team has now moved from processing the data to writing papers about the results.
But EPOCh isn’t through yet. Using the same data analysis technique developed for the mission and employed with GJ 436 data, researchers will soon be working with data from the Spitzer Space Telescope. The Deep Impact spacecraft may have Hartley 2 in its sights, but the EPOCh team will be looking to Spitzer’s investigation of the red dwarf system GJ 1214, some 42 light years from Earth. The Spitzer search, to be conducted in the spring of 2011, targets the habitable zone around this star, and the analytical tools developed for EPOCh coupled with Spitzer’s results could theoretically detect a planet here that is even smaller than the Earth.
Thus a spacecraft designed to drive an impactor into a comet — and one that has been re-purposed for a second comet encounter — produces exoplanet data and new analytical techniques that will now mesh with results from a space-based telescope (not to mention its studies of the Earth from space, that provide information about how to observe Earth-like planets at long range). No question about it, Deep Impact has given us plenty of bang for the buck.
You can find more about the EPOCh targets here, and ponder as I do what may turn up in the subsequent analysis of EPOXI and Spitzer data. The paper is Ballard et al., “A Search for Additional Planets in the NASA EPOXI Observations of the Exoplanet System GJ 436,” The Astrophysical Journal Vol. 716, Number 2 (20 June, 2010). Abstract available.
by Paul Gilster | Sep 8, 2010 | Exoplanetary Science |
We’re entering the era of the ‘super-Earths,’ when rocky planets larger than our own will pepper the lists of new discoveries. These smaller worlds will occasionally make a transit of their star, as does CoRoT-7b, and that’s when things really get interesting. After all, we know that secondary eclipses, in which a transiting exoplanet swings behind its star as seen from Earth, can be used to study distant atmospheres. The method collects light from both star and planet and, when the planet is hidden, subtracts the starlight to get the planetary signature.
Now Lisa Kaltenegger (Harvard-Smithsonian Center for Astrophysics) and colleagues Wade Henning and Dimitar Sasselov are advancing the idea that we can use near-term instrumentation like the James Webb Space Telescope to spot volcanic eruptions using these same methods. Their model is based on eruptions on an Earth-like planet, extrapolating from what happens on our world to suggest that sulfur dioxide from a major volcanic event on an exoplanet is measurable because it can be produced in huge quantity and is slow to wash out of the air.
Mount Pinatubo, which erupted in the Philippines in 1991, accounted for 17 million tons of sulfur dioxide blown into the atmosphere in a layer between 10 and almost 50 kilometers above the Earth’s surface. But we also have the example of the Tambora eruption of 1815, which was as much as ten times more powerful than Pinatubo. Tambora is the largest observed eruption in recorded history, an explosion that could be heard 2600 kilometers away. Fine ash particles stayed in the atmosphere for a period of years, and the summer of 1816 became known as ‘the year without a summer’ in the northern hemisphere , a climatic anomaly evidently related to the release of vast amounts of sulfur.
Eruptions of this magnitude don’t occur frequently on Earth, but there is no reason to think that young, rocky exoplanets would not be more volcanically active than our more mature world. And ponder the effects of tidal heating, not only on planets but also on their moons. From the paper:
Tidal heating may contribute significantly to volcanism for eccentric exomoons or eccentric planets in heliocentric periods of 20-30 days or less… Such tidal heating has the potential to generate from thousands to millions of times more internal heating than in the modern Earth. However, at the upper end of this heat range, magma is more likely to escape in a non-explosive pattern, or may simply emerge into lava lakes or magma oceans partly sustained by the high insolation values near stars, such as the magma ocean suggested for Corot 7b. Such extreme worlds may also be more likely to have a reducing atmosphere. Significant tidal activity can be stimulated in multi-body systems by secular perturbations, secular resonance crossings, or a deep and stable mean motion resonance, analogous to the Galilean moon system.
So the most extreme heating may well work against detectable volcanic activity, but tidal effects may be pronounced in more moderate scenarios:
Modest tidal heating cases may easily supply some exoplanets with both a 10x increase in the size and frequency of large eruptions, while simultaneously enhancing nonexplosive activity.
And that, of course, is only part of the picture. Volcanic activity may also be keyed to planetary age, with younger planets expected to have more residual accretion heat and higher radionuclides, while plate tectonic activity can increase the frequency of explosive volcanoes. The paper is careful to examine other mechanisms for heat escape in non-explosive events and notes that we don’t know whether hotter planets would necessarily show more explosive eruptions. Nonetheless, an Earth-like world less than thirty light years from the Sun should be a fair candidate for JWST studies that can help us put constraints on some of these variables.
Although it’s true that secondary eclipse studies give us a relatively crude picture of a planetary atmosphere, they do help us find particularly abundant molecules and provide us with a basic model that can be developed over time. This new work shows that in the best case scenario, volcanic features become visible at values between 1 to 10 times the Pinatubo eruption — the probability of observing an eruption of Pinatubo class is about 1 percent if four Earth-like planets are observed for one year, while a Tambora-size event could be detected with a 10 percent probability by observing roughly 50 such planets for two years. The paper concludes:
These observations becomes a very interesting option to characterize rocky planets, especially if one assumes larger, and/or more frequent eruptions than on Earth, or smaller host stars, where a planet in the HZ orbits closer to their stars, increasing the transit probability., or longer SO2 residence times than on Earth.
The paper is Kaltenegger et al., “Detecting Volcanism on Extrasolar Planets,” in press at The Astrophysical Journal (preprint).
