by Paul Gilster | Jan 10, 2014 | Deep Sky Astronomy & Telescopes |
How do you boost the velocity of a star up to 540 kilometers per second? Somewhere in that region, with a generous error range on either side, is the speed it would take to escape the galaxy if you left from our Solar System’s current position. Here on Centauri Dreams we often discuss exotic technologies that could propel future vehicles, but it’s hard to imagine mechanisms that would drive natural objects out of the galaxy at such speeds. Even so, there are ways, as explained by Vanderbilt University’s Kelly Holley-Bockelmann:
“It’s very hard to kick a star out of the galaxy. The most commonly accepted mechanism for doing so involves interacting with the supermassive black hole at the galactic core. That means when you trace the star back to its birthplace, it comes from the center of our galaxy.”
The mechanism works like this: A binary pair of stars moving a bit too close to the massive black hole at the center of the Milky Way loses one star to the black hole while flinging the other outward at high velocity. When you calculate that the black hole has a mass equal to some four million Suns, this works: Stars can indeed be accelerated to galactic escape velocity, and so far a number of blue hypervelocity stars have been found that could be explained this way.
But Holley-Bockelmann and grad student Lauren Palladino have run into something that casts doubt on this explanation, or at least makes us wonder about other methods for making stars travel this fast. Calculating stellar orbits with data from the Sloan Digital Sky Survey, the duo have found about twenty stars the size of the Sun that appear to be hypervelocity stars. Moreover, these are stars whose composition mirrors normal disk stars, leading the researchers to believe they were not formed in the galaxy’s central bulge or its halo.
Image: Top and side views of the Milky Way galaxy show the location of four of the new class of hypervelocity stars. These are sun-like stars that are moving at speeds of more than a million miles per hour relative to the galaxy: fast enough to escape its gravitational grasp. The general directions from which the stars have come are shown by the colored bands. (Graphic design by Julie Turner, Vanderbilt University. Top view courtesy of the National Aeronautics and Space Administration. Side view courtesy of the European Southern Observatory).
Holley-Bockelmann and Palladino are working on possible causes for the movement of these stars, including interaction with globular clusters, dwarf galaxies or even supernovae in the galactic disk. The list of possibilities is surprisingly long, as noted in the paper on this work (internal references omitted for brevity):
While the SMBH [supermassive black hole] at the Galactic center remains the most promising culprit in generating HSVs [hypervelocity stars], other hypervelocity ejection scenarios are possible, such as a close encounter of a single star with a binary black hole… In this case, the star gains energy from the binary black hole and is flung out of the Galaxy while the orbit of the black hole binary shrinks…Another alternative hypervelocity ejection model involves the disruption of a stellar binary in the Galactic disk; here a supernova explosion in the more massive component can accelerate the companion to hypervelocities…
We may know more soon, for the paper points out that a nearby supernova should have contaminated the spectrum of a hypervelocity star. As they delve into these and other possibilities, the researchers are also expanding their search for hypervelocity stars to a larger sample within the Sloan data to include all spectral types.
The findings were announced at the meeting of the American Astronomical Society in Washington this week. The paper is Palladino et al., “Hypervelocity Star Candidates in the SEGUE G and K Dwarf Sample,” The Astrophysical Journal Vol. 780, No. 1 (2014), with abstract and preprint available.
by Paul Gilster | Jan 9, 2014 | Exoplanetary Science |
“Weather on Other Worlds” is an observation program that uses the Spitzer Space Telescope to study brown dwarfs. So far 44 brown dwarfs have fallen under its purview as scientists try to get a read on the conditions found on these ‘failed stars,’ which are too cool to sustain hydrogen fusion at their core. The variation in brightness between cloud-free and cloudy regions on the brown dwarf gives us information about what researchers interpret as torrential storms, and it turns out that half of the brown dwarfs investigated show these variations.
Given the chance nature of their orientation, this implies that most, if not all, brown dwarfs are wracked by high winds and violent lightning. The image below could have come off the cover of a 1950’s copy of Astounding, though there it would have illustrated one of Poul Anderson’s tales with Jupiter as a violent backdrop (“Call Me Joe” comes to mind). Brown dwarfs are, of course, a much more recent find, and in many ways a far more fascinating one.
Image: This artist’s concept shows what the weather might look like on cool star-like bodies known as brown dwarfs. These giant balls of gas start out life like stars, but lack the mass to sustain nuclear fusion at their cores, and instead, fade and cool with time. Credit: NASA/JPL-Caltech/University of Western Ontario/Stony Brook University.
Storms like these inevitably suggest Jupiter’s Great Red Spot, too, but we want to be careful with analogies considering how much we still have to learn about brown dwarfs themselves. What we can say is this: Brown dwarfs are too hot for water rain, leading most researchers to conclude that any storms associated with them are made up of hot sand, molten iron or salts.
The idea that brown dwarfs have turbulent weather is not surprising, but it is interesting to learn that such storms are evidently commonplace on them. Even more interesting is what the Spitzer work has revealed about brown dwarf rotation. Some of the Spitzer measurements found rotation periods much slower than any previously measured. Up to this point the assumption had been that brown dwarfs began rotating quickly shortly after they formed, a rotation that did not slow down as the objects aged. Aren Heinze (Stony Brook University) had this to say:
“We don’t yet know why these particular brown dwarfs spin so slowly, but several interesting possibilities exist. A brown dwarf that rotates slowly may have formed in an unusual way — or it may even have been slowed down by the gravity of a yet-undiscovered planet in a close orbit around it.”
Whatever the case, brown dwarfs do seem to be opening a window into weather systems in exotic places, systems that can be studied and characterized by their variations in brightness. Heinze presented this work at the 223rd annual meeting of the American Astronomical Society in Washington for principal investigator Stanimir Metchev (University of Western Ontario).
by Paul Gilster | Jan 8, 2014 | Exoplanetary Science |
As the 223rd meeting of the American Astronomical Society continues in Washington, we’re continuing to see activity on the subject of mini-Neptunes and ‘super-Earths,’ the latter often thought to be waterworlds. Given how fast our picture of planets in this domain is changing, I was intrigued to see that Nicolas Cowan (Northwestern University) and Dorian Abbot (University of Chicago) have come up with a model that allows a super-Earth with active plate tectonics to have abundant water in its mantle and oceans as well as exposed continents.
If Cowan and Abbot are right, such worlds could feature a relatively stable climate even if the amount of water there is far higher than Earth. Focusing on the planetary mantle, the authors point to a deep water cycle that moves water between oceans and mantle, a movement made possible by plate tectonics. The Earth itself has a good deal of water in its mantle. The paper argues that the division of water between ocean and mantle is controlled by seafloor pressure, which is proportional to gravity.
As planetary size increases, in other words, a super-Earth’s gravity and seafloor pressure go up as well. Rather than a waterworld with surface completely covered by water, the planet could have many characteristics of a terrestrial-class world, leading Cowan to say: “We can put 80 times more water on a super-Earth and still have its surface look like Earth. These massive planets have enormous sea floor pressure, and this force pushes water into the mantle.”
Image: Artist’s impression of Kepler-62f, a potential super-Earth in its star’s habitable zone. Could a super-Earth like this maintain oceans and exposed continents, or would it most likely be a water world? Credit: NASA/Ames/JPL-Caltech.
The implications for habitability emerge when we regard super-Earth oceans as relatively shallow. Exposed continents allow a planet to undergo the deep carbon cycle, producing the kind of stabilizing feedback that cannot exist on a waterworld. A super-Earth with exposed continents is much more likely to have an Earth-like climate, with all of this being dependent on whether or not the super-Earth has plate tectonics, and on the amount of water it stores in its mantle. Cowan calls the argument ‘a shot from the hip,’ but it’s an interesting addition to our thinking about this category of planet as we probe our ever growing database of new worlds.
On the matter of the amount of water, not all super-Earths would fit the bill, and it shouldn’t be difficult to turn a planet into a waterworld — Cowan argues that if the Earth were 1 percent water by mass, not even the deep water cycle could save the day. Instead, the researchers are considering planets that are one one-thousandth or one ten-thousandth water. From the paper:
Exoplanets with sufficiently high water content will be water-covered regardless of the mechanism discussed here, but such ‘ocean planets’ may betray themselves by their lower density: a planet with 10% water mass fraction will exhibit a transit depth 10% greater than an equally-massive planet with Earth-like composition… Planets with 1% water mass fraction, however, are almost certainly waterworlds, but may have a bulk density indistinguishable from truly Earth-like planets. Given that simulations of water delivery to habitable zone terrestrial planets predict water mass fractions of 10-5 – 10-2… we conclude that most tectonically active planets — regardless of mass — will have both oceans and exposed continents, enabling a silicate weathering thermostat.
The paper is Cowan and Abbot, “Water Cycling Between Ocean and Mantle: Super-Earths Need Not be Waterworlds,” The Astrophysical Journal Vol. 781, No. 1 (2014). Abstract and preprint available.
by Paul Gilster | Jan 7, 2014 | Exoplanetary Science |
Yesterday’s look at the exoplanet KOI-314c showed us a world with a mass equal to the Earth, but sixty percent larger than the Earth in diameter. This interesting planet may be an important one when it comes to studying exoplanet atmospheres, for KOI-314c is a transiting world and we can use transmission spectroscopy to analyze the light that passes through the atmosphere as the planet moves in front of and then behind its star. A space-based observatory like the James Webb Space Telescope should be able to tease useful information out of KOI-314c.
But the American Astronomical Society meeting in Washington DC continues, and it’s clear that the technique of studying transit timing variations (TTV) is coming into its own as a tool for exoplanet investigation. David Kipping and colleagues use TTV to look for exomoons, and it was during such a search that they discovered KOI-314c. But consider the other AAS news. At Northwestern University, Yoram Lithwick has been measuring the masses of approximately sixty exoplanets larger than the Earth and smaller than Neptune.
Learn the mass and the size of a planet, and you can make a call on its density, and thus learn something about its probable composition. And guess what?
“We were surprised to learn that planets only a few times bigger than Earth are covered by a lot of gas,” said Lithwick. “This indicates these planets formed very quickly after the birth of their star, while there was still a gaseous disk around the star. By contrast, Earth is thought to have formed much later, after the gas disk disappeared.”
That resonates nicely with Kipping and company’s work on KOI-314c, and Lithwick, working with graduate student Sam Hadden, used transit timing variation to achieve his results. Among the duo’s sample, planets two to three times larger than the Earth have very low density (compare with KOI-314c, which turned out to be only thirty percent denser than water). These are worlds something like Neptune except smaller and covered in massive amounts of gas.
Image: Chart of Kepler planet candidates as of January 2014. Credit: NASA Ames.
Transit timing variations occur when two planets orbiting the same star pull on each other gravitationally, so that the exact time of transit for each planet is affected. These are complicated interactions, to be sure, but we’re beginning to see radial velocity measurements confirming trends that have been originally uncovered with TTV. I ran this by David Kipping, asking whether TTV wasn’t coming into its own, and he agreed. “My bet is that when we measure the mass of Earth 2.0,” Kipping wrote, “it will be via TTVs.”
We can also look at the work of Ji-Wei Xie (University of Toronto), who used TTV to measure the masses of fifteen pairs of Kepler planets. These ranged in size from close to Earth to a little larger than Neptune, The results appeared in The Astrophysical Journal in December and were presented at the AAS meeting. The work complements reports from the Kepler team at AAS presenting mass measurements of worlds between Earth and Neptune in size. Here the follow-up used for the Kepler findings was based on Doppler measurements. In fact, six of the planets under investigation are non-transiting and seen only in Doppler data.
So we’re seeing both radial velocity and TTV used to study this interesting category of planets. 41 planets discovered by Kepler were validated by the program of ground-based observation, and the masses of sixteen of these were determined, allowing scientists to make the call on planetary density. In the Kepler study, ‘mini-Neptune’ planets with a rocky core show up with varying proportions of hydrogen, helium and hydrogen-rich molecules surrounding the core. The variation is dramatic, and some of these worlds show no gaseous envelope at all.
Kepler mission scientist Natalie Batalha sums up the questions all this raises:
“Kepler’s primary objective is to determine the prevalence of planets of varying sizes and orbits. Of particular interest to the search for life is the prevalence of Earth-sized planets in the habitable zone. But the question in the back of our minds is: are all planets the size of Earth rocky? Might some be scaled-down versions of icy Neptunes or steamy water worlds? What fraction are recognizable as kin of our rocky, terrestrial globe?”
Plenty of questions emerge from these findings, but the Kepler team’s report tells us that more than three-quarters of the planet candidates the mission has discovered have sizes between Earth and Neptune. Clearly this kind of planet, which is not found in our own Solar System, is a major player in the galactic population, and learning how such planets form and what they are made of will launch numerous further investigations. The usefulness of transit timing variations at determining mass will likely place the technique at the forefront of this ongoing work.
The paper by Ji-Wei Xie is “Transit Timing Variation of Near-Resonance Planetary Pairs: Confirmation of 12 Multiple-Planet Systems,” Astrophysical Journal Supplement Series Vol. 208, No. 2 (2013), 22 (abstract). I don’t have the citation for the Kepler report, about to be published in The Astrophysical Journal, but will run it as soon as I can. Yoram Lithwick’s presentation at AAS was based on Hadden & Lithwick, “Densities and Eccentricities of 163 Kepler Planets from Transit Time Variations,” to be published in The Astrophysical Journal and available as a preprint.
by Paul Gilster | Jan 6, 2014 | Uncategorized |
Search for one thing and you may run into something just as interesting in another direction. That has been true in the study of exoplanets for some time now, where surprises are the order of the day. Today David Kipping (Harvard-Smithsonian Center for Astrophysics) addressed the 223rd meeting of the American Astronomical Society in Washington to reveal a planetary discovery made during the course of a hunt for exomoons, satellites of planets around other stars. Kipping’s team has uncovered the first Earth-mass planet that transits its host star.
Just how that happened is a tale in itself. Kipping heads up the Hunt for Exomoons with Kepler project, which mines the Kepler data looking for tiny but characteristic signatures. Transit timing variations are the key here, for a planet with a large moon may show telltale changes in its transits that point to the presence of the orbiting body. In the case of the red dwarf KOI-314, it became clear Kepler was seeing two planets repeatedly transiting the primary. No exomoon here, it turned out, but as David Nesvorny (Southwest Research Institute) puts it:
“By measuring the times at which these transits occurred very carefully, we were able to discover that the two planets are locked in an intricate dance of tiny wobbles giving away their masses.”
The star, located about 200 light years away, was orbited by KOI-314b, about four times as massive as the Earth and circling the star every thirteen days. Transit timing variations, it became clear, flagged the presence not of an exomoon but another planet further out in the system, dubbed KOI-314c. And while the latter turns out to have the same mass as the Earth, it is anything but an ‘Earth-like’ planet. KOI-314c is, in Kipping’s words, ‘the lowest mass planet for which we have a size *and* mass measurement.’ With size and mass in hand we can work out the density. This is a world only thirty percent denser than water, as massive as the Earth but sixty percent bigger, evidently enveloped in a thick atmosphere of hydrogen and helium.
Are we dealing with something like a ‘mini-Neptune,’ perhaps one that has lost some of its atmosphere over time due to radiation from the star? Whatever the answer, KOI-314c forces us to re-examine our assumptions about small planets and how they are made. In a recent email, Kipping told me:
“If you asked an astronomer yesterday what they would guess the composition of a newly discovered Earth-mass planet was, they probably would have said rocky. Today, we know that is not true since now we have an Earth-mass planet with a huge atmosphere sat on top of it. Nature continues to surprise us with the wonderful diversity of planets which can be built.”
Image: Exomoon hunter David Kipping, whose fine-tuning of TTV (transit timing variations) is opening up new possibilities in exoplanet and exomoon detection.
Exactly so, and what a potent lesson in minding our assumptions! Kipping continues:
“We now know that one can’t simply draw a line in the sand at X Earth masses and claim “everything below this mass is rocky”. It hints at the fact that the recipe book for building planets is a lot more complicated than we initially thought and so far we’ve perhaps only been looking at the first page.”
A humbling notion indeed. Finding solar systems like our own, with rocky worlds in an inner region and gas giants further out, has proven surprisingly difficult, making us think that our system may be anything but typical. Now we’re examining a small planet that forces questions about planetary mass and composition and leaves us without easy answers. Its discovery highlights the significance of transit timing variations, a technique that is clearly coming into its own as we reach these levels of precision studying low mass planets. We’re still looking for that first exomoon, but the hunt for these objects is pushing the envelope of detection.
Kipping’s email mentioned yet another significant fact. KOI-314c circles a red dwarf close enough for detailed observations with the upcoming James Webb Space Telescope. The size of the atmosphere around this world could make it ideal for detecting molecules in its atmosphere, adding to our knowledge of this particular planet but also giving us another venue on which to sharpen the tools of atmospheric characterization. A worthy find indeed, and a reminder of how much we have yet to discover in Kepler’s hoard of data.
