by Paul Gilster | Apr 21, 2020 | Exoplanetary Science |
We’re learning interesting things about 2I/Borisov, the first interstellar comet discovered entering our Solar System (‘Oumuamua may have been a comet as well, but the lack of an active gas and dust coma makes it hard to say for sure). Moving at 33 kilometers per second, 2I/Borisov is on a trajectory clearly indicating an interstellar origin. Now two different studies have shown that in terms of composition, the visiting object is unlike most of the comets found in our own system.
Both the Hubble Space Telescope and the Atacama Large Millimeter/submillimeter Array (ALMA) have found levels of carbon monoxide (CO) higher than expected, a concentration greater than any comet yet detected within 2 AU of the Sun (about 300 million kilometers). The ALMA team finds the CO concentration to be somewhere between 9 and 26 times higher than inner system comets, while Hubble sees levels at least 50 percent more abundant than the average of comets in the inner system. Dennis Bodewits (Auburn University) is lead author of the paper on the Hubble work, which appears in Nature Astronomy:
“The amount of carbon monoxide did not drop as expected as the comet receded from the Sun. This means that we are seeing the primitive layers of the comet, which really reflect what this object is made of. Because of the abundance of carbon monoxide ice that survived so close to the Sun, we think that comet Borisov comes from a much colder place and from a very different debris disk around a star than our own.”
Image: Comet 2I/Borisov, captured here in two separate images from NASA’s Hubble Space Telescope, including one with a background galaxy (left), is the second interstellar object known to enter our Solar System. New analysis of Borisov’s bright, gas-rich coma indicates the comet is much richer in carbon monoxide gas than water vapor, a characteristic very unlike comets from our Solar System. Credit: NASA, ESA and D. Jewitt (UCLA).
2I/Borisov isn’t completely off the charts — there is one Oort Cloud comet, the recently discovered C/2016 R2 (PanStarrs), that showed even higher CO levels than Borisov when it reached a distance of 2.8 AU from the Sun. But the Borisov results are intriguing and give us a preliminary benchmark against which to measure interstellar objects discovered in the future. The Hubble work was performed using the instrument’s Cosmic Origins Spectrograph, sensitive to ultraviolet, with data taken in four periods from Dec. 2019 to Jan. 2020. NASA’s Swift satellite was able to confirm Hubble’s readings with measurements taken over the same period.
Carbon monoxide is volatile enough that it begins to sublimate (turn from ice to gas) far out in the Solar System, perhaps as much as three times the distance between Pluto and the Sun. Water, on the other hand, remains an ice until somewhere between Mars and the inner edge of the main asteroid belt, and water is the prevalent signature of comets in the inner system. Adds Bodewits: “Borisov’s large wealth of carbon monoxide implies that it came from a planet formation region that has very different chemical properties than the disk from which our solar system formed.”
In a second study reported in the same issue of Nature Astronomy, scientists using ALMA data collected in the same time frame as Hubble’s (15-16 December 2019) detected both hydrogen cyanide (HCN) and CO in Borisov. The HCN showed up at levels that were not surprising to researchers, but the CO amounts had researchers speculating on the object’s origins. Martin Cordiner, along with Stefanie Milam, led an international team at NASA GSFC:
“Most of the protoplanetary disks observed with ALMA are around younger versions of low-mass stars like the Sun. Many of these disks extend well beyond the region where our own comets are believed to have formed, and contain large amounts of extremely cold gas and dust. It is possible that 2I/Borisov came from one of these larger disks… This is the first time we’ve ever looked inside a comet from outside our solar system, and it is dramatically different from most other comets we’ve seen before.”
Image: ALMA observed hydrogen cyanide gas (HCN, left) and carbon monoxide gas (CO, right) coming out of interstellar comet 2I/Borisov. The ALMA images show that the comet contains an unusually large amount of CO gas. ALMA is the first telescope to measure the gases originating directly from the nucleus of an object that travelled to us from another planetary system. Credit: ALMA (ESO/NAOJ/NRAO), M. Cordiner & S. Milam; NRAO/AUI/NSF, S. Dagnello.
The Hubble team considers a carbon-rich circumstellar disk around a cool red dwarf as one possibility for 2I/Borisov’s birthplace. Here, gravitational interactions with a planet orbiting in the outer reaches of the system could have ejected the comet. “These stars have exactly the low temperatures and luminosities where a comet could form with the type of composition found in comet Borisov,” says John Noonan (Lunar and Planetary Laboratory, University of Arizona, Tucson).
This is interesting speculation, but the real news here is that this is the first measurement of the carbon monoxide composition of a comet from another star. The work points to the entirely new area of exoplanetology that is opening up, the study of debris from other stellar systems that will be further enabled as new telescopes and survey methods come online. Future objects should help us clarify how to use such data as we explore planet formation around other stars.
The papers are Bodewits et al., “The carbon monoxide-rich interstellar comet 2I/Borisov,” Nature Astronomy 20 April 2020 (abstract) and Cordiner et al., “Unusually high CO abundance of the first active interstellar comet,” in the same issue of Nature Astronomy (full text).
by Paul Gilster | Apr 19, 2020 | Deep Sky Astronomy & Telescopes |
Back in January — and boy does that seem like another era — I wrote about the plan to look at two nearby stars with the help of the New Horizons spacecraft as well as observations from the general public. If you’d like to get involved, there is still time, but the date is fast approaching. Amateur equipment and digital cameras have reached the point where astronomy at a very high level can be conducted from small observatories and even back yards. Here’s another chance to make the case for the value of such work.
Tha planned observations take advantage of parallax, the apparent shift in position of nearby stars as measured using the radius of the Earth’s orbit. Friedrich Bessel’s groundbreaking work on stellar distances involved taking such measurements to calculate the distance of 61 Cygni, all this back in 1838. The apparent shift of the star against background stars allowed him to peg 61 Cygni’s distance at 10 light years, reasonably close to the modern figure of 11.4.
New Horizons gives us a baseline extending all the way to the Kuiper Belt. By the time we take the upcoming observations of Proxima Centauri and Wolf 359, the spacecraft will be 46 times farther from the Sun than Earth (some 8 billion kilometers out). Combining the New Horizons data with Earth-based images made on April 22 and 23 will yield a record-setting parallax measurement as the two stars seem to shift in position against the background.
“These exciting 3D images, which we’ll release in May, will be as if you had eyes as wide as the solar system and could detect the distance of these stars yourself,” said New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute, Boulder, Colorado. “It’ll be a truly vivid demonstration of the immense distance New Horizons has traveled, and a cool way to take advantage of the spacecraft’s unique vantage point out on the very frontier of our solar system!”
Image: This figure illustrates the phenomenon of stellar parallax. When New Horizons and observers on Earth observe a nearby star at the same time, it appears to be in different places compared to more distant background stars — this is because New Horizons has traveled so far out in space that it has to look in a different direction to see that star. The small images below Earth and New Horizons show each unique view. Note that the farther-away background stars stay in the same place, but the nearby star appears to move between the two vantage points. Credit: Pete Marenfeld, NSF’s National Optical-Infrared Astronomy Research Laboratory.
Those wishing to participate will need a camera equipped telescope with 6-inch aperture or larger. To learn more, click here. Proxima Centauri is 4.244 light years away, a distant companion to the primary Centauri A and B stars. It’s also at quite a large angle in the sky from Wolf 359, making both stars a useful reference as the team explores autonomous interstellar navigation. Robotic missions within the Solar System have used optical imagery for navigation before, but New Horizons takes the technique into interstellar trajectories as it heads out ever deeper into the Kuiper Belt. Thus New Horizons science team member Tod Lauer:
“For all of history, the fixed stars in the night sky have served as navigation markers. As we voyage out of the solar system and into interstellar space, how the nearer stars shift can serve as a new way to navigate. We will see this for the first time with New Horizons.”
Image: This figure, by New Horizons contributing scientist Brian May, shows the parallax as an effect of New Horizons’ travels deeper into the Kuiper Belt. Traditionally, parallax is measured as the Earth orbits around the sun. The two lines at left show the lines of sight from Earth to the star on either side of Earth’s orbit. This causes a small shift in the position of the nearby star compared to more distant stars. New Horizons is so far away that a much larger shift in the line of sight to the star occurs. Credit: Brian May.
As for Wolf 359, it’s a red dwarf 7.9 light years away in Leo. You may recall numerous science fiction references to the star, ranging from “Wolf 359”, an episode on the 60’s TV show The Outer Limits to Harry Harrison’s Captive Universe (1969), a generation ship novel, and a double episode of Star Trek: The Next Generation. More recently, there is the Hugo nominated Ken MacLeod short story “Who’s Afraid of Wolf 359?”
While Proxima Centauri is not visible for most northern hemisphere observers, Wolf 359 should be observable from both hemispheres with telescopes of sufficient aperture. Star charts of the Wolf 359 and Proxima Centauri fields are available at the project site.
by Paul Gilster | Apr 16, 2020 | Exoplanetary Science |
What intrigues me about Kepler-1649c, a newly discovered planet thrust suddenly into the news, isn’t the fact that it’s potentially in its star’s habitable zone, nor that it is close to being Earth-sized (1.06 times Earth’s radius). Instead, I’m interested in the way it was found. For this is a world turned up in exhaustive analysis of data from the original Kepler mission. Bear in mind that data from the original Kepler field ceased being gathered a full seven years ago.
I share Jeff Coughlin’s enthusiasm on the matter. Coughlin is an astronomer affiliated with the SETI Institute who is a co-author on the new paper, which appears in Astrophysical Journal Letters. One of the goals of the mission that began as Kepler and continued (on different star fields) as K2 was to find the fraction of stars in the galaxy that have planets in the habitable zone, using transit methods for initial detection and radial velocity follow-up on Earth. Of the new find, made with an international team of scientists, Coughlin says:
“It’s incredible to me that we just found [Kepler-1649c] now, seven years after data collection stopped on the original Kepler field. I can’t wait to see what else might be found in the rich dataset from Kepler over the next seven years, or even seventy.”
Yes, because we’re hardly through with a dataset that has taken in close to 200,000 stars. What the new planet reminds us is that the initial detection stage of Kepler’s work long ago ceded to analysis of how many of the acquired signals are due to systematic effects in the instrumentation, or variable stars, or perhaps binaries, for so many things can appear to be the signature of a planet when they are not. Making computer algorithms to automate this work has occurred only after intensive human study to learn the best ways to distinguish between signals.
But consider: The algorithm, called Robovetter, that was used to distinguish false positives was working on data in which only 12 percent of the transit signals turned out to be planets. We have false positives galore, out of which we now get the Kepler False Positive Working Group to serve as a double-check on the algorithm’s work. The KFPWG is a dedicated team indeed, taking years to review the thousands of signals from the original Kepler dataset. Kepler-1649c comes out of this review, a world that had been misclassified by the automated methods earlier on. Clearly, humans and machines learn from each other as they make such difficult calls.
Image: A comparison of Earth and Kepler-1649c, an exoplanet only 1.06 times Earth’s radius. Credit: NASA/Ames Research Center/Daniel Rutter.
I yield to the experts when it comes to habitability, and trust our friend Andrew LePage will weigh in soon with his own analysis. What we do know is that this planet receives about 75 percent of the amount of light Earth receives from the Sun, though in this case the star in question is a red dwarf of spectral type M5V, the same as Proxima Centauri, fully 300 light years out. As always with red dwarfs, we bear in mind that stellar flares are not uncommon. The planet orbits its star every 19.5 days. We have no information about its atmosphere.
Even so, lead author Andrew Vanderburg (University of Texas at Austin) notes the potential for habitability:
“The more data we get, the more signs we see pointing to the notion that potentially habitable and Earth-sized exoplanets are common around these kinds of stars. With red dwarf stars almost everywhere around our galaxy, and these small, potentially habitable and rocky planets around them too, the chance one of them isn’t too different than our Earth looks a bit brighter.”
There is a planet on an orbit interior to Kepler-1649c, a super-Earth standing in relation to the former much as Venus does to the Earth. The two demonstrate an uncommon nine-to-four orbital resonance, one that hints at the possibility of a third planet between them. This would give us a pair of three-to-two resonances, a much more common scenario, though no trace of the putative world shows up in the data. If there, it is likely not a transiting planet.
I rather like the image that comes out of NASA Ames, where the Kepler effort is managed, but I like it as a kind of science fictional art, the sort of thing to be found on SF paperbacks. It’s evocative and suggestive, but of course we have no idea what this world really looks like.
Image: An illustration of what Kepler-1649c could look like from its surface. Credit: NASA/Ames Research Center/Daniel Rutter.
The paper is Vanderburg et al., “A Habitable-zone Earth-sized Planet Rescued from False Positive Status,” Astrophysical Journal Letters Vol. 893, No. 1 (1 April 2020). Abstract.
by Paul Gilster | Apr 15, 2020 | Missions |
Get ready for the Polarimeter to UNify the Corona and Heliosphere (PUNCH) mission, which will begin popping up even as Centauri Dreams continues to consider heliophysics in relation to proposed missions far beyond the Solar System. We’ve seen recently that the Applied Physics Laboratory at Johns Hopkins is looking, under the leadership of Ralph McNutt, at a mission to 1000 AU, using an Oberth maneuver at the Sun as a possible way to reach such distances with a flight time of 50 years (see The 1000 AU Target). Thus do heliophysics and deep space intersect in unexpected ways, and not just at APL but JPL and elsewhere as we look toward the upcoming decadal survey.
As for PUNCH, it’s all about the solar wind and the connection between it and the Sun’s corona, says PUNCH principal investigator Craig DeForest of Southwest Research Institute’s Space Science and Engineering Division:
“For over 50 years, we’ve studied the solar corona by remote imaging and the solar wind by direct sampling. PUNCH will bridge that gap by imaging the solar wind itself as it leaves the outermost reaches of the Sun’s corona.”
Image: NASA has selected Southwest Research Institute to lead a microsatellite mission to image the Sun’s outer corona. PUNCH proposes a constellation of four suitcase-sized satellites that will orbit the Earth, studying how the Sun’s corona connects with the interplanetary medium, to better understand how coronal structures infuse the solar wind with mass and energy. Credit: Southwest Research Institute.
The four PUNCH spacecraft will orbit the Earth as a constellation in a polar orbit. Aboard one will be a coronagraph whose Narrow Field Imager will watch the Sun’s corona continuously, while wide-angle cameras and Wide Field Imagers optimized to image the solar wind with a 90 degree field of view are aboard the other three. The good news about PUNCH is that it has now passed the System Requirements Review/Mission Definition Review, a major step for this NASA Small Explorer (SMEX) mission.
The spacecraft will be built, along with the Wide Field Imagers, by the Southwest Research Institute (SwRI), while the US Naval Research Laboratory will produce the Narrow Field Imager, with additional contributions from the Rutherford Appleton Laboratory in Oxfordshire, England. The work has continued despite the extraordinary conditions forced by COVID-19. Says principal investigator Craig DeForest (SwRI):
“Preparing for the SRR/MDR review was an unexpected challenge. Team members have been working at home for over three weeks, yet they met the challenge and presented their design in written and oral presentations culminating in the positive decision from our review board. I’m very proud of this team for what they’ve accomplished during less than ideal collaboration conditions. While spacecraft are constructed out of aluminum and exotic metals, they are initially made of requirements documents.”
Image: SwRI developed and prototyped the Wide Field Imager for the PUNCH mission. The dark baffles in the top recess allow the instrument to image objects over a thousand times fainter than the Milky Way. Credit: Southwest Research Institute.
A major document requirement has now been passed. Insights into the solar wind are sure to flow from this mission. And from a deep space perspective, a number of concepts have been advanced that would ‘ride’ the solar wind despite its turbulence. More accurate modeling in such studies will surely be an ancillary result from PUNCH, as will improved understanding of conditions near the Sun for future spacecraft on close flybys.
by Paul Gilster | Apr 13, 2020 | Exoplanetary Science |
The interstellar object we call ‘Oumuamua was bound to be fascinating no matter what it actually was. You discover the first incoming object from interstellar space only once. But this one had its own share of peculiarities. Here was what was assumed to be a comet, but one that showed no outgassing as it reached perihelion and in fact seemed to be unusually dry. Here was an object of an apparently elongated shape, an aspect ratio with which we had nothing to compare in our own system. A tiny but detectable acceleration on the way out of the system seemed to indicate later outgassing, but how was that consistent with earlier data?
I think Harvard’s Avi Loeb was exactly right to point out that among the possible explanations of new objects, we can’t disregard the possibility of a technology from another civilization. That ‘Oumuamua was a natural object is an obvious default position, but we are at a stage in our understanding of the cosmos when we realize that the conditions for life occur elsewhere. We don’t know how often it forms — abiogenesis may be spectacularly rare. But maybe not.
But while we’ll never know with 100 percent certainty what ‘Oumuamua was, we can look forward to many more interstellar objects to evaluate as new instrumentation comes online. Meanwhile, that apparently elongated shape that so distinguished ‘Oumuamua has come under scrutiny from Yun Zhang (National Astronomical Observatories of the Chinese Academy of Sciences) and Douglas N. C. Lin (UC-Santa Cruz), who have run computer simulations that show how it could have formed and go on to explain its other oddball quirks.
“We showed that ‘Oumuamua-like interstellar objects can be produced through extensive tidal fragmentation during close encounters of their parent bodies with their host stars, and then ejected into interstellar space,” says Lin. “Our objective is to come up with a comprehensive scenario, based on well understood physical principles, to piece together all the tantalizing clues.”
Image: This illustration shows the tidal disruption process that can give rise to ‘Oumuamua-like objects. Credit: NAOC/Y. Zhang.
Tidal forces are the key to this work, which is described in a paper in Nature Astronomy. Think Shoemaker-Levy 9, whose spectacular disintegration produced the famous ‘chain of pearls’ as the comet was progressively torn apart by Jupiter’s gravity. In Zhang and Lin’s simulations, the modeling of the structural dynamics of an object passing close to its star produces extremely elongated fragments that are then ejected into deep space. The work shows that aspect ratios even greater than 10 to 1 are not out of the question.
Could such a shape be stable? Evidently so, for the scientists used thermal modeling to show that at tight stellar distances, the surface of such an object would melt and then recondense on its way out of the system, producing a crust that should remain cohesive. We get a ‘shard’ of material being flung into the cosmos in a random direction. Moreover, this is a process that contains within it other unusual aspects of the ‘Oumuamua encounter with our system.
“Heat diffusion during the stellar tidal disruption process also consumes large amounts of volatiles, which not only explains ‘Oumuamua’s surface colors and the absence of visible coma, but also elucidates the inferred dryness of the interstellar population,” Zhang said. “Nevertheless, some high-sublimation-temperature volatiles buried under the surface, like water ice, can remain in a condensed form.”
So we circle back to the fact that ‘Oumuamua showed no cometary activity but an apparent outgassing sufficient to produce a non-gravitational motion, albeit a tiny one. Under the modeling Zhang and Lin performed, the warmth of the approach to perihelion would have activated residual water stores that match the observed trajectory of ‘Oumuamua.
Image: A ‘Oumuamua-like object produced by a simulation of the tidal disruption scenario proposed by Zhang and Lin. Credit: NAOC/Y. Zhang; background: ESO/M. Kornmesser.
We should expect quite a few more interstellar objects in our future. Zhang and Lin believe that on average, a single planetary system should eject a total of about a hundred trillion objects like ‘Oumuamua. The tidal forces in play in these simulations work for any number of source objects, from super-Earths to long-period comets and other material from a debris disk. In other words, there is hardly a shortage of material out of which to form interstellar objects. Doubtless our Sun has spewed its share into nearby interstellar space, shard-like or not.
We’ve had a second interstellar object since ‘Oumuamua, the much more comet-like 2I/Borisov. Whether Zhang and Lin are right in their assessment may eventually become clear as we detect and study further examples. Given their apparent plenitude, we should by virtue of these simulations assume that some, if not many, will share some of the traits of ‘Oumuamua. Eventually we’ll build a catalog that may teach us something about planetary evolution.
The paper is Zhang and Lin, “Tidal fragmentation as the origin of 1I/2017 U1 (‘Oumuamua),” Nature Astronomy 13 April 2020 (abstract).
