by Paul Gilster | Jul 15, 2019 | Exoplanetary Science |
Some confusion has arisen about a possible circumplanetary disk in the system PDS 70, which I wrote about recently (see Exoplanet Moons in Formation?, from June 7). A team led by led by Valentin Christiaens at Monash (Australia) presented evidence for the kind of disk that may have formed the moons of Jupiter around the forming planet PDS 70b, using data from the Very Large Telescope, finding evidence for both the disk and a developing atmosphere here.
The finding was admittedly tentative, which should be kept in mind as we resolve the discrepancy between this and a separate observation, what Rice University is calling in a news release ‘the first observations of a circumplanetary disk of gas and dust…’ What we have in the Rice document is a report on a paper from the university’s Andrea Isella and colleagues, who studied millimeter wave radio signals from the Atacama Large Millimeter/submillimeter Array (ALMA) to identify a circumplanetary disk around the other forming planet here, PDS 70 c.
Image: Radio astronomers using the Atacama Large Millimeter/submillimeter Array of telescopes in Chile have found a disk of gas and dust (left) around exoplanet PDS 70 c, a still-forming gas giant that was obscured from view in the 2018 infrared image (right) that first revealed its sister planet, PDS 70 b. Credit: A. Isella, ALMA (ESO/NAOJ/NRAO).
I want to clear this up because several emails express confusion about which planet is doing what. What’s happening is that two separate teams are reporting on findings that deepen our interest in this young system. Circumplanetary disks are new terrain for exoplanetology, and most models of planet formation show that such disks last no more than 10 million years. Thus observing what may be two instances of moons forming around planets in the same system is an extraordinary opportunity. No wonder multiple teams are digging into it.
On the PDS 70b possibility, Isella reports the detection of a continuum emission “close, but not coincident, to the position of PDS 70 b.” His team has two sources of emission that are interesting, one near each forming world, and labels them PDS 70 csmm and PDS 70 bsmm respectively, the ‘smm’ standing for ‘submillimeter.’
In Isella’s work, the millimeter wavelength observations of this star from ALMA were backed by data from the VLT’s SPHERE instrument, and a second VLT instrument called MUSE, which observed in the visible wavelength called H-alpha, an emission that occurs when infalling hydrogen is ionized. “It’s complementary to the optical data,” said Isella, “and provides completely independent confirmation that there is something there,”
What Isella is arguing is that the two sources, one near each planet, might have different physical origins, with the source at PDS 70b offset from the planet itself. Here’s what the Isella paper, which appears in Astrophysical Journal Letters, says about PDS 70b:
The last item of discussion concerns the nature of PDS 70 bsmm. Its proximity to PDS 70 b suggests that the observed continuum emission might be somehow related to the planet. Due to the uncertainties on the position of the host star in the ALMA images, we cannot exclude that the sub-millimeter continuum arises for circumplanetary dust.
And here is a color-enhanced image of millimeter-wave radio signals showing the disk of gas and dust around PDS 70c:
Image credit: A. Isella, ALMA (ESO/NAOJ/NRAO).
Between the two papers, then, we’re looking at the possibility of two circumplanetary disks forming around separate worlds in this system. Both these planets appear to be 5 to 10 times larger than Jupiter, with the innermost at roughly the distance from Uranus to the Sun, while PDS 70c is in an orbit analogous to Neptune’s. It’s clear that this system is going to repay further study, for we are now at the point where our observatories can examine planetary systems in rich detail and observe moon formation in its nascent stages. Says Isella:
“There are a handful of candidate planets that have been detected in disks, but this is a very new field, and they are all still debated. (PDS 70 b and PDS 70 c) are among the most robust because there have been independent observations with different instruments and techniques.”
The Isella paper is “Detection of continuum submillimeter emission associated with candidate protoplanets,” Astrophysical Journal Letters Vol. 879, No. 2 (abstract / preprint). The Christiaens paper is “Evidence for a Circumplanetary Disk around Protoplanet PDS 70 b,” Astrophysical Journal Letters Vol. 877, No. 2 (3 June 2019). Abstract / Preprint.
by Paul Gilster | Jul 12, 2019 | Astrobiology and SETI |
A useful exercise for learning how to look for life elsewhere is to try to find it right here on Earth. Thus Carl Sagan’s observations of our planet via data taken during the 1993 flyby of the Galileo spacecraft, which was doing a gravity assist maneuver enroute to Jupiter. Sagan and team found pigments on the Earth’s surface with a sharply defined edge in the red part of the spectrum. What he was looking at was the reflection of light off vegetation. The ‘red edge’ has become well known in astrobiology circles and is considered a potential biosignature.
On Earth, vegetation is the most abundant reflecting surface indicating life (vegetation covers about 60% of present-day Earth’s land surface). The increase in reflectance shows up at about 700 nm, varying in strength depending upon the species of plant. But as Jack O’Malley-James and Lisa Kaltenegger (Cornell University/Carl Sagan Institute) point out, photosynthetic structures containing chlorophyll are found not just in vegetation but also in lichens, corals, algae and cyanobacteria.
This is helpful, because for anyone looking at the early Earth, the vegetation red edge would have been apparent only after the advent of land plants, while if we can detect a similar feature in other forms of photosynthetic life (call this a photosynthetic red edge, or PRE), we can extend the ability to detect such life back as far as 2 billion years and more (in the case of cyanobacteria). According to the scientists, lichen probably emerged at about the same time as algae, 1 billion years ago, while corals and modern vegetation begin to appear no earlier than 725 million years back.
This offers a much wider ‘window’ in which to observe red edge features on other worlds. The authors’ new paper in Astrophysical Journal Letters looks at what could produce a PRE spectrum aside from land vegetation and asks whether features like these would be detectable. Says Kaltenegger:
“If an alien had used color to observe if our Earth had life, that alien would see very different colors throughout our planet’s history – going back billions of years – when different life forms dominated Earth’s surface. Astronomers had concentrated only on vegetation before, but with a better color palette, researchers can now look beyond a half-billion years and up to 2.5 billion years back on Earth’s history to match like periods on exoplanets.”
Image: To understand where exoplanets are in their own evolution, astronomers can use Earth’s biological milestones as a Rosetta stone. Credit: Wendy Kenigsberg/Cornell Brand Communications.
The paper models how a planet’s spectrum would change depending upon the dominant organism on the surface. O’Malley-James refers to the authors’ use of the early Earth in this analysis as a kind of Rosetta Stone, one that extends back halfway as far as the Earth itself. Examining the spectra of Earth-like planets modeled with four different organisms — cyanobacteria, algae, and lichen, as well as deciduous vegetation (lichen, for example, would have cast a sage to mint green color, a distinctive red-edge signature of photosynthesis), the authors show that the addition of an atmosphere and clouds to the model can mask individual features but still produce enough data to reach a broader conclusion: From the paper:
…for similar surface coverage the PRE signal of other organisms that could be dominant on the surface of an exoplanet can be similar in strength to the signal produced by modern vegetation for Earth in our models, which is approximated using deciduous tree reflectance producing an estimated reflectance increase of ?4% (Table 1), falling within the lower end of the range of values (1%–10%) given for Earth’s VRE,,, Figure 1 shows that individually the different organisms can be distinguished with high spectral resolution. However, once we add a present-day Earth atmosphere as well as clouds to the model…, the individually distinguishing slope of the reflectivity of the organisms is no longer apparent. Thus a red edge detection, while not being specific to any one form of photosynthetic organism, can indicate a wider range of organisms than only vegetation.
Image: This is Figure 1 from the paper. Caption: Examples of red edge features—the increase in reflectance caused by chlorophyll, highlighted in the shaded region—exhibited by (A) corals, (B) deciduous vegetation (trees; representative of the present-day red edge feature in Earth’s spectrum), (C) the photosynthetic sea slug, Elysia viridis, (D) lichen (Acarospora sp.), (E) algae (Rhodosorus marinus), (F) cyanobacteria (Chroococcidiopsis sp.). Credit: Jack O’Malley-James/Lisa Kaltenegger.
The red edge would be a difficult biosignature detection but not beyond the reach of high-precision instruments as we move to the next generation of observatories. It also provides another tool for biodetection that in combination with atmosphere analysis offers a multi-pronged approach to our remote probing for life, lessening the potential ambiguity of the results.
The paper is O’Malley-James & Kaltenegger, “Expanding the Timeline for Earth’s Photosynthetic Red Edge Biosignature,” Astrophysical Journal Letters Vol. 879, No. 2 (10 July 2019). Abstract.
by Paul Gilster | Jul 11, 2019 | Exoplanetary Science |
We should always be on the lookout for new ways of finding exoplanets. Right now we’re limited by our methods to stars within the neighborhood of the Sun (in galactic terms), for both radial velocity and transit detections are possible only around brighter, closer stars. The exception here is gravitational microlensing, capable of probing deep into the galaxy, but here the problem is one of numbers. We simply don’t make enough detections this way to build up the kind of statistical sample that the Kepler mission has provided in terms of transiting planets.
So how significant is this kind of selection bias, which thus far has been forced upon us? Without knowing the answer, we would do well to explore ideas like those put forward by Nicola Tamanini (AEI Potsdam) and colleague Camilla Danielski (CEA/Saclay, Paris). The two scientists are looking at the possibilities of gravitational wave astronomy, looking toward the launch, in the 2030s, of LISA, the Laser Interferometer Space Antenna.
Image: Artistic representation of gravitational waves produced by a compact binary white dwarf system with a jovian mass planetary companion. Credit: © Simonluca Definis.
This is rarified air indeed, and the kind of target in play is likewise a rarity, giant exoplanets orbiting detached double white dwarf binaries (DWDs). These are intriguing objects, eclipsing double white dwarfs, remnants of stars like our Sun that have passed beyond their red giant phase. Short-period DWDs with orbital periods less than one hour are rarer still. But they’re worth seeking out because these short-period binaries generate powerful gravitational waves.
What the authors propose in their new paper in Nature Astronomy is to use gravitational waves to find circumbinary planets, worlds that orbit both stars in the binary. We have no planets around white dwarf binaries in our catalog at present, but LISA should be able to remedy that by identifying DWDs both inside and outside the Milky Way. Perturbations in the gravitational wave signal would then flag the presence of a third gravitationally bound object, a giant planet. Thousands of DWDs are expected to be found, producing no shortage of targets.
Tamanini likens the method to Doppler modulations of the kind we use with radial velocity studies, this being their gravitational wave analog. But significantly, gravitational waves are not affected by the kind of stellar activity that can confound radial velocity signals. Nor are we hampered by distance to the degree we are when using electromagnetic means, for gravitational wave perturbations should be apparent from anywhere in the galaxy and nearby galaxies as well. The scientist believes LISA could detect exoplanets down to about 50 Earth masses throughout this range.
If Tamanini’s conclusions are valid, the method would therefore bring the kind of large statistical sample we derived from Kepler to the domain of post-main sequence stellar systems, which means we are pushing into regions in what he calls the ‘planetary Hertzsprung-Russell diagram’ that have not yet been explored. Valid over the entire galaxy, the data would be free of selection effect. Moreover, the paper points out that follow-up observations of close-in DWDs will be helpful in confirming the LISA identification and deepening our knowledge of its characteristics:
Imaging of CBPs [circumbinary planets] around DWDs can be used to test the presence of a second generation of exoplanets in the outer regions of a planetary system, and consequently to provide constraints on migration theories. Emission spectra of these objects will furthermore allow us to estimate their temperature and the main molecular component of their atmosphere, making direct connections to chemical element distributions in the atmosphere of white dwarfs. This would also allow to better understand the observed white dwarf pollution effect. On the other hand, if an existing CBP accretes mass after a common-envelope stage, it becomes brighter, further decreasing the already low planet-to-white dwarfs contrast, meaning that also first-generation, more mature exoplanets can be imaged.
So we are looking at a kind of exoplanet about which we know nothing, if it indeed exists, but bear in mind that about half ot the stellar population occurs in multiple star systems. LISA is expected to measure gravitational waves from thousands of DWDs. Exoplanets here would yield insights into the kind of planet that survives a star’s red giant phase, while probing the regime of any second generation planets — those that form after the red giant phase is complete. We further our knowledge even if LISA finds no exoplanets around DWDs, for then we’ve set statistical constraints on the last phase of planetary evolution.
The paper is Tamanini et al., “The gravitational-wave detection of exoplanets orbiting white dwarf binaries using LISA,” Nature Astronomy 8 July 2019 (abstract).
by Paul Gilster | Jul 10, 2019 | Missions |
One of the many legacies of the Voyager spacecraft is the Interstellar Mapping and Acceleration Probe (IMAP). Scheduled for a 2024 launch, IMAP has as part of its charter the investigation of the solar wind’s interactions with the heliosphere, drawing on data from an area into which only the Voyagers have thus far ventured. Let me hasten to add that IMAP will stay much closer to home, orbiting the Sun-Earth L1 Lagrange point, but like the Interstellar Boundary Explorer (IBEX), it will help us learn more about a region physically reachable only by long-duration craft.
The fact that we’re still talking about Voyager as an ongoing mission is the story here. Launched in 1977, the doughty probes have kept surprising us ever since. In terms of their longevity, I noted in 2017 that when Voyager 1’s thrusters had begun to lose their potency (they’re needed to keep the spacecraft’s antenna pointed at Earth to return data), controllers were able to fire a set of backup thrusters that hadn’t been used for a whopping 37 years.
Even the Voyager Interstellar Mission, an extension to the primary, is long in the tooth, having begun when the two spacecraft had been in flight no more than twelve years. These days Voyager is all about power management, for we’re still getting good data. Voyager 1’s cosmic ray instrument is still at work, along with a plasma instrument, its magnetometer, and its low-energy charged particle instrument. Voyager 2 likewise studies cosmic rays, operates two plasma instruments, a magnetometer, and its own low-energy charged particle instrument.
Voyager instruments are proving to be as tenacious as bulldogs. Consider: Voyager 2’s cosmic ray subsystem (CRS) continues to run although engineers have turned off the heater that keeps it warm to save power, as part of a new power management plan for both spacecraft. The CRS now functions at -59 degrees Celsius, a good 15 degrees colder than it was originally tested for back in the days before launch. Voyager 1’s ultraviolet spectrometer continued to function for years after losing its heater as part of an earlier power strategy implemented in 2012.
Voyager project manager Suzanne Dodd (Jet Propulsion Laboratory) notes how important it is to make choices about power and instruments, given that the heat available from the three radioisotope thermoelectric generators (RTGs) aboard each craft decreases with time, for each spacecraft produces 4 fewer watts of electrical power each year. We’re down to 60 percent of the heat energy the RTGs could produce at launch, so hard decisions have to be made about which systems to keep operational. Says Dodd:
“It’s incredible that Voyagers’ instruments have proved so hardy. We’re proud they’ve withstood the test of time. The long lifetimes of the spacecraft mean we’re dealing with scenarios we never thought we’d encounter. We will continue to explore every option we have in order to keep the Voyagers doing the best science possible.”
Image: This artist’s concept depicts one of NASA’s Voyager spacecraft, including the location of the cosmic ray subsystem (CRS) instrument. Both Voyagers launched with operating CRS instruments. Credit: NASA/JPL-Caltech.
The cosmic ray subsystem had its heater turned off despite its role in detecting Voyager 2’s passage through and exit from the heliosphere, the ‘bubble’ produced by the outflow of solar wind particles from the Sun. One aspect of this difficult choice is that the CRS can only look in specific fixed directions, making its heater in this environment expendable. Voyager 2 is driving the new power plan because it has one more instrument collecting data than Voyager 1.
If there is one place where power remains essential to the last, it’s the fuel lines that power the Voyager thrusters. In addition, Voyager 2’s current thrusters are degrading, just as Voyager 1’s did, forcing a switch to trajectory correction maneuver (TCM) thrusters last used during the encounter with Neptune in 1989. That switch should take place later this month. Let’s give a nod to Aerojet Rocketdyne, whose MR-103 thrusters have performed beyond all expectation, as has the mission itself, originally slated to last but five years.
One day, probably in the coming decade though perhaps late in it, the amount of electrical power needed to keep both spacecraft operational will no longer be available. Here’s hoping we get at least eight more years out of the Voyagers so that they’re still with us on their 50th anniversary. My own hunch is that the gifted people managing the Voyager Interstellar Mission may just find enough tricks to get them through to 2030 before the flow of data ceases.
As I’m still delighted to say, this mission isn’t over. Go Voyager.
by Paul Gilster | Jul 9, 2019 | Exoplanetary Science |
We refine our terminology as we go when a field as new as exoplanetology is in play. Take the case of GJ 3470b. At 12.6 Earth masses, is this a ‘sub-Neptune’ or a ‘super Earth’? Neptune itself is 17 Earth masses, so I’d on balance give the nod to ‘sub-Neptune,’ though categories here get confusing. The planet is 0.031 AU out from its star, a red dwarf half the mass of our Sun. Oddly, it has a hydrogen/helium atmosphere in which heavier elements are all but absent.
We know this because scientists have been able to put data from both the Hubble instrument and Spitzer to work on an analysis of the atmosphere of the planet. This is done through a technique we’ve examined before, transmission spectroscopy, in which astronomers study the absorption of the star’s light as the planet passes across its face (a transit as seen from Earth), and then the loss of reflected planetary light as the planet moves behind the star (this is called a secondary eclipse).
Image: A comparison between transits and secondary eclipses (also sometimes called occultations). In a planetary transit, the planet crosses in front of the star (see lower dip) blocking a fraction of the star’s brightness. In a secondary eclipse, the planet crosses behind the star, blocking the planet’s brightness (see dip in the middle). The latter dip in brightness is fainter due to the faintness of the planet. Credit: astrobites/Josh Winn.
The atmospheric data come from observations of 12 transits and 20 eclipses, giving us a first look at the atmospheric composition of a world like this. GJ 3470b is, to say the least, unusual. Björn Benneke (University of Montreal) is lead author of the paper, now available in Nature Astronomy:
“This is a big discovery from the planet-formation perspective. The planet orbits very close to the star and is far less massive than Jupiter – 318 times Earth’s mass – but has managed to accrete the primordial hydrogen/helium atmosphere that is largely ‘unpolluted’ by heavier elements. We don’t have anything like this in the solar system, and that’s what makes it striking.”
The scientists expected to find heavier elements such as carbon and oxygen, out of which we would detect water vapor and methane. This would point to the Neptune model. But the data threw a curve: What was actually revealed was an atmosphere devoid of heavy elements, to such a degree that Benneke likens it to the hydrogen and helium composition of the Sun. With resemblances to a gas giant atmosphere in being hydrogen-dominated, it is nonetheless one that is depleted in methane.
Here’s how the paper handles the lack of methane, and its bearing on planet formation. One possibility is that there is interior heating that is not being accounted for, but there are others:
Evolution modeling of GJ 3470b indicates that internal heat from formation should have been radiated away within a few Myr, well below the estimated age of the system; however, tidal heating due to forced eccentricity from another unseen planet in the system, similar to the situation with Jupiter’s moon Io could be a possible explanation. The residual non-zero eccentricity of GJ 3470b as independently confirmed by our eclipse observations and radial velocity measurements support this hypothesis. Alternatively, GJ 3470b’s surprising lack of methane could potentially be the results of photochemical depletion due to catalytic destruction of CH4 in deeper atmospheric regions where photolysis of NH3 and H2S release large amounts of atomic hydrogen. The fact that ammonia is also depleted in comparison to expectations based on our chemical-kinetics modeling is consistent with this catalytic-destruction possibility.
Image: This artist’s illustration shows the theoretical internal structure of the exoplanet GJ 3470b. It is unlike any planet found in the Solar System. Weighing in at 12.6 Earth masses the planet is more massive than Earth but less massive than Neptune. Unlike Neptune, which is 4.5 billion kilometers from the Sun, GJ 3470b may have formed very close to its red dwarf star as a dry, rocky object. It then gravitationally pulled in hydrogen and helium gas from a circumstellar disk to build up a thick atmosphere. The disk dissipated many billions of years ago, and the planet stopped growing. The bottom illustration shows the disk as the system may have looked long ago. Observation by NASA’s Hubble and Spitzer space telescopes have chemically analyzed the composition of GJ 3470b’s very clear and deep atmosphere, yielding clues to the planet’s origin. Many planets of this mass exist in our galaxy. Credit: NASA, ESA, and L. Hustak (STScI).
The authors rule out migration of a world that formed beyond the snow line, for this origin would have produced the heavier elements we do not see here. Instead, they believe that GJ 3470b formed where it is today, showing that sub-Neptunes can form with atmospheres that are the result of direct accretion from the protoplanetary disk onto a rocky core. The implication is that we are looking at a planet-forming process that is essentially distinct from more massive planets, one in which the gas envelope is not enriched to any great degree by later collisions.
There is so much to learn about this, indicating just how far we have to go in our understanding of such low-mass, star-hugging planets. The authors point to GJ 3470b as a prime target for the James Webb Space Telescope. Every time I read something like this I think about how much is riding on JWST being launched successfully and can only keep my fingers crossed. For studying atmospheric chemistry is going to require powerful space-based resources as we start delving into atmospheres on worlds this small and aim at rocky worlds that are smaller still.
The paper is Benneke et al., “A Sub-Neptune Exoplanet with a Low-Metallicity Methane-Depleted Atmosphere and Mie-Scattering Clouds,” published online by Nature Astronomy 1 July, 2019 (preprint).
