by Paul Gilster | Apr 2, 2020 | Exoplanetary Science |
The Kepler mission gave us, along with plenty of exoplanetary scenarios, a statistical look at a particular patch of sky, one containing parts of Lyra, Cygnus and Draco. Some of the stars within that field were close (Gliese 1245 is just 15 light years out), but the intention was never to home in on nearby systems. Most of the Kepler stars ranged from 600 to 3,000 light years away. Instead, Kepler would produce an overview of planets around different stellar types, including some in the habitable zone of their stars.
As with all such observations, we’re limited by the methods chosen, which in Kepler’s case involved transits of the host star. TESS, the Transiting Exoplanet Survey Satellite, likewise uses the transit method, though with particular reference to broad sky coverage and close, bright stars. We can deploy the widely anticipated James Webb Space Telescope, to be launched next year, to follow up interesting finds, but let’s also consider how useful the Wide Field Infrared Survey Telescope (WFIRST) is going to be, for this instrument brings a new population of planets into the mix.
Not that WFIRST won’t be able to spot transits as well, but the real interest here is microlensing, a phenomenon a good deal less common than transits, but one holding the promise of finding planets far more distant than Kepler. Looking toward the passage of a star and planetary system in front of a background star, astronomers can catch the lensing effect produced by the warping of spacetime. That quick brightening can contain within it the signature of one or more planets.
As you would imagine, finding these occultations is tricky, for they don’t occur very often. David Bennett is head of the gravitational microlensing group at NASA’s Goddard Space Flight Center. Kepler monitored more than 150,000 stars in its primary mission, but WFIRST will have a lot more to play with, says Bennett :
“Microlensing signals from small planets are rare and brief, but they’re stronger than the signals from other methods. Since it’s a one-in-a-million event, the key to WFIRST finding low-mass planets is to search hundreds of millions of stars.”
Image: WFIRST will make its microlensing observations in the direction of the center of the Milky Way galaxy. The higher density of stars will yield more exoplanet detections. Credit: NASA’s Goddard Space Flight Center/CI Lab.
Slated for launch in the mid-2020s, WFIRST will round out what we’ve learned from previous missions and different exoplanet detection methods. Radial velocity is sensitive to planets close to the host star and, with ever greater spectroscopic precision, can tease out information about smaller worlds further out. Transits are excellent for finding small worlds in tight orbits. What microlensing brings to the table are planets of all sizes — and perhaps even large moons — orbiting at a wide range of distances from the host, as far out as Uranus and Neptune and potentially much farther.
Here the bias, if we want to call it that, is toward planets from the habitable zone outward, fully complementing our other methods, which function so much better in inner systems. We have no idea how common ice giants are, but WFIRST should help us build the census. We go from Kepler’s 115 square degree field of view of stars typically within 1,000 light years to a 3 square degree field that, because it’s toward galactic center, will track 200 million stars. Their average distance will be in the range of 10,000 light years, far beyond the reach of other methods.
Microlensing has produced its share of planets — 86 so far — but bear in mind that the observations that have turned them up have been in visible light, which means that looking toward the center of the galaxy, shrouded in dust, has not been feasible. That’s beginning to change with the United Kingdom Infrared Telescope (UKIRT), which from its vantage in Hawaii has begun mapping the region. Data from UKIRT will help determine the WFIRST microlensing observation strategy.
Savannah Jacklin (Vanderbilt University) has led studies using UKIRT data:
“Our current survey with UKIRT is laying the groundwork so that WFIRST can implement the first space-based dedicated microlensing survey. Previous exoplanet missions expanded our knowledge of planetary systems, and WFIRST will move us a giant step closer to truly understanding how planets – particularly those within the habitable zones of their host stars – form and evolve.”
But WFIRST microlensing goes beyond exoplanet discovery to take in everything from black holes to neutron stars, brown dwarfs and ‘rogue’ planets that have been ejected from their planetary systems. TESS is currently tracking 200,000 stars over the entire sky. The infrared studies of WFIRST will dramatically add to what Kepler and TESS have given us, using machine learning tools now being refined by UKIRT to comb through the data. An overview of planetary populations at all distances from the host star, and with a target field containing hundreds of millions of stars, is the much desired result.
by Paul Gilster | Nov 21, 2018 | Outer Solar System |
It makes sense that planets in other stellar systems would have moons, but so far it has been difficult to find them. That’s why Kepler-1625b, about 8,000 light years out in the direction of Cygnus, is so interesting. As we noted last month, David Kipping and graduate student Alex Teachey have compiled interesting evidence of a moon around this gas giant, which is itself either close to or within the habitable zone of its star. The massive candidate exomoon is the size of Neptune, and if confirmed, would mark the first exomoon detection in our catalog.
As the examination of Kepler-1625b and its transit timing variations continues, we have new work out of the University of Zürich, ETH Zürich and NCCR PlanetS that adds weight to the assumption that moons around large planets should be ubiquitous. Using computer simulations run at the Swiss National Supercomputing Centre (CSCS) in Lugano, a team of researchers led by Judit Szulágyi (University of Zurich and ETH Zurich) has determined that both gas giants and ice giants like Neptune and Uranus will produce moon-bearing circumplanetary disks.
Image: One of the computer simulations on the formation of moons (white bodies) around Neptune (blue sphere). Credit: Judit Szulágyi.
The issue is given point by the difference between Neptune and Uranus when it comes to moons. The five major moons of Uranus do not seem out of place when compared to what we see around Jupiter and Saturn. But we seem to see a different formation history at Neptune, whose solitary major moon, Triton, may well have been captured from the Kuiper Belt.
Szulágyi and team wondered whether the moons of Uranus were not themselves outliers, perhaps formed through a collision in the early days of the Solar System. Our own Moon is thought to have been the result of just such an ancient catastrophe. But the simulations the researchers ran pointed to both Uranus and Neptune originally having their own moon-forming disk of gas and dust. In each case, the simulations produced icy moons. This is a useful result as it has been widely believed that the two ice giants were too light to form such a disk.
The implication: Neptune was itself once orbited by a system of icy moons much like that of Uranus, one that would have been disrupted during the capture of the massive moon Triton. Bear in mind that Triton contains 99 percent of the mass of Neptune’s entire satellite system. The authors point to an earlier study showing that the capture of Triton would only have been possible if Neptune originally had a moon system with the mass of the Uranian moons.
From the paper:
We investigated CPD [circumplanetary disk]- and moon-formation around Uranus and Neptune with combining radiative hydrodynamical simulations with satellite population synthesis. We found that both Uranus and Neptune could form a gaseous disk at the end of their formation, when their surface temperature dropped below 500 K. These disks are able to form satellites in them within a few hundred thousand years. The masses of such satellite-systems for both planets were often similar to the current one around Uranus. All the formed moons must be icy in composition, given that they formed in a CPD that has a temperature below water freezing-point.
All of this has implications for the exomoon hunt, in that the formation of moons seems to be likely across the entire range from gas giant to ice giant. Bear in mind how often we’ve found Neptune-class planets among the population of exoplanet candidates. If such worlds are producing exomoons, then such moons form a larger population than we had realized. Our studies of the Jovian and Saturnian moons have shown how interesting they are in terms of astrobiological possibilities, a realm that now widens as we expand the discovery space.
Says Szulágyi: “[A] a much larger population of icy moons in the Universe means more potentially habitable worlds out there than it was imagined so far. They will be excellent targets to search for life outside the Solar System.”
The paper is Szulágyi, Cilibrasi and Mayer, “In situ formation of icy moons of Uranus and Neptune,” Astrophysical Journal Letters 868 (2018), L13. Abstract / Preprint. Among the papers on Triton as a disrupter of Neptune’s early system of moons, see in particular Rufu & Canup, “Triton’s Evolution with a Primordial Neptunian Satellite System,” Astronomical Journal Vol. 154, No. 5 (2017). Abstract.
by Paul Gilster | Jul 2, 2018 | Exoplanetary Science |
It’s always a shock for me when the soft air and fecund smells of spring slam into a parched and baked July, but seasonal change is inevitable. At least it is on Earth. We get such seasonal changes because of Earth’s obliquity, the angle of its spin axis relative to the plane of its orbit. For Earth, the angle has stayed pretty close to 23 degrees for a long time, although the tilt’s direction wobbles over cycles of thousands of years. And this very constancy of obliquity turns up in exoplanet discussions at times because it affects conditions on a planetary surface.
Some have argued that without the gravitational effects of the Moon, the tilt of the Earth would be changed by the gravitational pull of the Sun and planets, producing a potentially high degree of obliquity. Contrast our situation with that of Uranus, where we find a 90-degree tilt that leaves one pole in sunlight for half the Uranian year as the other remains in darkness. Without knowing how long the Moon has been able to stabilize Earth’s axial tilt, we can’t say how apparent equatorial ice sheets some 800 million years ago fit into this view of the Moon’s effect.
But obliquity as a factor in habitability continues to energize exoplanetary researchers. At Georgia Tech, a team led by Gongjie Li, working with graduate student Yutong Shan (Harvard-Smithsonian Center for Astrophysics) has developed computer simulations to analyze the spin axis dynamics of two exoplanets, Kepler 186f and Kepler 62f, two planets considered to be in or close to the habitable zone of their stars. The paper argues that without our Moon, Earth’s obliquity variation would range from 0 to 45 degrees over billion-year timescales.
Thus obliquity is an interesting data point. Bear in mind that so far, we have no reliable values for exoplanet obliquity, although ways to infer it from light curves and from high-contrast direct imaging have been proposed in the literature. The authors make the assumption that in both exoplanet systems studied, all planets have been identified. They then go on to study the evolution of the two five-planet systems. The ‘secular analytical framework’ they arrive at allows them to factor in planetary rotation rates, additional planets and satellites, and regions where resonant interactions within the system can produce large obliquity variations. For various realizations of planetary systems, the paper thus describes an ‘obliquity evolution.’
We know that Mars and Earth interact strongly with each other, as do Mercury and Venus; other than Earth, none of these worlds has a large moon. The authors point out that the orientation angle of a planet’s orbit around its host star can be made to oscillate through gravitational interactions. If the orbit oscillates at the same pace as the precession of the planet’s spin axis, large obliquity variations can be induced, the kind of thing our Moon dampens out.
Image: An artist’s depiction of Kepler-62f. Credit: NASA Ames/JPL-Caltech/T.Pyle.
For these two exoplanet systems, we get an interesting result, for even without a stabilizing moon (if none is present), these two planets could be experiencing relatively low changes in their axial tilt:
“It appears that both exoplanets are very different from Mars and the Earth because they have a weaker connection with their sibling planets,” said Li. “We don’t know whether they possess moons, but our calculations show that even without satellites, the spin axes of Kepler-186f and 62f would have remained constant over tens of millions of years. That’s not to say either exoplanet has water, let alone life. But both are relatively good candidates. Our study is among the first to investigate climate stability of exoplanets and adds to the growing understanding of these potentially habitable nearby worlds.”
As Li has just pointed out, we have no knowledge of surface conditions on either of these planets, making the lovely image above nothing more than a guess, and an optimistic one at that. The ‘super Earth’ Kepler 62f, about 40 percent larger and with a mass 2.8 times that of our planet, is in the constellation of Lyra, the outermost of the five planets orbiting a K2-class star some 1200 light years from Earth. Kepler-186f orbits a red dwarf about 550 light years out, part of a five-planet system in the constellation Cygnus. A stable axial tilt would make it likely that both worlds experience regular seasons and thus a stable climate.
But are large obliquity values necessarily inimical to life? Some recent work, considered by the authors, shows that variability in obliquity can keep a planet’s global temperature higher than it would otherwise have been, extending the outer edge of the habitable zone. But it does appear that obliquity variations can produce sharp transitions between climate states. From the paper:
Recently, Kilic et al. (2017) mapped out the various equilibrium climate states reached by an Earth-like planet as a function of stellar irradiance and obliquity. They find that, in this parameter space, the state boundaries (e.g. between cryo- and aqua-planets) are sharp and very sensitive to the climate history of the planet. This suggests that a variable obliquity can easily move the planet across state divisions, as well as alter the boundaries themselves, which would translate into a dramatic impact on instantaneous surface conditions and long-term climate evolution.
Planets with highly irregular seasons aren’t necessarily destined to be lifeless, but if we become capable of determining planetary obliquity, such a value could help us narrow the target list for future space telescopes. The authors also suggest that their framework can provide input parameters for existing global climate models as we analyze habitability in multi-planet systems.
The paper is Shan and Li, “Obliquity Variations of Habitable Zone Planets Kepler-62f and Kepler-186f,” <em>Astronomical Journal</em> Vol. 155, No. 6 (17 May 2018). Abstract / preprint.
by Paul Gilster | Jun 20, 2018 | Exoplanetary Science |
Looking through a recent Astrophysical Journal paper on gas giants in the habitable zone of their stars, I found myself being diverted by the distinction between a conservative habitable zone (CHZ) and a somewhat more optimistic one (OHZ). Let’s pause briefly on this, because these are terms that appear frequently enough in the literature to need some attention.
The division works like this (and I’ll send you to the paper for references on the background work that has developed both concepts): The OHZ in our Solar System is considered to be roughly 0.71 to 1.8 AU, which sees Venus as the inner cutoff (a world evidently barren for at least a billion years) and Mars as the outer edge, given that it appears to have been habitable in the early days of the system, perhaps some 3.8 billion years ago. ‘Habitable’ in both HZ categories is defined as the region around a star where water can exist in a liquid state on a planet with sufficient atmospheric pressure (James Kasting has a classic 1993 paper on all this).
The CHZ’s inner edge is considered to be at the ‘runaway greenhouse limit,’ where the breakdown of water molecules by solar radiation allows free hydrogen atoms to escape, drying out the planet at approximately 0.99 AU in our own system. Its outer edge, says the paper:
…consists of the maximum greenhouse effect, at 1.7 AU in our solar system, where the temperature on the planet drops to a point where CO2 will condense permanently, which will in turn increase the planet’s albedo, thus cooling the planet’s surface to a point where all water is frozen (Kaltenegger & Sasselov 2011).
It goes without saying that boundaries like these are going to vary from one planetary system to another, and it’s likewise clear that most of our thinking about habitable zone planets has gone in the direction of small rocky worlds as we mount the search for Earth analogues. What Stephen Kane (University of Southern Queensland), working with an undergraduate student at the university named Michelle Hill as well as colleagues at the University of California, Riverside has done is to identify 121 giant planets in Kepler data that could host habitable moons.
To be sure, the gas giants themselves aren’t considered candidates for life as we know it (though obviously we can’t rule out exotic species adapted to extreme conditions, like Edwin Salpeter’s ‘gasbags,’ free-floating lifeforms that might populate dense atmospheres — see Edwin Salpeter and the Gasbags of Jupiter for more). But the real focus is on those rocky moons that occur in such abundance in our own system.
“There are currently 175 known moons orbiting the eight planets in our solar system. While most of these moons orbit Saturn and Jupiter, which are outside the Sun’s habitable zone, that may not be the case in other solar systems,” says Kane. “Including rocky exomoons in our search for life in space will greatly expand the places we can look.”
Image: This is an artist’s illustration of a potentially habitable exomoon orbiting a giant planet in a distant solar system. Credit: NASA GSFC: Jay Friedlander and Britt Griswold.
As we consider the different dimensions of habitable zones around other stars, we should also keep in mind the fact that the moons that may emerge in these systems can be as various as our own. Earth’s Moon, for example, seems to be the result of a giant impact early in the system’s formation. Most moons are thought to have formed by accretion within the dust disks around planets, but others can be captured by a planet’s gravitational pull — Triton seems to be an example of this. Thus we could find moons of considerably different composition than their host planet. Considering how many moons we see orbiting our gas giants, the assumption that moons exist around other such worlds in exoplanetary systems seems reasonable.
We still have no exomoon detections, but the search continues, and I always scan the latest papers from the Hunt for Exomoons with Kepler project that David Kipping runs with anticipation, along with those of exomoon theorist René Heller. Having a database of the giant planets we’ve identified thus far as being in the habitable zone of their star may help us target future observations to refine the expected properties of their moons, assuming these exist. Such moons would receive energy from the primary star, of course, but would also receive reflected radiation from the planet they orbit. René Heller has proposed that exomoons in a habitable zone could provide a better environment for life than Earth itself. Let me quote the Hill paper:
Exomoons have the potential to be what [Heller] calls “super-habitable” because they offer a diversity of energy sources to a potential biosphere, not just a reliance on the energy delivered by a star, like earth. The biosphere of a super-habitable exomoon could receive energy from the reflected light and emitted heat of its nearby giant planet or even from the giant planet’s gravitational field through tidal forces. Thus exomoons should then expect to have a more stable, longer period in which the energy received could maintain a livable temperate surface condition for life to form and thrive in.
Discussing the difficulties of exomoon detection, such as the fact that multiple moons around a single planet may eliminate a useful transit timing signal (this is Jean Schneider’s work) and the problems of direct imaging, it’s interesting to see that microlensing remains a candidate. It’s also intriguing to ponder the fate of exomoons, as this paper does, in terms of migrating gas giants and the likelihood that their moons will be lost. We still have much to learn about the movement of giant planets and the effect of their migration upon their own moons as well as other planets.
Once we have a firm exomoon detection, we can begin to characterize the possibilities. As we await improvements in our technology, deepening our knowledge of potential exomoon host planets is the best we can do, and that would begin, as this paper suggests, with radial velocity follow-up observations on gas giant habitable zone candidates like the ones compiled by the authors.
The paper is Hill et al., “Exploring Kepler Giant Planets in the Habitable Zone,” The Astrophysical Journal, 2018; 860 (1): 67. Abstract / preprint. The Kasting paper mentioned above is “Habitable Zones around Main Sequence Stars,” Icarus Vol. 101, Issue 1 (1993), pp. 108-128 (abstract). For René Heller’s work on ‘superhabitable’ moons, see Heller & Armstrong, “Superhabitable Worlds,” Astrobiology January 2014 (preprint). Jean Schneider’s paper on exomoon detection problems is Schneider & Sartoretti, “On the detection of satellites of extrasolar planets with the method of transits,” Astronomy & Astrophysics. Suppl. Ser. Vol. 134, No. 3 (1 February), pp. 553-560 (abstract).
by Paul Gilster | Mar 7, 2018 | Exoplanetary Science |
How do we go from seeing an exoplanet as a dip on a light curve or even a single pixel on an image to a richly textured world, with oceans, continents and, perhaps, life? We’ve got a long way to go in this effort, but we’re already having success at studying exoplanet atmospheres, with the real prospect of delving into planets as small as the Earth around nearby red dwarfs in the near future. Atmospheric detection and analysis can help us in the search for biosignatures.
But I was surprised when reading a recent paper to realize just how many proposals are out there to analyze planetary surfaces pending the development of next-generation technologies. Back in 2010, for example, I wrote about Tyler Robinson (University of Washington), who was working on how we might detect the glint of exo-oceans (see Light Off Distant Oceans for more on Robinson’s work). And Robinson’s ideas are joined by numerous other approaches. I won’t go into detail on any of these, but l do want to illustrate the range of possibilities here:
- Exomoon detection (see Sartoretti & Schneider, 1999, or the Hunt for Exomoons with Kepler and papers from David Kipping);
- Planetary oblateness — i.e., having an equatorial diameter greater than the distance between poles (Seager & Hui 2002; Carter & Winn 2010);
- Light from alien cities (Loeb & Turner 2012);
- Plant pigments (Berdyugina et al 2016);
- Industrial pollution (Lin, Gonzalez Abad, & Loeb 2014);
- Circumplanetary rings (Arnold & Schneider 2006).
I’ve pulled this list with references out of a paper suggesting yet another target, the surface topography of exoplanets. The work of graduate student Moiya McTier and David Kipping (Columbia University), the paper points out that while many of these effects are beyond the reach of current equipment, they are nonetheless valuable in pushing the limits of exoplanet characterization and helping us understand what technologies we will need going forward.
So is it really possible to detect surface features like mountains, trenches and craters on a distant exoplanet? McTier and Kipping make the case that we can draw conclusions about a planetary surface through what they call its ‘bumpiness,’ which should show up in a planetary transit as a scattering in the light curve produced as its silhouette gradually changes (assuming, of course, that we are dealing with a rotating planet in transit). We would obtain not the image of a specific mountain or other surface feature but a general analysis of overall topography.
The paper’s method is to model planetary transits for known bodies — the Earth, the Moon, Mars, Venus, Mercury — to see what it would take to tease out such a signature. We have ample elevation data for rocky planets in our Solar System. Using this information, we can model what would happen if one of them transited a nearby white dwarf. The researchers used thse values to find a general relationship between bumpiness and transit depth scatter.
In terms of bumpiness, the paper argues:
…the definition should encode the planet’s radius. An Everest-sized mountain on an otherwise featureless Mercury provides more contrast to the average planet radius than an Everest on an otherwise featureless Earth, and should result in a higher bumpiness value.
What we are after here is what the paper calls “an assessment of global average features,” one that incorporates the largest feature on a planet (an enormous mountain, for example) but also includes the contribution to the lightcurve scatter produced by all the planet’s features.
Mars, because of its small size and low surface gravity, turns out to be the bumpiest of these planets. A Mars-sized planet orbiting a white dwarf in its habitable zone proves to be an optimal situation for detecting bumpiness. Why white dwarfs? We learn that even huge ground-based telescopes planned for future decades such as the Extremely Large Telescope and Colossus would be unable to detect bumpiness on planets around stars like the Sun or M-dwarfs because of astrophysical noise and the limitations of the instruments. False positives through pulsations on the star’s surface, for example, can likewise appear as extra scatter in the light curve.
White dwarfs, on the other hand, appear unlikely to have convective star spots, but even if they do occur, McTier and Kipping argue that they can be detected and filtered out. Orbiting moons could similarly cause variations in the transit depth that could be mistaken for topography, but here the signature of the exomoons shows up just outside the ingress and egress points in the transit curve, unlike the topographical signature, which appears only in the in-transit data.
It turns out that the largest of our next generation of big telescopes would be able to work with a white dwarf planet, which the paper models as orbiting at 0.01 AU in the center of the star’s habitable zone. If we assume a mass typical of such stars (0.6 M☉), we get an orbital period of just over 11 hours. 20 hours of observing time covering some 400 transits with a telescope like the 74-meter combined aperture of Colossus should be able to detect topography.
Image: This is Figure 8 from the paper, showing an oceanless Earth transiting a white dwarf. The caption: Top: Transits of a dry Earth with features (in red) and an idealized spherical Earth (in black) in front of a .01R☉ white dwarf with noise of 20 ppm added (20σ detection). The exaggerated silhouettes of Earth at different rotational phases are shown in brown. Middle: Zoomed-in frame of the bottom of the light curve in the top panel. Bottom: Residual plot showing the difference between the realistic and idealized transits. Grey shadows show the error bars on the residuals equal to 50 ppm. Dashed lines are to illustrate that residuals deviate from 0ppm only inside the transit. Credit: McTier & Kipping.
Surface features should tell us a good deal about a planet’s composition. From the paper:
…a detection of bumpiness could lead to constraints on a planet’s internal processes. Mountain ranges like the Himalayas on Earth form from the movement and collision of tectonic plates (Allen 2008). Large volcanoes like Olympic Mons on Mars form from the uninterrupted buildup of lava from internal heating sources. A high-bumpiness planet is likely to have such internal processes, with the highest bumpiness values resulting from a combination of low surface gravity, volcanism, and a lack of tectonic plate movement. Truly low-bumpiness planets are less likely to have these internal processes. On such planets, surface features are likely caused by external factors like asteroid bombardment.
I like the phrase the authors use in closing the paper, referring to their mission “of adding texture to worlds outside our own.” Texture indeed, for we are beginning to move into the realm of deeper planetary analysis, like a painter gradually applying detail to the roughest of sketches. Because of the magnitude of the challenge, we are coming at the question of exoplanet characterization from numerous different directions, as the list at the beginning of this post suggests. Synergies between their methods will be key to exoplanet surface discoveries.
The paper is McTier and Kipping, “Finding Mountains with Molehills: The Detectability of Exotopography,” accepted at Monthly Notices of the Royal Astronomical Society (preprint).
