by Paul Gilster | Sep 11, 2019 | Exoplanetary Science |
We’re beginning to probe the atmospheres of planets other than gas giants, a step forward that the next generation of space- and ground-based instruments will only accelerate. This morning we have word that the habitable zone ‘super-Earth’ eight times as massive as Earth orbiting the star K2-18 has been found to have water vapor in its atmosphere, making it the only exoplanet known to have water as well as temperatures that could sustain that water as a liquid on the surface. This is also our first atmospheric detection of any kind for a planet orbiting in the habitable zone of its star.
Angelos Tsiaras (University College London Centre for Space Exochemistry Data) is lead author on this work, which appears today in Nature Astronomy:
“Finding water in a potentially habitable world other than Earth is incredibly exciting. K2-18b is not ‘Earth 2.0’ as it is significantly heavier and has a different atmospheric composition. However, it brings us closer to answering the fundamental question: Is the Earth unique?”
Image: UCL’s Angelos Tsiaras. Credit: University College London.
If Dr. Tsiaras’ name seems familiar, it’s because you’re recalling his work on the super-Earth 55 Cancri e, reported in these pages back in 2016 [see Light, Dry Atmosphere of a ‘Super-Earth’]. Tsiaras and team used a methodology they developed to detect significant amounts of hydrogen and helium in this world, working with transmission spectroscopy data from the Hubble telescope. Now, collaborating with UCL colleagues, Tsiaras again uses Hubble, to analyze starlight filtering through the atmosphere of K2-18b as it crosses the face of the star as seen from Earth.
The molecular signature of water vapor is clear, as is the evidence for both hydrogen and helium, while such molecules as nitrogen and methane, if present, are undetectable at these levels. It will be fascinating to see whether we can move on from these observations to estimate the planet’s cloud cover and percentage of atmospheric water. The star is a cool red dwarf 110 light years away in the constellation Leo. And it presages the kind of work that the study of such nearby stars will generate, as co-author Ingo Waldmann (UCL) notes:
“With so many new super-Earths expected to be found over the next couple of decades, it is likely that this is the first discovery of many potentially habitable planets. This is not only because super-Earths like K2-18b are the most common planets in our Galaxy, but also because red dwarfs — stars smaller than our Sun — are the most common stars.”
Thus we move on to the next generation, which includes the James Webb Space Telescope as well as the interesting ARIEL (Atmospheric Remote-sensing Infrared Exoplanet Large-survey), slated for a 2028 launch by the European Space Agency. ARIEL’s charter will be to observe at least 1,000 known transiting exoplanets, going to work on their atmospheric composition, chemistry and thermal properties. K2-18b will obviously be one of its targets as we dig further into conditions on its surface. Meanwhile, we can expect TESS (the Transiting Exoplanet Survey Satellite) to detect hundreds more super-Earths as it continues its mission. No shortage of targets!
Image: Exoplanet K2-18b. This artist’s impression shows the planet K2-18b, its host star and an accompanying planet in this system. K2-18b is now the only super-Earth exoplanet known to host both water and temperatures that could support life. UCL researchers used archive data from 2016 and 2017 captured by the NASA/ESA Hubble Space Telescope and developed open-source algorithms to analyse the starlight filtered through K2-18b’s atmosphere. The results revealed the molecular signature of water vapour, also indicating the presence of hydrogen and helium in the planet’s atmosphere. Credit: ESA/Hubble, M. Kornmesser.
K2-18b was detected in 2015 through light curve analysis made possible by Kepler in its reconfigured K2 phase, its mass later constrained by radial velocity data from HARPS, leaving researchers to believe it is either a large, rocky planet or a water planet with an ice crust. The latter work, led by Ryan Cloutier (University of Toronto), also discovered a second, non-transiting super-Earth in the system, moving on an orbit interior to K2-18b.
The paper is Tsiaras et al., “Water Vapour in the Atmosphere of the Habitable-Zone Eight Earth-Mass Planet K2-18 b,” Nature Astronomy 11 September 2019 (abstract). Also see Benneke et al, “Water vapor on the habitable-zone exoplanet K2-18b,” submitted to Astronomical Journal (preprint). Björn Benneke (Université de Montréal) is a co-author on the Tsiaras paper. [My mistake, looking at the wrong abstract].
The Cloutier paper referenced above is “Characterization of the K2-18 multi-planetary system with HARPS: A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit,” Astronomy & Astrophysics Vol. 608, A35 (5 December 2017). Abstract.
by Paul Gilster | Sep 10, 2019 | Outer Solar System |
The vast amount of data returned to Earth from the Cassini mission continues to pay off with new research angles, a process that will continue for years to come. Today we learn of a possible explanation for an odd feature of some methane-filled lakes on Saturn’s moon Titan. As viewed in Cassini radar data, we can see what appear to be sharp ridges, along with cratered edges, raised rims and ramparts. Interestingly, some of the steeper ridges are considerably higher than Titan’s liquid sea level. Winnipeg Lacus, a small lake near Titan’s north pole, is but one example.
The model currently in play about Titan’s lakes is that liquid methane dissolved a bedrock of ice and solid organic compounds — essentially creating the reservoirs which it then fills. The process is similar to karstic lakes found on Earth as the result of bodies of water dissolving surrounding limestone, dolomite or gypsum, with distinctive sinkholes and caves.
But an international team of researchers headed by Giuseppe Mitri of Italy’s d’Annunzio University now offers an alternative. In this scenario, changes in temperature over the aeons on Titan could cause pockets of liquid nitrogen below the surface to become a pressurized, explosive gas, blowing out the craters which later warming events would fill with methane.
This process accounts for features that the karstic lake basin theory fails to address. Jonathan Lunine (Cornell University) is a co-author of the paper on this work:
“You either need gas that ignites explosively or a gas that builds up enough pressure so that it just pops like a cork from a champagne bottle. On Titan, there is nothing that will create a fiery explosion because that moon has no free oxygen,” says Lunine. “Thus, a pressurized explosion model, we argue, is a better model for those kinds of lakes. Craters are created and they fill with liquid methane.”
Image: This artist’s concept of a lake at the north pole of Saturn’s moon Titan illustrates raised rims and rampart-like features such as those seen by NASA’s Cassini spacecraft around the moon’s Winnipeg Lacus. New research using Cassini radar data and modeling proposes that lake basins like these are likely explosion craters, which could have formed when liquid molecular nitrogen deposits within the crust warmed and quickly turned to vapor, blowing holes in the moon’s crust. This would have happened during a warming event (or events) that occurred in a colder, nitrogen-dominated period in Titan’s past. The new research may provide evidence of these cold periods in Titan’s past, followed by a relative warming to conditions like those of today. Although Titan is frigid compared to Earth, methane in the atmosphere provides a greenhouse effect that warms the moon’s surface. Credit: NASA/JPL-Caltech.
To make the effect work, we have to consider the interplay between nitrogen and methane, for the geophysical history of Titan, the researchers believe, includes eras when methane is depleted, with nitrogen as the primary constituent of a much colder atmosphere. Cooling nitrogen would produce liquid rain which could collect in pockets beneath the moon’s crust. The catalyst for explosion could well be geologic heating. The craters thus produced would later be filled with methane. Lunine likens the process to what Voyager II saw at Neptune’s large moon Triton.
“We saw these big deposits of nitrogen with streaks of black that looked like cigarette burns on the nitrogen ice. It was dust under the nitrogen being heated by the sun, allowing the nitrogen to explode outward.”
Are we then, seeing a Titan today that is in a much warmer phase than the frigid moon of earlier times in its evolution? Methane has acted as a greenhouse gas on this moon for perhaps as long as a billion years, but the model advocated here is that Titan has gone through cycles of heating and cooling, with methane first depleted and then resupplied. Thus steep edges and raised rims would speak to an epoch when liquid nitrogen was plentiful on the surface.
The key question, then, is how far we can take this hypothesis of climate change on Titan. Here is a snip from the paper addressing the matter:
The photochemical lifetime of methane in Titan’s atmosphere is several tens of million years, and the value of 12C/13C in Titan’s atmosphere indicates that the current atmospheric inventory of methane probably outgassed ~107-108 yr ago. Evolution models suggest that Titan’s crust is composed of water ice and clathrate hydrate, with mostly methane as primary guest species, and that episodic methane outgassing could be a cause for the continued presence of methane in the atmosphere. However, in the absence of continuous or frequent outgassing from the crust or deeper methane sources, Titan’s atmosphere in the past may have been episodically methane depleted [italics mine]. Under such conditions, the greenhouse effect would have been limited to the collision-induced absorption of N2-N2, resulting in a Titan that was colder than at present, with a mean surface temperature lower than 81 K…
The supposition that infrequent methane outgassing events would have allowed intervals between them when the surface was pocked with liquid nitrogen lakes, along with crustal aquifers of nitrogen in the polar regions, is thus key. We would, then, wind up with a stable nitrogen cycle at lower temperatures that parallels the methane cycle we see on Titan today. Small temperature variations could then produce the explosions presumed to create the lake beds that show up in Cassini’s data.
The authors make the case that if the lakes we see today are the result of explosive eruptions, then this itself is evidence of a cold nitrogen-dominated epoch followed by subsequent rewarming. A 2 K increase in Titan’s temperature between 1989 and 1998 along with an increase in atmospheric pressure may have been the result of rising nitrogen levels:
On Triton, the venting of nitrogen and possibly methane and an increase of atmospheric pressure are probably due to warming events that may mirror processes that happened in Titan’s nitrogen-dominated past.
The paper is Mitri et al., “Possible explosion crater origin of small lake basins with raised rims on Titan,” Nature Geoscience 9 September 2019 (abstract).
by Paul Gilster | Sep 9, 2019 | Exoplanetary Science |
Our thinking on how planetary systems form includes the accretion of rocky bodies within a disk surrounding a young star, and we’re examining such disks in numerous systems, such as the well studied Beta Pictoris. But the idea of accretion leaves many issues unsettled, such as what happens when large rocky bodies collide in the violent endgame of system formation. The Earth evidently underwent such a collision, with our own Moon being the tangible result.
Caltech postdoc Simon Lock has been working with Sarah Stewart (UC-Davis) to study how such giant impacts unfold, running simulations of early planetary materials whose collisions can form bodies with masses between 0.9 and 1.1 Earth masses. The energy involved in such impacts is thought to allow, in some cases, the two colliding bodies to form a ‘synestia,’ or a rotating torus of planetary materials that will later cool into one or more spherical planets.
The synestia is, however, but one outcome out of many produced by these simulations. What the researchers found is that when a major collision occurs, the internal pressure of the resulting objects is much lower than we would expect. Lock explains how the computer modeling changes earlier thinking about pressure within a young planet:
“Previous studies have incorrectly assumed that a planet’s internal pressure is simply a function of the mass of the planet, and so it increases continuously as the planet grows. What we’ve shown is that the pressure can temporarily change after a major impact, followed by a longer term increase in pressure as the post-impact body recovers. This finding has major implications for the planet’s chemical structure and subsequent evolution.”
Image: Artist’s depiction of a synestia, a rotating donut of planetary material that can result from the collision of two planetary bodies. In time, the synestia cools back into one or more spherical planetary bodies. Credit: Ron Miller/Scientific American.
Lock and Stewart’s modeling indicates that the lower post-impact pressure is the result of rapid rotation induced by the collision, which could push material away from the object’s spin axis. Moreover, the now partially vaporized object is hot and its density is correspondingly reduced.
Modeling like this is important in the study of early stellar systems because we have no observational data as yet on how Earth-class planets grow, and according to this work, the physical properties of a planet as shaped by collisions are highly variable. Lower internal pressure (and the pressure inside the Earth not long after the impact that formed the Moon was, the researchers believe, half that of present-day Earth) could help us tie together geochemical analysis of the Earth’s mantle with the collision model of planet formation now current.
The paper argues that the internal pressure of a planet like ours helps to determine the chemical composition of the mantle. Objects colliding with the early Earth would have delivered metals into the mantle which, along with other elements already there, would sink to the core. By studying the amount of elements dissolved into the metal, we can determine Earth’s internal pressure at the time. Thus today’s mantle is a window into the mantle pressure at play during the planet’s formation.
The contradiction has been that studies of the metal content of the mantle show the pressure during formation would have been the same as found in the middle of the mantle today. But our models of giant impacts show that most of the mantle should melt, thus producing much higher pressure, equivalent to that found near the core. The researchers resolve the problem by showing that the pressure would be lower after these impacts, rising as the planet recovers. Evidence in the mantle should show how it solidified and how the earliest planetary crust formed. As the paper notes, this is an area that has been little explored before now:
…we show that cooling and tidal evolution of satellites after giant impacts can lead to increases in pressure. Pressure increases in the lower mantle could lead to partial melting. Pressure-induced melting provides a new mechanism for the creation of chemical heterogeneity early in Earth’s history and could explain the existence of seismologically anomalous regions in the present-day lower mantle. Pressure-induced phase transitions during the recovery of a body after a giant impact are a previously unrecognized phenomenon in planet formation.
So we have a new model, one that examines the changing nature of internal pressure in planetary bodies late in the planet-formation process. The lower average pressures that Lock and Stewart find help to explain the concentrations of elements between the core and mantle and reconcile them with the melting expected after giant impacts. The way thus opens to test various Moon formation scenarios using geochemical and geophysical observations of today’s Earth because how the mantle solidifies varies depending on the kind of impact. Adds Lock:
“We have shown that the pressures in planets can increase dramatically as a planet recovers, but what effect does that have on how the mantle solidifies or how Earth’s first crust formed? This is a whole new area that has yet to be explored.”
The paper is Lock et al., “Giant impacts stochastically change the internal pressures of terrestrial planets,” Science Advances 5 (9), 2019. Abstract/full text.
by Paul Gilster | Sep 6, 2019 | Astrobiology and SETI |
Reykjavik is an old haunt of mine, a favorite place to which I have not returned in all too long. I was delighted, then, to hear from Angelle Tanner, who in August attended the Extreme Solar Systems IV conference there. I had the pleasure of getting to know Dr. Tanner in Knoxville when we both spoke at a biosignatures session at the 2017 symposium of the Tennessee Valley Interstellar Workshop. Dr. Tanner received her PhD at UCLA, did postdoc work at both Caltech and Georgia State, and is now an associate professor at Mississippi State University. Her work specializes in exoplanet detection and programs devoted to understanding the properties of stars that host planets, as well as the architecture of the systems that evolve around them. It’s a pleasure to turn today’s essay over to Dr. Tanner for a look at exoplanetary events in Iceland’s capital.
by Angelle Tanner
Mid-August marked the fourth meeting of the Extreme Solar Systems conference — this one in Reykjavik, Iceland – touted as one of the biggest exoplanet-themed astronomy conferences of the year. Meetings I-III were hosted in Santorini, Greece; Jackson Hole, Wyoming and The Big Island of Hawaii – all parts of the world with extreme geology. Reykjavik was no exception, with glaciers, volcanoes and geysers within driving distance of the conference venue at the Harpa Concert Hall and Conference Center. It is a beautiful building right on the water with sharp cliffs and fishing and excursion boats in plain view.
To keep with the conference theme, I’ll cover some of the science highlights by investigating the extreme properties of exoplanets, their host stars and our own Solar System. Some of these results may sound familiar as they were announced globally via press releases that were embargoed until their presentation at the meeting.
Image: Harpa Concert Hall and Conference Center in Reykjavik. Credit: Angelle Tanner.
Common vs. Rare
As expected, planets around M-dwarfs were in full force at this meeting as we heard about exciting new discoveries on the first day. David Charbonneau (Harvard), speaking for his postdoc Jennifer Winters, told us about the complex triple M-dwarf system LTT 1445. It has a TESS-discovered transiting planet orbiting the A stellar component of the system. While the mass of the planet around LTT 1445 A still needs to be determined, the fact that it transits its M-dwarf host star makes it a good target for JWST.
We will be able to determine the composition of the atmosphere of the planet from transit spectroscopy not currently possible with ground or space-based telescopes as we can’t quite reach the necessary signal-to-noise yet. M-dwarfs constitute the majority of stars in our galaxy and have become high priority targets for planet search programs. This system will join the Trappist-1, GJ 1214 and GJ 518 M-dwarf planetary systems, to name a few which are getting plenty of attention, as we characterize each planet and further investigate these types of systems. They are not only the most common in our galaxy but may be our best chance to look for life outside of our Solar System.
At the other population extreme are those rare planets in orbits with very high eccentricities. Shasha Hinkley (University of Exeter) spoke about what could eventually be the first directly imaged hot Jupiter planet. How? Well, turns out the planet orbiting HD 20782 has an orbital eccentricity of 0.97! With an orbital period of ~600 days, the planet gets as close as 0.04 AU and as far as 2.7 AU (0.072″) from its host G1.5V star (data from exoplanet.eu).
With its widest separation of 0.07″ at apastron, this planet is tantalizing close to the limit on the inner working angle of many coronagraphic systems available today like SPHERE on the VLT or GPI on Gemini. Hinkley’s team was not able to image the planet due to a little too much stellar saturation in their non-coronagraphic images, but I’m sure they will try again. They were able to determine that there are not any 20-60 MJ objects further out from the star, which makes us wonder why this planet still retains such a high orbital eccentricity.
Young vs Old
Discoveries of new planets around young stars were some of the highlights of the meeting. Young stars are problematic for planet search programs, as their tendency to be photometrically active hinders planet discovery via radial velocities, and the large distances to young stellar clusters (50 – 100 pc) requires sophisticated coronagraphic direct imaging instruments and analysis tools.
Marie LaGrange (INST) announced the detection of an additional planet around Beta Pictoris. This young star has been in the news for decades, as its edge-on debris disk was first observed with the IRAS telescope in 1983 and the planet beta Pic b, which is in an edge-on orbit but does not quite transit, was first confirmed in 2018. This time the existence of the second planet, beta Pic c, is inferred from radial velocity measurements. Getting a thorough census of the planets in this system is important to help understand planet formation and evolution and also how the planets influence the structure of the disk, which has moving clumps and is warped.
There was another announcement of the discovery of a planet (or two) around a well-known young star with an edge-on disk. Peter Plavchan (George Mason University) announced the discovery of the planets via both TESS transit photometry, and follow-up radial velocity measurements with the new iSHELL infrared echelle spectrometer on the NASA IRTF telescope. Due to the fact that the paper is still under review by Nature (I’m a co-author), the name of the host star is still hush, hush. Stay tuned!
At the other age extreme, we have the remains of dead planets being detected in the atmospheres of white dwarfs. White dwarfs have very simple spectra, with mostly just absorption lines from H and He. Over the past few years, astronomers have detected multiple heavy elements including Fe, Mg, Ca and Si. This meeting hosted a few different speakers including Christopher Manser (University of Warwick) who announced the first detection of a planetesimal orbiting a white dwarf. His team tracked changes in the shape of the Ca II line and noticed variations over a timeframe of just 2 hours. They then determined that the material causing the variations is a solid object and, therefore, is a chunk of material still in orbit around the white dwarf [see White Dwarf Debris Suggests a Common Destiny]. This is a young and super exciting sub-field of exoplanet research which allows us to study the composition of the insides of rocky planets while we are still developing new methods to study their atmospheres.
Image: Iceland’s Blue Lagoon, a popular geothermal spa. There is a volcano caldera in the background. Credit: Angelle Tanner.
Hot vs Cold
One of the hottest extrasolar planets both literally and figuratively at this meeting was Kelt 9b. At an orbital distance of 0.034 AU around an A-class star, this planet has an average surface temperature of 4050 K. The intense heat this planet is receiving from its star results in a unique set of elements detectable in its atmosphere. Enric Palle (Gran Telescopio Canarias) updated us on the Carmenes planet atmosphere survey of 25 known transiting planets. Kelt 9b is on their target list and they have detected ionized iron and titanium in the atmosphere of this super-heated planet. This survey has also detected He in the atmospheres of three different planets, which is good to know as we make plans to travel across the galaxy.
From the extremely hot to the extremely cold (kinda), Sarah Blunt discussed the detection of a planet around the G0V star HR 5183 using radial velocity measurements. Proto-doctor Blunt made some audience members groan as she mentioned that the first RV measurement in her data set was collected before she was born (1997); however, our persistence as observers has paid off as the RV signal shot up over the past few years, revealing the presence of an extremely eccentric planet at a current distance of 18 AU from the host star. It will reach a distance of 30 AU over the course of its ~74 year orbit. We again are left to ask why this planet has such a high eccentricity with no known other planets in the system and no stellar companion close enough to cause its current configuration. Additional direct imaging and Gaia astrometric measurements will give us more information about this puzzling system [see HR 5183 b: Pushing Radial Velocity Techniques Deeper into a Stellar System].
Image: An iceberg from Iceland’s Jökulsárlón, or ‘Glacier’s-River-Lagoon’. Credit: Angelle Tanner.
Bright vs. Dark
Our final set of extremes comes from pitting the bright against the dark. We start with thinking about objects which are so bright we can see them with binoculars – the Galilean moons! Laura Mayorga opened the Thursday session reminding us that there are objects in our own Solar System that we can study to help us better understand exoplanets. She looked at the phase curve of the reflected light from Io and concluded that we need to stop using Lambertian curves (consider a cueball) to model reflected light curves. She also determined that an Earth-sized planet with a surface similar to Io would have a reflected light contrast of 10-9 to 10-10. This is indeed the contrast ratio being targeted by upcoming space missions such at LUVOIR and the Origins telescope, intent on direct imaging of Earth-like worlds. While we’ve known these contrast values are necessary for future direct imaging missions it is still useful to have observational confirmation and new phase curves for direct comparisons to future data sets.
We finish with one of the biggest announcements of the meeting, as Laura Kreidberg gave an exciting talk about measuring the first thermal phase curve of a terrestrial planet. Her team observed the transiting planet, LHS 3844 b with the Spitzer Space Telescope. Over the course of the planet’s very close 0.006 AU orbit around the M-dwarf host, Spitzer can detect changes in the amount of infrared light from the planet as seen here in our Solar System similar to the change in the phase of Venus we see on Earth as we both orbit the Sun [see LHS 3844b: Rocky World’s Atmosphere Probed].
From the significant amplitude of the change in thermal emission over the planet’s orbit, we can infer that this planet not only lacks any appreciable atmosphere but is most likely composed of a dark, basaltic rock like the moon and Mercury. In this golden age of exoplanet science, we are not only detecting and characterizing the atmospheres of extrasolar planets now but are now making informed assertions on the composition of the surface materials!
There were two other results that caught my attention but could not be cleverly placed in the context of extremes. Eric Nielsen (Stanford) and a few other folks reported on the statistical results of direct imaging programs like GPI and SHINE. There appears to be a difference in the types of stars that have planetary companions on wide orbits versus those with brown dwarfs.
Stars with masses > 1.5 Msun are more likely to host planetary mass companions, which suggests these objects were formed via the core accretion mode of planet formation. Brown dwarfs, on the other hand, seem to form more around stars < 1.5 Msun, favoring the gravitational instability planet formation model. These trends are observed from their 300-star GPI direct imaging survey.
Finally, Lisa Kaltenegger (Cornell University) stayed true to her appointment at the Carl Sagan Institute and went way out there with a trip into alien oceans. She talked about the different depths to which light would penetrate the oceans on a habitable planet around early vs late-type stars. The light from an M-dwarf star, mostly in the infrared, would penetrate only the top few meters of an alien ocean, while light from an F-type star would make it into deeper depths.
Most intriguing was her suggestion that life in some of these M-dwarf worlds would need to somehow break down the UV light expected from the flares that are common for these types of stars. Recall that the habitable zone for M-dwarfs can be a fraction of an AU and well within the reach of life-damaging flares. She then showed a cool animation of the surface of an alien world fluorescing in reaction to a UV flare, which made us all wonder if this could one day be detectable from own humble world [see Looking for Life Under Flaring Skies].
This was a small sample of the menagerie of juicy exoplanet news covered during a subset of the talks presented at this meeting. There were also a few hundred posters covering anything from new planets to new instruments to upcoming NASA exoplanet missions. The best is yet to come! There has been no news yet about the location of Extreme Solar Systems V… I’m rooting for New Zealand!
by Paul Gilster | Sep 4, 2019 | Astrobiology and SETI |
SETI, the Search for Extraterrestrial Intelligence, has kept its focus on the stars, through examination of electromagnetic wavelengths from optical to radio signals. But Jim Benford has been advocating that we consider near-Earth objects as potential SETI targets, prompted by Ronald Bracewell’s thoughts in a 1960 paper advancing the ‘sentinel hypothesis.’ A Bracewell probe could linger in a target system for millions of years, monitoring developments on worlds with the potential for life. Couple that thought with the rarely studied co-orbital’ objects that approach the Earth both frequently and closely and you have a map for a realm of SETI that is only now coming into investigation.
What follows is a news release from The Astrophysical Journal covering Benford’s new paper, one we discussed on Centauri Dreams back in March [see A SETI Search of Earth’s Co-Orbitals]. I want to get this out now because Benford will be delivering the 2019 Eugene Shoemaker Memorial Lecture tomorrow, Thursday September 5, at 1900 MST (0100 UTC). The lecture, at Marston Exploration Theater in Tempe, AZ can be accessed online at https://asunow.asu.edu/asulive. For those not familiar with it, the Shoemaker Lecture has been set up by the BEYOND Center for Fundamental Concepts in Science as a special award to a leading scientist to honor the life and work of Eugene Shoemaker who, together with his wife Carolyn, pioneered research in the field of asteroid and comet impacts.
The most recently discovered group of rocky bodies nearby Earth are termed co-orbital objects. These may have been an attractive location for extraterrestrial intelligence (ETI) to locate a probe to observe Earth throughout our deep past. Co-orbital objects approach Earth very closely every year at distances much shorter than anything except the moon. They have the same orbital period as Earth. These near-Earth objects provide an ideal way to watch our world from a secure natural object. Co-orbitals provide resources an ETI might need: materials, constant solar energy, a firm anchor, concealment.
Co-orbitals have been little studied by astronomy and not at all by SETI or planetary radar observations. James Benford has proposed both passive and active observations of them as possible sites for ET probes that may be quite ancient.
A ‘Lurker’ is a hidden, unknown and unnoticed observing probe. They may respond to an intentional signal and may not, depending on unknown alien motivations. Lurkers would likely be robotic, like our own Voyager and New Horizons probes.
Long-lived robotic lurkers could have been sent to observe Earth long ago. As they would remain there after their energy supply runs out, this is extraterrestrial archaeology. If we find nothing there, this gives us a profound result: no one has come to look at the life of Earth, which has been evident in our atmosphere’s spectral lines over interstellar distances for over a billion years.
Co-orbitals are attractive targets for SETI searches because of their proximity. Benford thinks we should move forthrightly toward observing them, both in the electromagnetic spectrum of microwaves and light, and planetary radar. And we can visit them with probes. The most attractive target is ‘Earth’s Constant Companion’ 2016 HO3, the smallest, closest, and most stable (known) quasi-satellite of Earth. Getting there from Earth orbit requires little rocket propulsion and can be done in short trips. China has announced they are going to send a probe to 2016 HO3.
The well-regarded astronomy journal of record, Astrophysical Journal, is publishing Benford’s paper “Looking for Lurkers: Co-orbiters as SETI Observables” in the near future.
This is the latest in an agenda Benford has carried out in imaginative searches for interstellar communication. His first work with other family members yielded the term ‘Benford Beacons’—short microwave bursts to attract attention, like lighthouses. Later he pointed out the uses of powerful electromagnetic beams to send light spacecraft, ‘sails’, in interplanetary exploration. His Lurkers proposal moves on to actual relic alien spacecraft that may have been nearby for longer than humans have existed.
Interstellar travel is challenging – no craft engineered by humans has yet traveled further than the outskirts of our own Solar System. One project working to change that is Breakthrough Starshot, which aims to send a gram-sized spacecraft to a nearby star system at around 20% of the speed of light. “Within the next few decades we hope humanity will become an interstellar species,” remarked Breakthrough Initiatives Chairman Dr. Pete Worden. “If intelligence arose elsewhere in our galaxy, it may well have sent out similar probes. It’s intriguing to think that some of these may already have reached our own Solar System.”
The paper is Benford, “Looking for Lurkers: Objects Co-orbital with Earth as SETI Observables,” soon to be published by the Astrophysical Journal (preprint). To attend Benford’s lecture at the University of Arizona online, the address is https://asunow.asu.edu/asulive.
