by Paul Gilster | Jan 13, 2017 | Astrobiology and SETI |
A recent snowfall followed by warming temperatures produced a foggy night recently, one in which I was out for my usual walk and noticed a beautiful Moon trying to break through the fog layers. The scene was silvery, almost surreal, the kind of thing my wife would write a poem about. For my part, I was thinking about the effect of the Moon on life, and the theory that a large single moon might have an effect on our planet’s habitability. Perhaps its presence helps to keep Earth’s obliquity within tolerable grounds, allowing for a more stable climate.
But that assumes we’ve had a single moon all along, or at least since the ‘big whack’ the Earth sustained from a Mars-sized protoplanet that may have caused the Moon’s formation. Is it possible the Earth has had more than one moon in its past? It’s an intriguing question, as witness a new paper in Nature Geoscience from researchers at the Technion-Israel Institute of Technology and the Weizmann Institute of Science. The paper suggests the Moon we see today is the last of a series of moons that once orbited the Earth.
“Our model suggests that the ancient Earth once hosted a series of moons, each one formed from a different collision with the proto-Earth,” says co-author Assistant Prof. Perets (Technion). “It’s likely that such moonlets were later ejected, or collided with the Earth or with each other to form bigger moons.”
To explore alternatives to giant impact theories, the researchers have produced simulations of early Earth impacts, varying the values for the impactor’s velocity, mass, angle of impact and the initial rotation of the target. The process that emerges involves multiple impacts that would produce small moons, whose gravitational interactions would eventually cause collisions and mergers, to produce the Moon we see today. Here’s how the paper describes the process:
… we consider a multi-impact hypothesis for the Moon’s formation. In this scenario, the proto-Earth experiences a sequence of collisions by medium- to large-size bodies (0.01–0.1M?). Small satellites form from the impact-generated disks and migrate outward controlled by tidal interactions, faster at first, and slower as the body retreats away from the proto-Earth. The slowing migration causes the satellites to enter their mutual Hill radii and eventually coalesce to form the final Moon. In this fashion, the Moon forms as a consequence of a variety of multiple impacts in contrast to a more precisely tuned single impact.
Here’s a graphic from the paper (listed as Figure 1) that shows the process at work:
Image (click to enlarge): a,b, Moon- to Mars-sized bodies impact the proto-Earth (a) forming a debris disk (b). c, Due to tidal interaction, accreted moonlets migrate outward. d,e, Moonlets reach distant orbits before the next collision (d) and the subsequent debris disk generation (e). As the moonlet–proto-Earth distance grows, the tidal acceleration slows and moonlets enter their mutual Hill radii. f, The moonlet interactions can eventually lead to moonlet loss or merger. The timescale between these stages is estimated from previous works.
The Hill radius mentioned above describes the gravitational sphere of influence of an object; in this case, meshing Hill radii can produce interactions that sometimes lead to mergers. The paper notes that in head-on impacts, the rotation of the planet is important because the disk needs angular momentum resulting from the rotation to stay stable. With increased rates of rotation, the angular momentum of the disks increases. Moons like ours emerge from many of the simulations:
We find that debris disks resulting from medium- to large-size impactors (0.01–0.1M?) have sufficient angular momentum and mass to accrete a sub-lunar-size moonlet. We performed 1,000 Monte Carlo simulations of sequences of N = 10, 20 and 30 impacts each, to estimate the ability of multiple impacts to produce a Moon-like satellite. The impact parameters were drawn from distributions previously found in terrestrial formation dynamical studies. With perfect accretionary mergers, approximately half the simulations result in a moon mass that grows to its present value after ~20 impacts.
If the multi-moon hypothesis proves credible, how would it affect the larger astrobiology question? In Ward and Brownlee’s Rare Earth (Copernicus, 2000), after a discussion of obliquity and the Moon’s effect on the Earth’s early history, the authors say this:
If the Earth’s formation could be replayed 100 times, how many times would it have such a large moon? If the great impactor had resulted in a retrograde orbit, it would have decayed. It has been suggested that this may have happened for Venus and may explain that planet’s slow rotation and lack of any moon. If the great impact had occurred at a later stage in Earth’s formation, the higher mass and gravity of the planet would not have allowed enough mass to be ejected to form a large moon. If the impact had occurred earlier, much of the debris would have been lost to space, and the resulting moon would have been too small to stabilize the obliquity of Earth’s spin axis. If the giant impact had not occurred at all, the Earth might have retained a much higher inventory of water, carbon and nitrogen, perhaps leading to a Runaway Greenhouse atmosphere.
The idea of a series of impacts eventually leading to a larger moon significantly muddies the waters here. It is true that in our Solar System, the inner planets are nearly devoid of moons, but we have no way of extending this situation to exoplanets without collecting the necessary data, which will begin with our first exomoon detections. Certainly if numerous collisions in an early planetary system can produce a large moon, as this paper argues, then we can expect similar collisional scenarios in many systems, making such moons a frequent outcome.
The paper is Rufu, Oharonson & Perets, “A Multiple Impact Hypothesis for Moon Formation,” published online by Nature Geoscience 9 January 2017 (abstract).
by Paul Gilster | Jan 12, 2017 | Exoplanetary Science |
Moving groups are collections of stars that share a common origin, useful to us because we can study a group of stars that are all close to each other in age. Among these, the Beta Pictoris moving group is turning out to be quite productive for the study of planet formation. These are young stars, aged in the tens of millions of years (Beta Pictoris itself is between 20 and 26 million years old). Within the moving group, we’ve detected planets around 51 Eridani and Beta Pictoris, while infalling, star-grazing objects have been found around Beta Pictoris.
Evidence of comet activity around another of these stars was discussed at the American Astronomical Society meeting in Texas. The star HD 172555, 23 million years old and about 95 light years from Earth, shows the presence of the vaporized remnants of cometary nuclei, marking the third extrasolar system where such activity has been traced. All the stars involved are under 40 million years old, giving us a glimpse of the kind of activity that happens during the era when young terrestrial planets have begun to emerge in their systems.
Image: This illustration shows several comets speeding across a vast protoplanetary disk of gas and dust and heading straight for the youthful, central star. The comets will eventually plunge into the star and vaporize. The comets are too small to photograph, but their gaseous spectral “fingerprints” on the star’s light were detected by NASA’s Hubble Space Telescope. The gravitational influence of a suspected Jupiter-sized planet in the foreground may have catapulted the comets into the star. This star, called HD 172555, represents the third extrasolar system where astronomers have detected doomed, wayward comets. The star resides 95 light-years from Earth. Credit: NASA, ESA, and A. Feild and G. Bacon (STScI).
Carol Grady (Eureka Scientific/NASA GSFC) led the study reported on at the AAS. Her thoughts:
“Seeing these sun-grazing comets in our solar system and in three extrasolar systems means that this activity may be common in young star systems. This activity at its peak represents a star’s active teenage years. Watching these events gives us insight into what probably went on in the early days of our solar system, when comets were pelting the inner solar system bodies, including Earth. In fact, these star-grazing comets may make life possible, because they carry water and other life-forming elements, such as carbon, to terrestrial planets.”
The deflection of comets by the gravitational influence of a massive gas giant in an emerging planetary system is a vivid picture, one clarified by Grady and team’s work with the Hubble Space Telescope Imaging Spectrograph (STIS) and the Cosmic Origins Spectrograph (COS) in 2015. The team’s spectrographic analysis, using Hubble data collected from two observing runs separated by six days, detected carbon gas and silicon in the light of HD 172555 moving across the face of the star at a speed of 160 kilometers per second.
This work follows up a French study that first found exocomets transiting the same star in archival data from the HARPS spectrograph. That work detected signs of calcium. Grady and team have extended the analysis with a spectrographic analysis in ultraviolet light. They believe they are seeing gaseous debris left behind as comets disintegrated, vaporized materials that contain large chunks of the original comet. Helpfully, the disk around HD 172555 is seen almost edge-on from Earth, offering Hubble a clear view of the highly dispersed activity.
“As transiting features go, this vaporized material is easy to see because it contains very large structures,” Grady said. “This is in marked contrast to trying to find a small, transiting exoplanet, where you’re looking for tiny dips in the star’s light.”
To confirm that they are seeing the disintegration of icy comets as opposed to rocky asteroids, Grady’s researchers hope to use the STIS again to search for oxygen and hydrogen, a composition that would add further weight to these conclusions.
by Paul Gilster | Jan 11, 2017 | Missions |
Nothing built by humans has ever gotten as far from our planet as Voyager 1, which is now almost 21 billion kilometers from Earth. We’ve talked about the future of both Voyagers before in these pages — Voyager 1 passes within about 1.6 light years of the star Gliese 445 in some 40,000 years, its closest approach to a neighboring star. Voyager 2, which is now almost 17 billion kilometers out, closes to within 1.7 light years of Ross 248 in the same 40,000 years.
My case for doing what Carl Sagan once discussed, giving each Voyager a final kick with its remaining hydrazine, so that those closing distances could be reduced, can be found in Voyager to a Star. It would be a symbolic and philosophical act rather than a scientific one, as both Voyagers are losing their ability to transmit data and will be silent in about a decade. And nothing can reduce those huge timeframes, which means that any such symbolic statement would be made to the future, a way of saying we are learning to be a starfaring species.
Image: In this artist’s conception, NASA’s Voyager 1 spacecraft has a bird’s-eye view of the solar system. The circles represent the orbits of the major outer planets: Jupiter, Saturn, Uranus, and Neptune. Launched in 1977, Voyager 1 visited the planets Jupiter and Saturn. The spacecraft is now 21 billion kilometers from Earth, making it the farthest and fastest-moving human-made object ever built. In fact, Voyager 1 is now zooming through interstellar space, the region between the stars that is filled with gas, dust, and material recycled from dying stars. Credit: NASA, ESA, and J. Zachary and S. Redfield (Wesleyan University); Artist’s Illustration Credit: NASA, ESA, and G. Bacon (STScI).
Meanwhile, we still have two viable spacecraft in the outer reaches of our Solar System, taking data on interstellar material, magnetic fields and cosmic ray hits and giving us a sense of what the local interstellar medium (LISM) is like. That’s crucial information, of course, for one day we hope to have not just a few but many spacecraft operating on the edge of interstellar space, and going beyond our system will require us to know the nature of the medium through which they move. On that score, the best book I know is Bruce Draine’s Physics of the Interstellar and Intergalactic Medium (Princeton, 2010). I enjoyed talking to Draine (Princeton University) at the latest Breakthrough Starshot sessions.
As you can imagine, learning more about the interstellar medium is a prerequisite if you’re thinking of pushing something up to 20 percent of lightspeed, as Breakthrough Starshot is, so the topic was a lively one at those meetings. At the recent American Astronomical Society meetings in Texas, we learned that astronomers have been using Hubble data to supplement what Voyager has been giving us, charting the hydrogen clouds and other elements of the LISM. Seth Redfield (Wesleyan University), who leads the study, offers this comment:
“This is a great opportunity to compare data from in situ measurements of the space environment by the Voyager spacecraft and telescopic measurements by Hubble. The Voyagers are sampling tiny regions as they plow through space at roughly 38,000 miles per hour [61,000 kph). But we have no idea if these small areas are typical or rare. The Hubble observations give us a broader view because the telescope is looking along a longer and wider path. So Hubble gives context to what each Voyager is passing through.”
Image: In this illustration, NASA’s Hubble Space Telescope is looking along the paths of NASA’s Voyager 1 and 2 spacecraft as they journey through the solar system and into interstellar space. Hubble is gazing at two sight lines (the twin cone-shaped features) along each spacecraft’s path. The telescope’s goal is to help astronomers map interstellar structure along each spacecraft’s star-bound route. Each sight line stretches several light-years to nearby stars. Credit: NASA, ESA, and Z. Levy (STScI).
The Hubble work makes it clear that in two thousand years or so, Voyager 2 will move out of the interstellar cloud that surrounds the Solar System before moving into another cloud, in which it will remain for as much as 90,000 years. The astronomers find slight variations in the abundances of the chemical elements in these clouds, which could chart a history involving different paths to formation. We do know that as the solar wind pushes against the interstellar medium, the heliosphere can be compressed, only to expand again when the Sun moves through lower-density matter. For more, see this Hubblesite news release.
We still haven’t built the next generation LISM explorer, one crafted from the outset as an interstellar data gatherer. As much as the Voyagers continue to give us, we have to remember that they were designed as planetary probes, their survival to this point being an amazing and unexpected gift, but one that has to be adapted to the medium through which the spacecraft move. A spacecraft fine-tuned for exploration beyond the heliopause is a goal that continues to see its share of study (more on this soon), but when it will fly remains an open question.
by Paul Gilster | Jan 10, 2017 | Exoplanetary Science |
Breakthrough Starshot, the research and engineering effort to lay the groundwork for the launch of nanocraft to Alpha Centauri within a generation, is now investing in an attempt to learn a great deal more about possible planets around these stars. We already know about Proxima b, the highly interesting world orbiting the red dwarf in the system, but we also have a K- and G-class star here, either of which might have planets of its own.
Image: The Alpha Centauri system. The combined light of Centauri A (G-class) and Centauri B (K-class) appears here as a single overwhelmingly bright ‘star.’ Proxima Centauri can be seen circled at bottom right. Credit: European Southern Observatory.
To learn more, Breakthrough Initiatives is working with the European Southern Observatory on modifications to the VISIR instrument (VLT Imager and Spectrometer for mid-Infrared) mounted at ESO’s Very Large Telescope (VLT). Observing in the infrared has advantages for detecting an exoplanet because the contrast between the light of the star and the light of the planet is diminished at these wavelengths, although the star is still millions of times brighter.
To surmount the problem, VISIR will be fitted out for adaptive optics. In addition, Kampf Telescope Optics of Munich will deliver a wavefront sensor and calibration device, while the University of Liège (Belgium) and Uppsala University (Sweden) will jointly develop a coronagraph that will mask the light of the star enough to reveal terrestrial planets.
Image: Paranal at sunset. This panoramic photograph captures the ESO Very Large Telescope (VLT) as twilight comes to Cerro Paranal. The enclosures of the VLT stand out in the picture as the telescopes in them are readied for the night. The VLT is the world’s most powerful advanced optical telescope, consisting of four Unit Telescopes with primary mirrors 8.2 metres in diameter and four movable 1.8-metre Auxiliary Telescopes (ATs), which can be seen in the left corner of the image. Credit: ESO.
According to the agreement signed by Breakthrough Initiatives executive director Pete Worden and European Southern Observatory director general Tim de Zeeuw, Breakthrough Initiatives will pay for a large part of the technology and development costs for the VISIR modifications. Meanwhile, the ESO will provide the necessary telescope time for a search program that will be conducted in 2019. The VISIR work, according to this ESO news release, should provide a proof of concept for the METIS instrument (Mid-infrared E-ELT Imager and Spectrograph), the third instrument on the upcoming European Extremely Large Telescope.
by Paul Gilster | Jan 9, 2017 | Exoplanetary Science |
What effect does the composition of a star have on the planets that form around it? Enough of one that we need to take it into account as we assess exoplanets in terms of astrobiology. So says a study that was presented at the American Astronomical Society meeting in Texas last week, looking at ninety specific stars identified by Kepler as having evidence of rocky planets.
We know about the composition of these stars because they are part of the 200,000 star dataset compiled by APOGEE, the Apache Point Observatory Galactic Evolution Experiment spectrograph mounted on the 2.5m Sloan Foundation telescope in New Mexico. APOGEE allows us to examine the spectra of stellar atmospheres to identify their elements.
Modeling the formation of planets around these stars shows us the implications for astrobiology. Johana Teske (Carnegie Observatories) explains:
“Our study combines new observations of stars with new models of planetary interiors. We want to better understand the diversity of small, rocky exoplanet composition and structure — how likely are they to have plate tectonics or magnetic fields?”
At the AAS meeting, Teske described how the team of astronomers and geoscientists she is working with focused on Kepler 102 and Kepler 407, the former a star slightly less luminous than the Sun hosting five known planets, the latter hosting two planets orbiting a star of roughly the Sun’s mass. The APOGEE data show that in terms of chemical composition, Kepler 102 is similar to the Sun, while Kepler 407 is much richer in silicon.
Geophysicist Cayman Unterborn (Arizona State) ran computer simulations of planet formation incorporating the APOGEE data. The result:
“We took the star compositions found by APOGEE and modeled how the elements condensed into planets in our models. We found that the planet around Kepler 407, which we called ‘Janet,” would likely be rich in the mineral garnet. The planet around Kepler 102, which we called ‘Olive,’ is probably rich in olivine, like Earth.”
Image: The picture shows what minerals are likely to occur at several different depths. Kepler 102 is Earth-like, dominated by olivine minerals, whereas Kepler 407 is dominated by garnet, so less likely to have plate tectonics. Credit: Robin Dienel, Carnegie DTM.
In Unterborn’s view, the difference is significant because garnet, a far stiffer mineral than olivine, flows more slowly, implying a garnet planet would be unlikely to have long-term plate tectonics. Like the Earth, the planet around Kepler 102 could sustain tectonics, which are thought to be essential for life because atmospheric recycling through geological processes like volcanoes and ocean ridge formation regulates the atmosphere’s composition. Without such geological processes, life would not necessarily have the chance to evolve.
Centauri Dreams‘ take: The interplay of the two datasets — APOGEE and Kepler — is deeply productive, but we’re only at the beginning of the analysis. APOGEE’s 200,000 stars include others known to host small planets, so similar methods can now be put to work on the mineral content of these worlds. Those most Earth-like in their mineral content would rank higher on our list for further astrobiological study, helping us refine our targets for future observation.
