Centauri Dreams
Imagining and Planning Interstellar Exploration
An Earth-sized Planet for TESS
If Kepler’s task was to give us a first statistical cut at the distribution of exoplanets in the galaxy, TESS (Transiting Exoplanet Survey Satellite) has a significantly different brief, to use its four cameras to study stars that are near and bright. Among these we may hope to find the first small, rocky planets close enough that their atmospheres may be examined by space telescopes and the coming generation of extremely large telescopes (ELTs) on Earth.
Thus the news that TESS has found its first planet of Earth size is heartening, even if the newly found world orbiting HD 21749 is in a tight 7.8 day orbit, making it anything but clement for life. What counts, of course, is the demonstrated ability of this mission to locate the small worlds we had hoped to find. Diana Dragomir is a postdoc at MIT’s Kavli Institute for Astrophysics and Space Research, as well as lead author on the paper describing the latest TESS planet:
“Because TESS monitors stars that are much closer and brighter, we can measure the mass of this planet in the very near future, whereas for Kepler’s Earth-sized planets, that was out of the question. So this new TESS discovery could lead to the first mass measurement of an Earth-sized planet. And we’re excited about what that mass could be. Will it be Earth’s mass? Or heavier? We don’t really know.”
Image: NASA’s Transiting Exoplanet Survey Satellite (TESS), shown here in a conceptual illustration, will identify exoplanets orbiting the brightest stars just outside our Solar System. Credit: NASA’s Goddard Space Flight Center.
Dragomir considers the new planet around HD 21749, some 52 light years from Earth, a milestone in being the mission’s first planet of Earth size, though the expectation is for at least a few dozen more among the nearest and brightest stars as TESS continues sweeping the sky in overlapping sectors. This is a two-year mission that has already discovered 10 planets smaller than Neptune, four of which now have estimated masses. We’re in the early days of this mission, and that’s a good sign. As noted in the paper, “All of these discoveries are based on only the first two sectors of TESS data, suggesting many more are to be found.”
The star is a K-class dwarf in the southern constellation Reticulum that is known to host a second planet, recently confirmed, that is about three times the size of Earth. The paper reports on the discovery and confirmation of HD 21749b and the discovery of HD 21749c, but it is the latter, given its small size, that is receiving the lion’s share of coverage.
The paper notes that spectroscopic and photometric data have made the confirmation of the larger planet possible, while the Earth-sized HD 21749c would be a challenging observation for radial velocity confirmation, if possibly feasible with a dedicated campaign using the combination of the Very Large Telescope (VLT) and the ESPRESSO spectrograph. But density measurements of both planets should be useful. From the paper:
…the density of HD 21749b indicates that it is likely surrounded by a substantial atmosphere. By measuring the density of these two planets (and other similar planets that TESS may find) more precisely, we can begin to observationally constrain the maximum core mass a planet can reach during its formation before accreting a volatile envelope.
The paper is Dragomir et al., “TESS Delivers Its First Earth-sized Planet and a Warm Sub-Neptune,” Astrophysical Journal Letters Vol. 875, No. 2 (15 April 2019. Abstract.
New Planet Detected in Circumbinary System
The transit method has proven invaluable for exoplanet detection, as the runaway success of the Kepler/K2 mission demonstrates. But stars where planets have been detected with this method are still capable of revealing further secrets. Consider Kepler-47. Here we have a circumbinary system some 3340 light years away in the direction of the constellation Cygnus, and as we are now learning about circumbinaries — planets that orbit two stars — the alignment of the orbital plane of the planet is likely to change with time.
Let’s pause for a moment on the value of the detection method. Transits detected in the lightcurve have helped us identify 10 transiting circumbinary planets, with the benefit of allowing astronomers to measure the planets’ radius even as variations in the duration of transits and deviations from the expected timing of the transits establish the circumbinary orbit.
At Kepler-47, we’re looking at the only known multi-planet circumbinary system. Moreover, the orbital period of the binary stars is in the range of 7.5 days, making this the shortest known orbital period for any known circumbinary system. The first two Kepler planets were detected in 2012, but San Diego State University astronomers now find a middle planet between these, Kepler-47d, its strengthening transit signal the result of orbital plane adjustment. In fact, the transit depth for the hitherto undetectable world has become the deepest of the three planets.
Jerome Orosz (SDSU) is the paper’s lead author.
“We saw a hint of a third planet back in 2012, but with only one transit we needed more data to be sure. With an additional transit, the planet’s orbital period could be determined, and we were then able to uncover more transits that were hidden in the noise in the earlier data.”
And according to co-author and SDSU colleague William Welsh, this planet defied expectations by showing up not exterior to the previously known planets but between them. “We certainly didn’t expect it to be the largest planet in the system. This was almost shocking,” said Welsh.
Image: Artist’s impression of the third planet in the Kepler-47 circumbinary system. Credit: NASA/JPL Caltech/T. Pyle.
So what we see at Kepler-47, at least at this juncture, is an inner planet 3.1 times the size of Earth, in an orbit taking 49 days around this G-class star orbiting an M-dwarf. The other planets here are, respectively, 7 times Earth-size on an 187-day orbit (Kepler-47d), and 4.7 Earth-size with a 303 day orbit. Remember that we are talking about planets orbiting two stars, in a system compact enough to fit inside the Earth’s orbit of the Sun.
Kepler-47’s system may be telling us something interesting about planet formation. From the paper:
This is the first detection of a dynamically packed region in a circumbinary system, and it further confirms suspicions that planet formation and subsequent migration can proceed much like that around a single star, at least when far from the binary (Pierens & Nelson 2008, 2013; Kley & Haghighipour 2014, 2015). We also find that, although they are close to having integer commensurate periods, the middle and outer planets are not in a mean-motion resonance-and yet they are gravitationally interacting and exchanging angular momentum, as indicated by their anti-phased oscillations in inclination and eccentricity.
The authors find the planetary configuration dynamically stable for at least 100 million years, adding:
This nearly circular, co-planar, packed configuration is unlikely to have arisen as an outcome of strong gravitational scattering of the planets into their current orbits. Rather, the observations suggest that the planetary configuration is the result of relatively gentle migration in a circumbinary protoplanetary disk.
Image: This is Figure 28 from the paper. Caption: The conservative (dark green) and optimistic (light green) habitable zone regions are shown for the Kepler-47 system. The red circle shows the critical stability radius (Holman & Wiegert 1999), interior to which planetary orbits are most likely unstable. Credit: Jerome Orosz/William Welsh et al.
On the matter of habitability, there is little reason to expect life here. These are low density worlds, all three being less dense than Saturn, which implies substantial hydrogen and helium atmospheres. The outer planet receives an average insolation from its two stars that is 86.5 percent of what the Earth receives. But while that puts this world within the boundaries of the circumbinary habitable zone, the density implies a world somewhere in the range between Neptune and Saturn. The newly discovered middle planet skirts the inner edge of the habitable zone, but again its density makes life unlikely.
The paper is Orosz et al., “Discovery of a Third Transiting Planet in the Kepler-47 Circumbinary System,” Astronomical Journal Vol. 157, No. 5 (16 April 2019). Abstract / Preprint.
Huge White Light Flare on a Tiny Star
About 250 light years away there is a faint object that is on the borderline between brown dwarf and star. Only a tenth of the radius of our Sun, ULAS J224940.13-011236.9 was actually too faint for most telescopes to observe until a huge flare lit it up, turning this L dwarf, among the lowest mass objects that can still be considered a star, 10,000 times brighter than it was before. Very cool compared to the average red dwarf, L dwarfs emit radiation primarily in the infrared.
But this story also has to do with visible light, and the question of how such a small object can produce such a powerful explosion. This was a ‘white light’ flare, a type of flare that displays associated brightening in the visible light spectrum. Astronomers believe flares are driven by magnetic energy, the sudden release of which can cause charged particles to heat plasma. In this case the resulting optical, ultraviolet and X-ray radiation was copious.
James Jackman, a PhD student in physics at the University of Warwick (UK) and lead author of the paper on these observations, points to the rare nature of this flare:
“The activity of low mass stars decreases as you go to lower and lower masses and we expect the chromosphere (a region of the star which support flares) to get cooler or weaker. The fact that we’ve observed this incredibly low mass star, where the chromosphere should be almost at its weakest, but we have a white-light flare occurring, shows that strong magnetic activity can still persist down to this level.”
Image: A superflare on an L-dwarf. Credit & Copyright: University of Warwick/Mark Garlick.
And note this from the paper on this work, on the unusual strength of some L dwarf flares:
While seen regularly on GKM stars, observations of white-light flares on L dwarfs remain rare, with only a handful of stars showing them to date (e.g. Paudel et al. 2018). However, those observed have included some of the largest amplitude flares ever recorded, reaching up to V ? ?11 (Schmidt et al. 2016). This shows that white-light flaring activity persists into the L spectral type, despite previous studies of L dwarfs showing their chromospheres and magnetic activity to be diminished compared to those of late M dwarfs.
So a borderline brown dwarf/star is giving us an interesting lesson, perhaps on the difference between the two, because we may be able to determine whether flares like these are limited to actual stars, learning at just what point the activity ceases. Are there other tiny stars, like ULAS J224940.13-011236.9 about the same size as Jupiter, that mark the limits of such flares, below which none occur? Whatever the case, few L dwarfs have produced a flare of this magnitude.
Embedded in future work will surely be the question of how tiny stars like this one store energy in magnetic fields, and their level of chromospheric activity. From the paper:
Ultracool dwarfs are also known to exhibit auroral activity…, which may account for observed H? [hydrogen alpha] emission [when a hydrogen electron falls from its third to second lowest energy level], in these systems. It is expected that the transition from predominantly chromospheric to auroral H? emission occurs during the L spectral type… Many ultracool dwarfs that show activity such as radio emission and flaring also tend to be fast rotators, with rotation periods on the order of hours. However, neither the L1 dwarf SDSSp J005406.55?003101.8 (Gizis et al. 2017a) nor the L0 dwarf J12321827?0951502 (Paudel et al. 2018) showed any sign of rapid rotation when observed by K2, despite showing large amplitude white-light flares. Consequently, we do not attempt to predict whether ULAS J2249?0112 is a fast rotator. Regardless of this, studies of white-light flares such as from ULAS J2249?0112 can aid in understanding exactly how far into the L spectral type chromospheric activity persists.
The J224940.13-011236.9 data come from the Next Generation Transit Survey (NGTS) facility at the European Southern Observatory’s Paranal Observatory, with further data from the Two Micron All Sky Survey (2MASS) and Wide-field Infrared Survey Explorer (WISE), a total observation period of 146 days. The flare occurred on 13 August 2017, with an energy equivalent of 80 billion megatonnes of TNT, surpassing the largest flare (the Carrington event of 1859) ever observed on the Sun.
The paper is Jackman et al., “Detection of a giant white-light flare on an L2.5 dwarf with the Next Generation Transit Survey,” Monthly Notices of the Royal Astronomical Society: Letters Vol. 485, Issue 1 (May 2019), L136-L140 (abstract).
Chinese Mission to an Earth Co-Orbital
This morning’s entry resonates with Jim Benford’s recent work on objects that are co-orbital with Earth (see A SETI Search of Earth’s Co-Orbitals). You’ll recall that Benford argues for close study of co-orbitals like Cruithne (3753), a 5-kilometer object with closest approach to Earth of 0.080 AU, and 2010 TK7, which oscillates around the Sun-Earth Lagrangian point L4. A number of other such objects are known in a 1:1 orbital resonance with Earth, but they are seldom studied or even mentioned in the literature.
Calling for SETI observations at radio and optical wavelengths, as well as lighting up the objects with planetary radar, Benford gives a nod to Ronald Bracewell, who speculated that one way for an extraterrestrial intelligence to study a stellar system would be to plant a probe within it that could inform the home civilization about events there. The Earth co-orbitals are made to order for such observation, so why not give them a look with all the tools in our SETI arsenal?
Now we learn that China plans to explore the near-Earth asteroid 2016 HO3, along with a main-belt comet designated 133P. An interesting move — 2016 HO3 is the closest, most stable quasi-satellite of Earth, with a minimum distance of 0.0348 AU. Also known as Kamo?oalewa — a Hawaiian word for an oscillating object in the sky — 2016 HO3 has a minimum orbital intersection distance of 0.0348 AU (5,210,000) km, which is 13.6 times as far away as the Moon, although it seldom comes closer than about 38 lunar distances from us. The Center for Near Earth Object Studies (CNEOS) calculates this one has been in a stable orbit of our planet for about a century and will remain in its orbital pattern for centuries.
Image: Orbit of 2016 HO3. Credit: James Benford.
According to Liu Jizhong, director of the Lunar Exploration and Space Program Center of the China National Space Administration (CNSA), the current plan is to study 2016 HO3 from space before landing on it to collect samples for return to Earth. Following delivery of the sample return capsule, the probe is to proceed to comet 133P by means of gravity assists at Earth and Mars, in a mission lasting on the order of 10 years.
China is now soliciting proposals for eight types of scientific instruments for the mission among universities, research organizations and private companies both in China and abroad, according to a CNSA news release. Among the instruments needed, Liu says, are a color camera with an intermediate field of view, thermal emission spectrometer, visible and infrared imaging spectrometer, multispectral camera, detection radar, magnetometer, charged and neutral particle analyzer and dust analyzer. Quoting from the news release:
[Liu] said there might be two forms of onboard schemes. One possible scheme is to carry an independent detector on the rocket. After China’s main probe enters the orbit, the onboard detector will separate from the rocket and then perform independent tasks. Its mass should not exceed 200 kg. The other possible option is to let China’s main probe carry the onboard detector to the near-Earth asteroid or the main-belt comet and then release it. The detector could either perform independent scientific exploration or coordinate with the main probe.
If the onboard detector does not separate with the main probe, its mass should not exceed 20 kg. If the detector separates from the main probe near the asteroid, its mass should be no more than 80 kg. If it separates from the main probe near the comet, its mass should not exceed 20 kg.
The deadline for proposals is August 31, 2019, with those interested asked to contact CNSA.
Image: An animation of 2016 HO3’s orbit around Earth 2000-2300. Credit: Phoenix7777 – Own work. Data source: HORIZONS System, JPL, NASA. CC BY-SA 4.0.
This will not be China’s first experience with an asteroid mission. In December of 2012, its second lunar probe, Chang’e-2, made a close approach and flyby of asteroid 4179 Toutatis after completing its primary mission, approaching to within 3.2 kilometers and returning images. Now we have an ambitious mission to give us a close-up look at an Earth co-orbital, with comet operations to follow. We should learn a lot, for right now even the size of 2016 HO3 is not firmly established, though it is believed to be between 40 and 100 meters, depending on assumptions about its albedo, and we do know that it is a fast rotator.
TRAPPIST-1: Of Flux and Tides
Seven planets of roughly Earth-size make TRAPPIST-1 a continuing speculative delight, as witness the colorful art it generates below. And with three of the planets arguably in the star’s habitable zone, this diminutive star attracts the attention of astrobiologists anxious to examine the possible parameters under which they orbit. One thing that is only now receiving attention is the question of planet-to-planet tidal effects, as opposed to the star’s tidal effects on its planets.
Image: An artist’s impression of the perpetual sunrise that might greet visitors on the surface of planet TRAPPIST-1f. If the planet is tidally locked, the “terminator region” dividing the night side and day side of the planet could be a place where life might take hold, even if the day side is bombarded by energetic protons. In this image, TRAPPIST-1e can be seen as a crescent in the upper left of the image, d is the middle crescent, and c is a bright dot next to the star. Credit: NASA/JPL-Caltech.
In our Solar System, we’ve become familiar with the idea that tidal deformation can cause interior heating, a fact that could well support both Europa at Jupiter and Enceladus at Saturn with energy needed to retain temperatures suitable for life below their icy surfaces. The effects are extreme at Io (though hardly life-inducing!) and also noteworthy on Neptune’s large moon Triton. Here again TRAPPIST-1 stands out, because we know of no other system where planets, not moons, are so tightly wound that they can raise significant tides on each other.
Consider TRAPPIST-1g, the sixth planet in the system, which according to a study performed by Hamish Hay and Isamu Matsuyama (Lunar and Planetary Laboratory, University of Arizona) experiences the mixed effects of tidal heating from the central star and the other planets more strongly than any other planet in the system.
Tides from the other planets in a planetary system are rarely seen as a factor, say the scientists, but heating due to tidal deformation is definitely in play here. From the paper:
Such tides are typically negligible because the mass of the central tide raising body is usually far greater than other bodies in the system, and also because the distances between these bodies are vast and the strength of tidal forces decreases with the distance between them cubed. The seven planet extrasolar system TRAPPIST-1…is the first system to be discovered where this is not the case. The separation distance at conjunction is small enough that tides raised by neighbouring planets can become significant, and heating must occur as a result.
Similarly, TRAPPIST-1’s two inner planets come close enough to raise powerful tides on each other, possibly sustaining volcanic activity on worlds that would be too hot on the day side to support life. An atmosphere maintained by volcanic eruptions could move heat to the night side, assuming tidal lock.
Image: An artist’s concept for a view of the TRAPPIST-1 system from near TRAPPIST-1f. The system is located in the constellation Aquarius and is just under 40 light-years away from Earth. Credit: NASA/JPL-Caltech.
The Trouble with TRAPPIST-1e
We’ve also recently looked at Lisa Kaltenegger’s work on the effect of intense radiation on M-dwarf planets (see M-Dwarfs: Weighing UV Radiation and Habitability). Kaltenegger (Cornell University/Carl Sagan Institute) has been investigating possible ways for life to survive the intense flares and ultraviolet radiation that pummel such worlds. Various mechanisms suggest themselves, enough to keep open the possibility that planets like these could sustain life.
What Federico Fraschetti (Harvard Smithsonian Center for Astrophysics) and colleagues have been studying is the ability of a star so much cooler and less massive than the Sun to emit such quantities of radiation. The scientists have simulated the path of high-energy protons through the magnetic field of the star, finding that the first of the three TRAPPIST-1 planets thought to be in the habitable zone (TRAPPIST-1e) is receiving up to 1 million times more flux than Earth.
We’re fortunate, of course, in being protected by our planet’s magnetic field from our star’s energetic proton bath, but Fraschetti’s calculations show that to have the same effect at TRAPPIST-1e, the planet’s magnetic field would need to be hundreds of times more powerful than Earth’s. The conclusion is based on the star’s most likely field alignment, which brings its energetic protons directly to the surface of TRAPPIST-1e, where damaging biological effects could occur. But much depends upon how the star’s magnetic field is angled away from its axis of rotation, making this a key datapoint for future investigations. From the paper:
Based on the scaling relation between far-UV emission and energetic protons for solar flares by Youngblood et al. (2017), we estimate that the innermost putative habitable planet, TRAPPIST-1e, is bombarded by a proton flux up to 6 orders of magnitude larger than experienced by the present-day Earth. Such a bombardment of planets in this study is found to result largely from the misalignment of the B-field/rotation axis assumed for the star-proxy. Since the exact magnetic morphology and alignment of the magnetic field is currently unknown for TRAPPIST-1, and for M dwarfs in general, our results indicate that determination of these quantities for exoplanet hosts would be of considerable value for understanding their radiation environments.
TRAPPIST-1e, then, may need some of Lisa Kaltenegger’s proposed solutions to the radiation flux problem if it is to be considered habitable. Lithophilic life, or perhaps life beneath an ocean, is one solution among those that Kaltenegger has proposed, and of course there is the possibility of tidal lock, which could keep the ‘dark’ side of the planet free of the flux. Habitability, as we continue to learn, is by no means an easy call, no matter where a planet is located within or without the putative habitable zone of its host.
The papers are Fraschetti et al., “Stellar Energetic Particles in the Magnetically Turbulent Habitable Zones of TRAPPIST-1-like Planetary Systems,” Astrophysical Journal Vol. 874, No. 1 (18 March 2019) (abstract / preprint); and Hay & Matsuyama, “Tides Between the TRAPPIST-1 Planets,” Astrophysical Journal Vol. 875, No. 1 (9 April 2019) (abstract / preprint).
Detection of an Interstellar Meteor
Do we have a second interstellar visitor, following on the heels of the controversial ‘Oumuamua? If so, the new object is of a much different nature, as was its detection. In 2014, a meteor north of Manus Island, off the coast of Papua New Guinea produced a powerful blast that, upon analysis, implied a ? 0.45m meter object massing about 500 kg. Events like this, not uncommon in our skies, are cataloged by the Center for Near Earth Object Studies (CNEOS); this one shows up as being detected at 2014-01-08 17:05:34 UTC.
Image: This gorgeous wide-angle photo from the 1997 Perseid shower captures a 20-degree-long fireball meteor and another, fainter meteor trail in a rich area of the northern summer Milky Way. Showers like these are predictable, but could some solitary fireballs mark the end of a meteor with an interstellar origin? Credit & Copyright: Rick Scott & Joe Orman.
Now the CNEOS catalog, which covers the last three decades, is useful indeed, for it takes advantage of detectors maintained by the U.S. government to analyze the sound and light of the passage of objects through the atmosphere, producing information on velocity and position at the time of impact. Harvard’s Avi Loeb, a familiar face in the media thanks to the ‘Oumuamua discussion, worked with undergraduate student Amir Siraj, whom he set to calculating. What could we learn about the prior trajectory of meteors in the catalog, homing in on the fastest?
In a paper submitted to Astrophysical Journal Letters, Loeb and Siraj note the latter’s identification of the 2014 Manus Island meteor as interstellar in origin. The paper finds no substantial gravitational interactions between the meteor and any planet other than Earth. Indeed, based on the CNEOS-reported impact speed of 44.8 km s-1, Loeb and Siraj calculate a speed of 43.8 km s-1 outside the Solar System. For the object to be bound, the observed speed at impact would have to be off by more than 45%.
This meteor, then, was on an unbound hyperbolic orbit. We can go on from here to note the object’s relation to another useful metric. For measured relative to the Local Standard of Rest, this meteor entered the Solar System with a speed of 60 kilometers per second.
The Local Standard of Rest (LSR) is produced by averaging the motion of all stars in the Sun’s neighborhood. Siraj and Loeb speculate that this velocity could indicate ejection from a planetary system, specifically from the inner regions where orbital speeds are high. The object’s speed would imply a position inside the orbit of Mercury were it to come from a star like our own, but a red dwarf like Proxima Centauri would have an ejection speed from its habitable zone in this very regime. Recall that the habitable zone around Proxima Centauri is 20 times closer to the star than the HZ in our own system. So here’s an interesting thought: “Since dwarf stars are most common, the detection of this meteor offers new prospects for ‘interstellar panspermia,’ namely the transfer of life between planets that reside in the habitable zones of different stars.”
What I’m quoting from above is an as yet unpublished summation Loeb has recently written of the paper’s findings, one that goes on to speculate about its implications. Panspermia would require a larger object because it would have to survive the fiery passage through the atmosphere, but the notion that objects could be passed from star to star in this way is interesting (and note that Loeb is not identifying a Proxima Centauri origin for this meteor, but rather pointing to possible scenarios between stars). The point is that dwarf stars are the most common in the universe, and the detection of an interstellar meteor could point to what is perhaps a common form of transfer between stars.
Beyond that, consider the possibilities in studying interstellar materials when we may find them entering our own atmosphere. Says Loeb:
Using the Earth’s atmosphere as a detector for interstellar objects offers new prospects for inferring the composition of the gases they leave behind as they burn up in the atmosphere. In the future, Astronomers may establish an alert system that triggers follow-up spectroscopic observations to an impact by a meteor of possible interstellar origin. Alert systems already exist for gravitational wave sources, gamma-ray bursts, or fast radio bursts at the edge of the Universe. Even though interstellar meteors reflect the very local Universe, they constitute a “message in a bottle” with fascinating new information about nurseries which may be very different from the Solar System. Some of them might even represent defunct technological equipment from alien civilizations, which drifted towards Earth by chance, just like a plastic bottle swept ashore on the background of natural seashells.
Thus spectroscopy of gaseous debris burning up in the Earth’s atmosphere could offer us a way to make interstellar investigations of the kind we’ve been assuming would be decades (at least) off, assuming we can make a timely identification of likely targets.
The paper is Siraj & Loeb, “Discovery of a Meteor of Interstellar Origin,” submitted to Astrophysical Journal Letters (preprint).
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