Centauri Dreams

What intrigues me about Kepler-1649c, a newly discovered planet thrust suddenly into the news, isn’t the fact that it’s potentially in its star’s habitable zone, nor that it is close to being Earth-sized (1.06 times Earth’s radius). Instead, I’m interested in the way it was found. For this is a world turned up in exhaustive analysis of data from the original Kepler mission. Bear in mind that data from the original Kepler field ceased being gathered a full seven years ago.

I share Jeff Coughlin’s enthusiasm on the matter. Coughlin is an astronomer affiliated with the SETI Institute who is a co-author on the new paper, which appears in Astrophysical Journal Letters. One of the goals of the mission that began as Kepler and continued (on different star fields) as K2 was to find the fraction of stars in the galaxy that have planets in the habitable zone, using transit methods for initial detection and radial velocity follow-up on Earth. Of the new find, made with an international team of scientists, Coughlin says:

“It’s incredible to me that we just found [Kepler-1649c] now, seven years after data collection stopped on the original Kepler field. I can’t wait to see what else might be found in the rich dataset from Kepler over the next seven years, or even seventy.”

Yes, because we’re hardly through with a dataset that has taken in close to 200,000 stars. What the new planet reminds us is that the initial detection stage of Kepler’s work long ago ceded to analysis of how many of the acquired signals are due to systematic effects in the instrumentation, or variable stars, or perhaps binaries, for so many things can appear to be the signature of a planet when they are not. Making computer algorithms to automate this work has occurred only after intensive human study to learn the best ways to distinguish between signals.

But consider: The algorithm, called Robovetter, that was used to distinguish false positives was working on data in which only 12 percent of the transit signals turned out to be planets. We have false positives galore, out of which we now get the Kepler False Positive Working Group to serve as a double-check on the algorithm’s work. The KFPWG is a dedicated team indeed, taking years to review the thousands of signals from the original Kepler dataset. Kepler-1649c comes out of this review, a world that had been misclassified by the automated methods earlier on. Clearly, humans and machines learn from each other as they make such difficult calls.

Image: A comparison of Earth and Kepler-1649c, an exoplanet only 1.06 times Earth’s radius. Credit: NASA/Ames Research Center/Daniel Rutter.

I yield to the experts when it comes to habitability, and trust our friend Andrew LePage will weigh in soon with his own analysis. What we do know is that this planet receives about 75 percent of the amount of light Earth receives from the Sun, though in this case the star in question is a red dwarf of spectral type M5V, the same as Proxima Centauri, fully 300 light years out. As always with red dwarfs, we bear in mind that stellar flares are not uncommon. The planet orbits its star every 19.5 days. We have no information about its atmosphere.

Even so, lead author Andrew Vanderburg (University of Texas at Austin) notes the potential for habitability:

“The more data we get, the more signs we see pointing to the notion that potentially habitable and Earth-sized exoplanets are common around these kinds of stars. With red dwarf stars almost everywhere around our galaxy, and these small, potentially habitable and rocky planets around them too, the chance one of them isn’t too different than our Earth looks a bit brighter.”

There is a planet on an orbit interior to Kepler-1649c, a super-Earth standing in relation to the former much as Venus does to the Earth. The two demonstrate an uncommon nine-to-four orbital resonance, one that hints at the possibility of a third planet between them. This would give us a pair of three-to-two resonances, a much more common scenario, though no trace of the putative world shows up in the data. If there, it is likely not a transiting planet.

I rather like the image that comes out of NASA Ames, where the Kepler effort is managed, but I like it as a kind of science fictional art, the sort of thing to be found on SF paperbacks. It’s evocative and suggestive, but of course we have no idea what this world really looks like.

Image: An illustration of what Kepler-1649c could look like from its surface. Credit: NASA/Ames Research Center/Daniel Rutter.

The paper is Vanderburg et al., “A Habitable-zone Earth-sized Planet Rescued from False Positive Status,” Astrophysical Journal Letters Vol. 893, No. 1 (1 April 2020). Abstract.

The interstellar object we call ‘Oumuamua was bound to be fascinating no matter what it actually was. You discover the first incoming object from interstellar space only once. But this one had its own share of peculiarities. Here was what was assumed to be a comet, but one that showed no outgassing as it reached perihelion and in fact seemed to be unusually dry. Here was an object of an apparently elongated shape, an aspect ratio with which we had nothing to compare in our own system. A tiny but detectable acceleration on the way out of the system seemed to indicate later outgassing, but how was that consistent with earlier data?

I think Harvard’s Avi Loeb was exactly right to point out that among the possible explanations of new objects, we can’t disregard the possibility of a technology from another civilization. That ‘Oumuamua was a natural object is an obvious default position, but we are at a stage in our understanding of the cosmos when we realize that the conditions for life occur elsewhere. We don’t know how often it forms — abiogenesis may be spectacularly rare. But maybe not.

But while we’ll never know with 100 percent certainty what ‘Oumuamua was, we can look forward to many more interstellar objects to evaluate as new instrumentation comes online. Meanwhile, that apparently elongated shape that so distinguished ‘Oumuamua has come under scrutiny from Yun Zhang (National Astronomical Observatories of the Chinese Academy of Sciences) and Douglas N. C. Lin (UC-Santa Cruz), who have run computer simulations that show how it could have formed and go on to explain its other oddball quirks.

“We showed that ‘Oumuamua-like interstellar objects can be produced through extensive tidal fragmentation during close encounters of their parent bodies with their host stars, and then ejected into interstellar space,” says Lin. “Our objective is to come up with a comprehensive scenario, based on well understood physical principles, to piece together all the tantalizing clues.”

Image: This illustration shows the tidal disruption process that can give rise to ‘Oumuamua-like objects. Credit: NAOC/Y. Zhang.

Tidal forces are the key to this work, which is described in a paper in Nature Astronomy. Think Shoemaker-Levy 9, whose spectacular disintegration produced the famous ‘chain of pearls’ as the comet was progressively torn apart by Jupiter’s gravity. In Zhang and Lin’s simulations, the modeling of the structural dynamics of an object passing close to its star produces extremely elongated fragments that are then ejected into deep space. The work shows that aspect ratios even greater than 10 to 1 are not out of the question.

Could such a shape be stable? Evidently so, for the scientists used thermal modeling to show that at tight stellar distances, the surface of such an object would melt and then recondense on its way out of the system, producing a crust that should remain cohesive. We get a ‘shard’ of material being flung into the cosmos in a random direction. Moreover, this is a process that contains within it other unusual aspects of the ‘Oumuamua encounter with our system.

“Heat diffusion during the stellar tidal disruption process also consumes large amounts of volatiles, which not only explains ‘Oumuamua’s surface colors and the absence of visible coma, but also elucidates the inferred dryness of the interstellar population,” Zhang said. “Nevertheless, some high-sublimation-temperature volatiles buried under the surface, like water ice, can remain in a condensed form.”

So we circle back to the fact that ‘Oumuamua showed no cometary activity but an apparent outgassing sufficient to produce a non-gravitational motion, albeit a tiny one. Under the modeling Zhang and Lin performed, the warmth of the approach to perihelion would have activated residual water stores that match the observed trajectory of ‘Oumuamua.

Image: A ‘Oumuamua-like object produced by a simulation of the tidal disruption scenario proposed by Zhang and Lin. Credit: NAOC/Y. Zhang; background: ESO/M. Kornmesser.

We should expect quite a few more interstellar objects in our future. Zhang and Lin believe that on average, a single planetary system should eject a total of about a hundred trillion objects like ‘Oumuamua. The tidal forces in play in these simulations work for any number of source objects, from super-Earths to long-period comets and other material from a debris disk. In other words, there is hardly a shortage of material out of which to form interstellar objects. Doubtless our Sun has spewed its share into nearby interstellar space, shard-like or not.

We’ve had a second interstellar object since ‘Oumuamua, the much more comet-like 2I/Borisov. Whether Zhang and Lin are right in their assessment may eventually become clear as we detect and study further examples. Given their apparent plenitude, we should by virtue of these simulations assume that some, if not many, will share some of the traits of ‘Oumuamua. Eventually we’ll build a catalog that may teach us something about planetary evolution.

The paper is Zhang and Lin, “Tidal fragmentation as the origin of 1I/2017 U1 (‘Oumuamua),” Nature Astronomy 13 April 2020 (abstract).

The brown dwarf 2MASS J10475385+2124234 is about the size of Jupiter, but maybe 40 times more massive. 33.2 light years from Earth, this object is in that category between planet and star, not massive enough to launch the same kind of nuclear reactions that power the Sun, but considerably more massive than any planet. Combining two tools — the Very Large Array (VLA) and NASA’s Spitzer Space Telescope — scientists have now measured the wind speed here.

Katelyn Allers (Bucknell University), who led the research team, realized that the combination of radio observations (VLA) and infrared (Spitzer) would make this kind of measurement possible, and expressed surprise that no one else had thought to do it before. After all, we already knew that the rotation period of Jupiter found through radio measurements differs from the period found at visible and infrared wavelengths. That disparity is key to the new measurement.

For the difference is the result of two separate phenomena. Radio emissions from Jupiter are caused by electrons interacting with the planet’s huge magnetic field, located deep in the interior. The infrared emission is measured at the top of the atmosphere. Because the atmosphere is rotating more quickly than the interior, we have a difference in velocity due to atmospheric winds. The same kind of measurement is now put to use at a brown dwarf.

“Since the magnetic field originates deep in the planet, or in this case brown dwarf, the radio data allows us to determine the interior period of rotation,” Allers says. “When you have an interior rotation rate and an atmospheric rotation rate, you can compare them to see how fast the wind is blowing.”

Image: Artist’s conception of a brown dwarf and its magnetic field. The magnetic field, rooted deep in its interior, rotates at a different rate than the top of the atmosphere. The difference allowed astronomers to determine the object’s wind speed. Credit: Bill Saxton, NRAO/AUI/NSF.

Allers and company observed 2MASS J10475385+2124234 with Spitzer in 2017 and 2018, noting the interesting fact that its infrared brightness varies regularly, indicating a long-lived feature of some sort in the upper atmosphere. Does this brown dwarf have something analogous to Jupiter’s Great Red Spot? That we can’t know at this point, but a set of VLA observations from 2018 measured the rotation period of the brown dwarf’s interior, so we do know about the winds.

The Jupiter analogy holds, for the brown dwarf’s atmosphere does indeed rotate faster than its interior, allowing the calculation of a wind speed of about 2300 kilometers per hour. By comparison, Jupiter’s wind speed clocks in at around 370 kilometers per hour. According to Allers, the difference is in agreement with theory and simulations of brown dwarf wind speeds.

This initial measurement may alert us to a technique that can measure winds not only on other brown dwarfs but on some giant exoplanets, although as co-author Peter K.G. Williams notes, the magnetic fields of such worlds are weaker than those of brown dwarfs, so the radio measurements would be conducted at lower frequencies than those used in the 2MASS J10475385+2124234 work. Williams (Center for Astrophysics | Harvard & Smithsonian) led the radio astronomical observations at the VLA.

“For the first time ever, we measured the speed of the winds of a brown dwarf — too big to be a planet, too small to be a star,” Williams adds. “The results rule out a few unusual models and prove that this new technique works and can be applied to more objects… This new technique opens the way to better understanding the behavior of atmospheres that are unlike anything found in our solar system.”

Image: Brown dwarf, left, and Jupiter, right. Artist’s conception of brown dwarf illustrates magnetic field and atmosphere’s top, which were observed at different wavelengths to determine wind speeds. Credit: Bill Saxton, NRAO/AUI/NSF.

The paper is Allers et al., “A Measurement of the Wind Speed on a Brown Dwarf,” Science Vol. 368, Issue 6487 (10 April 2020). Abstract.

We looked recently at Voyager 2’s flyby of Uranus, via a new paper that examined the craft’s magnetometer data to draw out information about the planet’s magnetic environment. Science fiction author Stanley Weinbaum, author of the highly influential “A Martian Odyssey” in 1935, christened Uranus ‘The Planet of Doubt’ in a short story of the same name. Weinbaum couldn’t have known about the world’s magnetic field axis, which we’ve learned is tilted 60 degrees away from its spin axis. The latter itself is 98 degrees off its orbital plane. Doubtful planet indeed.

Here we have a world that is spinning on its side, one that demands answers as to how it got that way. A giant impact at some point in its history is a natural assumption, but how do we explain the fact that the Uranian moons as well as the planet’s ring system all show the same 98 degree orbital tilt as their parent? Back in 2011, a team led by Alessandro Morbidelli (Observatoire de la Cote d’Azur) ran a variety of simulations to test impact scenarios and discovered that a sufficiently early impact could have reformed the entire protoplanetary disk, leading to its 27 moons being in the position we see today. See the Centauri Dreams post A New Slant on the Planet of Doubt for more on the Morbidelli et al. paper.

Image: Uranus is uniquely tipped over among the planets in our Solar System. Uranus’ moons and rings are also orientated this way, suggesting they formed during a cataclysmic impact which tipped it over early in its history. Credit: Lawrence Sromovsky, University of Wisconsin-Madison/W.W. Keck Observatory/NASA.

Now we have another look at the problem, this from a team led by Shigeru Ida (Earth-Life Science Institute, Tokyo Institute of Technology). The key to this new paper is the understanding that while impacts would have been more common in the early Solar System than now, the outer planets would have experienced impacts that were different in result than those in the inner system. A rocky Mars-sized object might smack into a rocky Earth to create our Moon, but collisions between two icy objects have different results in the outer Solar System.

Out here, we’re dealing with planets with an abundance of volatiles, elements that would be gases or liquids in the warmer regions of the inner system, but are frozen at large distances from the Sun. The temperature needed to vaporize water ice is low, and the team assumes, reasonably, that both Uranus and its impactor were dominated by ices. Thus an impact early in the formation history of Uranus would have vaporized this ice, with leftover materials remaining gaseous and becoming incorporated primarily into the forming planet. Ida’s computer modeling shows that such impacts produce not one or two large moons but a number of small ones.

The inclination of both ring and moon system at Uranus makes it clear that the impact was early and formative for the entire Uranian system. Bear in mind that the ratio of the planet’s mass to its moons is larger than the ratio of Earth’s mass to its Moon by a factor of more than one hundred. Working with substantial water vapor mass loss, the researchers’ simulation reproduces the observed mass-orbit configuration of the Uranian satellites by incorporating the predicted distribution of ices as they re-condense. The results parallel the system we see today, indicating it is the result of the evolution of this volatile-laden impact-generated disk.

The authors contrast this with the giant impact model for Earth’s Moon, arguing that about half of the solid or liquid disk created by the strike was integrated into the Moon. The difference is the high condensation temperature at Earth’s orbital distances, meaning that the rocky and liquid material of the Earth-Moon impact would have solidified quickly, allowing the Moon to collect a significant amount of the debris created by the collision due to its gravity shortly after impact.

The work may have applications in other stellar systems. From the paper:

We have shown that the current Uranian major satellites are beautifully reproduced by the derived analytical formulas based on viscous spreading and cooling of the disk generated by an impact that is constrained by the spin period and the tilted spin, independent of details of the initial disk parameters. Although we have focused on Uranus, the model here provides a general scenario for satellite formation around ice giants with the scaling by the mass and the physical radius of a central planet, which is totally different from satellite formation scenarios around terrestrial planets and gas giants. It could also be applied for the inner region of Neptune’s satellite system, where we can neglect the effect of Triton that may have been captured. Observations suggest that many of [the] discovered super-Earths in exoplanetary systems may consist of abundant water ice, even in close-in (warm) orbits. The model here may also give a lot of insights into possible icy satellites of super-Earths.

The paper is Shigeru Ida et al., “Uranian satellite formation by evolution of a water vapour disk generated by a giant impact,” Nature Astronomy 30 March 2020 (abstract / preprint).