Centauri Dreams
Imagining and Planning Interstellar Exploration
New Work on Planetary Inflation
Once in space in 2018, the Transiting Exoplanet Survey Satellite (TESS) will be observing, among many other things, hundreds of thousands of red giant stars across the entire sky. Planets around red giants are an interesting topic, because such stars point to an evolutionary outcome our own Sun will share, and we’d like to know more about what happens to existing planets in such systems as the host star swells and reddens, engulfing inner worlds.
New work out of the University of Hawaii Institute for Astronomy now examines two ‘hot Jupiters’ around red giants, stellar systems where we see the gas giants swelling up as the result of processes that remain controversial. The inflated size of planets like these can be explained in at least two ways, one of which involves a slowing of the cooling in the planet’s atmosphere, which causes the planet to inflate soon after formation. But the data presented here, drawn from NASA’s K2 mission, tend to corroborate the thinking of co-author Eric Lopez (NASA GSFC) that direct energy input from the host star is the dominant cause of this planetary inflation.
Image: Upper left: Schematic of the K2-132 system on the main sequence.
Lower left: Schematic of the K2-132 system now. The host star has become redder and larger, irradiating the planet more and thus causing it to expand. Sizes not to scale. Main panel: Gas giant planet K2-132b expands as its host star evolves into a red giant. The energy from the host star is transferred from the planet’s surface to its deep interior, causing turbulence and deep mixing in the planetary atmosphere. The planet orbits its star every 9 days and is located about 2000 light years away from us in the constellation Virgo. Credit: Karen Teramura, UH IfA.
The work is now available in The Astronomical Journal, where lead author Samuel Grunblatt and team show that each of the two planets is about 30 percent larger than Jupiter, though in each case only about half as massive. The two planets — K2-132b and K2-97b — are similar in orbital periods, radii and masses. Each orbits its red giant star in about nine days, with planetary radii being calculated at 1.31 ± 0.11 RJ and 1.30 ± 0.07 RJ respectively.
The researchers used models to analyze the evolution of planets like these over time, determining that their radii are typical for planets receiving their current level of radiation, but calculating back to main sequence values of radiation, they find the gas giants would have been considerably smaller. Stellar flux flowing to the planets’ deep convective interiors could therefore explain their current size, an indication that planet ‘inflation’ is directly tied to stellar irradiation rather than delayed atmospheric cooling after the planets’ formation.
But other factors remain to be tested, metallicity in particular. From the paper:
Further studies of planets around evolved stars are essential to confirm the planet re-inflation hypothesis. Planets may be inflated by methods that are more strongly dependent on other factors such as atmospheric metallicity than incident flux. An inflated planet on a 20 day orbit around a giant star would have been definitively outside the inflated planet regime when its host star was on the main sequence, and thus finding such a planet could more definitively test the re-inflation hypothesis. Similarly, a similar planet at a similar orbital period around a more evolved star will be inflated to a higher degree (assuming a constant heating efficiency for all planets). Thus, discovering such a planet would provide more conclusive evidence regarding these phenomena.
Also in play is the issue of heating efficiency, which may well vary between planets depending on their composition. Back to TESS, whose investigations should complement these results. Grunblatt and team point out that TESS should be able to observe additional planets in roughly 10 day orbits around more evolved stars, including oscillating red giants. The data should allow us to distinguish between the delayed cooling possibility and stellar irradiation scenarios.
The paper is Grunblatt et al., “Seeing Double with K2: Testing Re-inflation with Two Remarkably Similar Planets around Red Giant Branch Stars,” Astronomical Journal Vol. 154, No. 6 (27 November 2017). Abstract / preprint.
Cassini’s Exquisite Last View
The release of Cassini’s last images of Saturn and its rings is a welcome event, a capstone to the mission that has taught us so much. What we see below is a series of images that have been grafted together, 42 red, green and blue images that allow us to see a wide-angle mosaic of Cassini’s view. The images were taken by the spacecraft’s wide-angle camera on September 13, and include the moons Prometheus, Pandora, Janus, Epimetheus, Mimas and Enceladus.
Image: After more than 13 years at Saturn, and with its fate sealed, NASA’s Cassini spacecraft bid farewell to the Saturnian system by firing the shutters of its wide-angle camera and capturing this last, full mosaic of Saturn and its rings two days before the spacecraft’s dramatic plunge into the planet’s atmosphere. During the observation, a total of 80 wide-angle images were acquired in just over two hours. This view is constructed from 42 of those wide-angle shots, taken using the red, green and blue spectral filters, combined and mosaicked together to create a natural-color view. Credit: NASA/JPL-Caltech/SSI.
I was glad to see some Cassini team members reminiscing about Voyager, whose journeys opened up the outer Solar System to close view, and continue to inform us about the interstellar medium. Thus Carolyn Porco, former Voyager imaging team member and Cassini’s imaging team leader at the Space Science Institute in Boulder, Colorado:
“For 37 years, Voyager 1’s last view of Saturn has been, for me, one of the most evocative images ever taken in the exploration of the solar system. In a similar vein, this ‘Farewell to Saturn’ will forevermore serve as a reminder of the dramatic conclusion to that wondrous time humankind spent in intimate study of our Sun’s most iconic planetary system.”
Here is a brightened view, processed to bring out detail in the fainter areas of the image. The six moons mentioned above show up faintly, but the annotations should help.
Image: The ice-covered moon Enceladus — home to a global subsurface ocean that erupts into space — can be seen at the 1 o’clock position. Directly below Enceladus, just outside the F ring (the thin, farthest ring from the planet seen in this image) lies the small moon Epimetheus. Following the F ring clock-wise from Epimetheus, the next moon seen is Janus. At about the 4:30 position and outward from the F ring is Mimas. Inward of Mimas and still at about the 4:30 position is the F-ring-disrupting moon, Pandora. Moving around to the 10 o’clock position, just inside of the F ring, is the moon Prometheus. Credit: NASA/JPL-Caltech/SSI.
We’re looking toward the sunlit side of the rings from about 15 degrees above the ring plane, with Cassini at approximately 1.1 million kilometers from the planet and on its final approach.
Titan’s Polar Vortex
Although Cassini is gone, we have vast amounts of data that will continue to generate new discoveries for quite some time, as witness the latest, a paper out of the University Of Bristol that discusses the atmospheric chemistry on Saturn’s largest moon, Titan. Lead author Nick Teanby has been studying Titan’s upper atmosphere, where in the polar regions we are seeing unexpected cooling, a process that differs from what we see on Earth, Venus and Mars.
Indeed, Titan’s polar vortex seems to be extremely cold. What we see on the other worlds is that the high altitude polar atmosphere on a planet’s winter hemisphere is warmed as a result of sinking air heating as it is compressed. It was Cassini’s Composite Infrared Spectrometer (CIRS) instrument that produced the observations Teanby has used to study this anomaly on Titan.
CIRS data showed the expected polar hot-spot beginning to develop in 2009, but temperatures dropped significantly by 2012 and remained as low as 120 K until late 2015, after which the expected hot-spot returned. Teanby explains what’s happening this way:
“For the Earth, Venus, and Mars, the main atmospheric cooling mechanism is infrared radiation emitted by the trace gas CO2 and because CO2 has a long atmospheric lifetime it is well mixed at all atmospheric levels and is hardly affected by atmospheric circulation. However, on Titan, exotic photochemical reactions in the atmosphere produce hydrocarbons such as ethane and acetylene, and nitriles including hydrogen cyanide and cyanoacetylene, which provide the bulk of the cooling.”
Image: Titan’s winter polar vortex imaged by the Cassini Spacecraft’s ISS camera. The vortex is now in deep winter and can only be seen because the polar clouds within the vortex extend high above Titan’s surface into the sunlight. The vortex was extremely cold from 2012-2015, giving rise to unusual nitrile ice clouds. Credit: NASA/JPL-Caltech/Space Science Institute/Jason Major.
Hydrocarbons and nitriles appear high in the atmosphere and are strongly susceptible to vertical atmospheric circulation, meaning that over the southern winter they accumulate in great amounts over the pole, creating the cooling Teanby and team are studying. The work also draws on Cassini data from 2014, when hydrogen cyanide ice clouds were observed over the pole, a reminder not only of Titan’s intriguing chemistry but the continuing vitality of Cassini data.
The paper is Teanby et al., “The formation and evolution of Titan’s winter polar vortex,” published online by Nature Communications 21 November 2017 (full text).
A Thought for the Weekend
From Arthur C. Clarke’s Interplanetary Flight: An Introduction to Astronautics (London: Temple Press Limited, 1960):
There is no way back into the past; the choice, as Wells once said, is the universe-or nothing. Though men and civilizations may yearn for rest, for the dream of the lotus-eaters, that is a desire that merges imperceptibly into death. The challenge of the great spaces between the worlds is a stupendous one; but if we fail to meet it, the story of our race will be drawing to its close.
Email Delivery Problems
Several readers have told me that their email deliveries of Centauri Dreams have not been coming through. This has been an on-again, off-again problem for some time and I’m now trying to switch providers to take care of it. Bear with me, as I hope to have the problem resolved within a few days.
Shards, Axis Ratios and Interstellar Objects
It being the day after the Thanksgiving holiday here in the States, I hadn’t planned a post, but a few more things have cropped up about ‘Oumuamua that I can quickly tuck in here. Now that I’ve learned how to pronounce it (oh MOO-uh MOO-uh), it doesn’t seem nearly as intimidating — it’s the lineup of vowels that trips up the unwary. On the other hand, Jim Benford suggested on Wednesday that we avoid the vowels altogether and call this thing ‘the Shard.’
Here’s the photo Jim sent of the Shard, a 95-story London skyscraper sometimes called The Shard of Glass. It’s 309.7 metres high, the tallest building in the United Kingdom, featuring 11,000 panes of glass with a total surface area of 56,000 square metres. What draws Jim’s attention is the 6 to 1 aspect ratio, with ‘Oumuamua’s thought to be 10 to 1. Jim might also have referenced London’s BT Tower (8 to 1), but what the Guinness Book of World Records calls the “most slender tower” turns out to be the i360 observation tower in Brighton, at a whopping 41 to 1.
With ‘Oumuamua, though, we now have to ask whether the 10 to 1 ratio is actually correct, as Jason Wright noted in a recent post. The problem here is that, unlike the situation with Boyajian’s Star, we have a small dataset to work with, and according to Wright (Penn State), researchers are getting different aspect ratios, ranging all the way from the aforementioned 10 to 1 down to a relatively ordinary 3 to 1.
If the latter is the case, the interstellar object may look something more like Haumea than the Shard. “I’ll need to see a lot more data and hard, critical analysis of the anomalies in ‘Oumuamua before I get interested in the SETI angle at the level I am for Tabby’s Star,” adds Wright.
Greg Laughlin (UC-Santa Cruz) also weighed in on ‘Oumuamua with a new paper (citation below) and accompanying article in Scientific American. Laughlin dubs our visitor “exhilaratingly bizarre” and goes on to describe its unusual arrival, in which, despite accruing 20 kilowatts of energy per square meter at perihelion, it showed no evidence of cometary activity. It was fun to see that Greg also refers to ‘Oumuamua at one point as “a crazily elongated shard.”
But what drew my attention in Greg’s post was how difficult the ejection of debris from a newly forming planetary system seems to be. Getting such a shard free from the host star demands a gravitational assist from a massive planet located at a large distance from the star — terrestrial worlds in our Solar System would fall far short, though the gas giants could manage the feat.
If objects like ‘Oumuamua are common and if they are made predominantly of ice, as we would expect in an outer stellar system, then it implies, says Laughlin, that almost every star in the Milky Way hosts a Neptune-class planet at roughly the distance of our own Neptune from the Sun.
And if it really is rock or metal? Then we deal with another scenario entirely:
…in the highly unlikely event ‘Oumuamua is indeed a refractory slab of rock or metal, as suggested by its complete lack of coma, then its appearance is extremely hard to understand. Only a few percent of stars host planets that are capable of ejecting volatile-free debris from warm regions deep within a gravitational well. They flat-out can’t generate the vast overall swarm implied by ‘Oumuamua’s recent passage, suggesting that another visit by a similar object won’t happen for a very long time.
A long time indeed. According to Laughlin’s calculations of that scenario, ‘Oumuamua could travel for 10 quadrillion years before coming into similar proximity to another star.
At that far distant time, the galaxy will be a very different place, in which all the stars that now shine warmly down on planets will be expired white dwarfs, warmed a few degrees above absolute zero by the flicker of proton decay.
The paper is Laughlin, G. & Batygin, K. (2017) “On the Consequences of the Detection of an Interstellar Asteroid,” submitted to Research Notes of the AAS (abstract).
`Oumuamua: Listening To An Interstellar Interloper
The buzz about `Oumuamua, our first known visitor from another stellar system, seems likely to continue given yesterday’s news that the object’s axis ratio is a startling 10 to 1. Given all that, Jim Benford wondered whether there were SETI implications here. Was anyone on the case from our major SETI organizations? The answer is below, as we learn that the effort is ongoing. A frequent contributor to these pages, Jim is President of Microwave Sciences in Lafayette, California, which deals with high power microwave systems from conceptual designs to hardware. He also heads up the critical sail subcommittee for Breakthrough Starshot, the effort to send small beamed sails with miniaturized payloads to a nearby star.
By James Benford
I contacted Jill Tarter and Andrew Siemion about whether SETI researchers are conducting observations of the interstellar interloper, Oumuamua. Both say yes.
Jill said that the Allen Telescope Array has been looking at it for a while. Andrew said that Breakthrough Listen was using the Green Bank Telescope for a few hours last weekend. This was actually looking for water via hydroxyl lines using broadband 1.1-1.9 GHz data. No water was immediately evident in the coarse spectra from the standard data reduction. Breakthrough Listen is working on incorporating the appropriate windowing capabilities necessary to analyze this data, so as to use their data analysis pipeline.
Therefore there are some observations in parts of the microwave spectrum.
Image: This diagram shows the orbit of the interstellar asteroid ‘Oumuamua as it passes through the Solar System. Unlike all other asteroids and comets observed before, this body is not bound by gravity to the Sun. It has come from interstellar space and will return there after its brief encounter with our star system. Its hyperbolic orbit is highly inclined and it does not appear to have come close to any other Solar System body on its way in. Credit: ESO/K. Meech et al.
Besides astronomical observations of this unique object, there is also this remote possibility: That this interloper is an interstellar survey probe, having perhaps dropped down to interplanetary-scale velocities in order to take data during its transit of our solar system, before going on to another star.
If this is the case, then perhaps we ought to be looking rather broadly in the electromagnetic spectrum for any signal it might send to us, having easily detected leakage from Earth. That assumes it would try to respond to us using frequencies it knows we use. That would certainly include the microwave bands 1-10 GHz, where most of our radiation leakage radiation is.
I think at present the frequencies most observable coming from Earth are leakage of uplink transmissions to our satellites, of which there are now about 1200 active in orbit. Those frequencies tend to be in the upper end of the microwave where the wavelength is smaller, so we can use smaller apertures on both Earth and satellite. Downlinks, of course, would be absorbed in the Earth and not observable from afar.
Or, because they know enough about our atmosphere’s transmission windows and the Sun’s radiation spectrum, they might be signaling in the visible. Therefore our SETI optical observatories ought to be watching as well.
I would look for a pulsed beacon signal, which is more noticeable. That would be like a pulsar, but of course with no interstellar dispersion.
This matter has a very low probability of success, of course. However, it’s our first opportunity to observe at close range a truly interstellar object.
Image: This plot shows how the interstellar asteroid `Oumuamua varied in brightness during three days in October 2017. The large range of brightness — about a factor of ten (2.5 magnitudes) — is due to the very elongated shape of this unique object, which rotates every 7.3 hours. The different coloured dots represent measurements through different filters, covering the visible and near-infrared part of the spectrum. The dotted line shows the light curve expected if `Oumuamua were an ellipsoid with a 1:10 aspect ratio, the deviations from this line are probably due to irregularities in the object’s shape or surface albedo. Credit: ESO/K. Meech et al.
Follow with RSS or E-Mail
