Posted on by Jon Voisey

Plausibility Check – Habitable Planets around Red Giants

Betelgeuse is a red giant star easily visible in our night sky. Betelgeuse is actally a red super-giant, meaning it has enough mass that it will end as a supernova, rather than as a white dwarf with a planetary nebula. Image credit: Hubble Space Telescope
Betelgeuse is a red super-giant, meaning it has enough mass that it will end as a supernova, rather than as a white dwarf with a planetary nebula. New research suggests that the star could've consumed a smaller companion star. Image credit: Hubble Space Telescope

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While planets orbiting twin stars are a staple of science fiction, another is having humans live on planets orbiting red giant stars. The majority of the story of Planet of the Apes takes place on a planet around Betelgeuse. Planets around Arcturus in Isaac Asimov’s Foundation series make up the capital of his Sirius Sector. Superman’s home planet was said to orbit a the fictional red giant, Rao. Races on these planets are often depicted as being old and wise since their stars are aged, and nearing the end of their lives. But is it really plausible to have such planets?

Stars don’t last forever. Our own Sun has an expiration date in about 5 billion years. At that time, the amount of hydrogen fuel in the core of the Sun will have run out. Currently, the fusion of that hydrogen into helium is giving rise to a pressure which keeps the star from collapsing in on itself due to gravity. But, when it runs out, that support mechanism will be gone and the Sun will start to shrink. This shrinking causes the star to heat up again, increasing the temperature until a shell of hydrogen around the now exhausted core becomes hot enough to take up the job of the core and begins fusing hydrogen to helium. This new energy source pushes the outer layers of the star back out causing it to swell to thousands of times its previous size. Meanwhile, the hotter temperature to ignite this form of fusion will mean that the star will give off 1,000 to 10,000 times as much light overall, but since this energy is spread out over such a large surface area, the star will appear red, hence the name.

So this is a red giant: A dying star that is swollen up and very bright.

Now to take a look at the other half of the equation, namely, what determines the habitability of a planet? Since these sci-fi stories inevitably have humans walking around on the surface, there’s some pretty strict criteria this will have to follow.

First off, the temperature must be not to hot and not to cold. In other words, the planet must be in the Habitable zone also known as the “Goldilocks zone”. This is generally a pretty good sized swath of celestial real estate. In our own solar system, it extends from roughly the orbit of Venus to the orbit of Mars. But what makes Mars and Venus inhospitable and Earth relatively cozy is our atmosphere. Unlike Mars, it’s thick enough to keep much of the heat we receive from the sun, but not too much of it like Venus.

This diagram shows the distances of the planets in the Solar System (upper row) and in the Gliese 581 system (lower row), from their respective stars (left). The habitable zone is indicated as the blue area, showing that Gliese 581 d is located inside the habitable zone around its low-mass red star. Based on a diagram by Franck Selsis, Univ. of Bordeaux. Credit: ESO

The atmosphere is crucial in other ways too. Obviously it’s what the intrepid explorers are going to be breathing. If there’s too much CO2, it’s not only going to trap too much heat, but make it hard to breathe. Also, CO2 doesn’t block UV light from the Sun and cancer rates would go up. So we need an oxygen rich atmosphere, but not too oxygen rich or there won’t be enough greenhouse gasses to keep the planet warm.

The problem here is that oxygen rich atmospheres just don’t exist without some assistance. Oxygen is actually very reactive. It likes to form bonds, making it unavailable to be free in the atmosphere like we want. It forms things like H2O, CO2, oxides, etc… This is why Mars and Venus have virtually no free oxygen in their atmospheres. What little they do comes from UV light striking the atmosphere and causing the bonded forms to disassociate, temporarily freeing the oxygen.

Earth only has as much free oxygen as it does because of photosynthesis. This gives us another criteria that we’ll need to determine habitability: the ability to produce photosynthesis.

So let’s start putting this all together.

Firstly, the evolution of the star as it leaves the main sequence, swelling up as it becomes a red giant and getting brighter and hotter will mean that the “Goldilocks zone” will be sweeping outwards. Planets that were formerly habitable like the Earth will be roasted if they aren’t entirely swallowed by the Sun as it grows. Instead, the habitable zone will be further out, more where Jupiter is now.

However, even if a planet were in this new habitable zone, this doesn’t mean its habitable under the condition that it also have an oxygen rich atmosphere. For that, we need to convert the atmosphere from an oxygen starved one, to an oxygen rich one via photosynthesis.

So the question is how quickly can this occur? Too slow and the habitable zone may have already swept by or the star may have run out of hydrogen in the shell and started contracting again only to ignite helium fusion in the core, once again freezing the planet.

The only example we have so far is on our own planet. For the first three billion years of life, there was little free oxygen until photosynthetic organisms arose and started converting it to levels near that of today. However, this process took several hundred million years. While this could probably be increased by an order of magnitude to tens of millions of years with genetically engineered bacteria seeded on the planet, we still need to make sure the timescales will work out.

It turns out the timescales will be different for different masses of stars. More massive stars burn through their fuel faster and will thus be shorter. For stars like the Sun, the red giant phase can last about 1.5 billion years, so ~100x longer than is necessary to develop an oxygen rich atmosphere. For stars twice as massive as the Sun, that timescale drops to a mere 40 million years, approaching the lower limit of what we’ll need. More massive stars will evolve even more quickly. So for this to be plausible, we’ll need lower mass stars that evolve slower. A rough upper limit here would be a two solar mass star.

However, there’s one more effect we need to worry about: Can we have enough CO2 in the atmosphere to even have photosynthesis? While not nearly as reactive as oxygen, carbon dioxide is also subject to being removed from the atmosphere. This is due to effects like silicate weathering such as CO2 + CaSiO3 –> CaCO3 + SiO2. While these effects are slow they build up with geological timescales. This means we can’t have old planets since they would have had all their free CO2 locked away into the surface. This balance was explored in a paper published in 2009 and determined that, for an Earth mass planet, the free CO2 would be exhausted long before the parent star even reached the red giant phase!

So we’re required to have low mass stars that evolve slowly to have enough time to develop the right atmosphere, but if they evolve that slowly, then there’s not enough CO2 left to get the atmosphere anyway! We’re stuck with a real Catch 22. The only way to make this feasible again is to find a way to introduce sufficient amounts of new CO2 into the atmosphere just as the habitable zone starts sweeping by.

Fortunately, there are some pretty large repositories of CO2 just flying around! Comets are composed mostly of frozen carbon monoxide and carbon dioxide. Crashing a few of them into a planet would introduce sufficient CO2 to potentially get photosynthesis started (once the dust settled down). Do that a few hundred thousand years before the planet would enter the habitable zone, wait ten million years, and then the planet could potentially be habitable for as much as an additional billion years more.

Ultimately this scenario would be plausible, but not exactly a good personal investment since you’d be dead long before you’d be able to reap the benefits. A long term strategy for the survival of a space faring species perhaps, but not a quick fix to toss down colonies and outposts.

Posted on by Jon Voisey

First Super-Earth Atmosphere Observed

Artist’s impression of GJ 1214b
Artist’s impression of GJ 1214b

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With the recent milestone of the discovery of the 500th extra solar planet the future of planetary astronomy is promising. As the number of known planets increases so does our knowledge. With the addition of observations of atmospheres of transiting planets, astronomers are gaining a fuller picture of how planets form and live.

Thus far, the observations of atmospheres have been limited to the “Hot-Jupiter” type of planets which often puff up, extending their atmospheres and making them easier to observe. However, a recent set of observations, to be published in the December 2nd issue of Nature, have pushed the lower limit and extended observations of exoplanetary atmospheres to a super-Earth.

The planet in question, GJ 1214b passes in front of its parent star when viewed from Earth allowing for minor eclipses which help astronomers determine features of the system such as its radius and also its density. Earlier work, published in the Astrophysical Journal in August of this year, noted that the planet had an unusually low density (1.87 g/cm3). This ruled out an entirely rocky or iron based planet as well as even a giant snowball composed entirely of water ice. The conclusion was that the planet was surrounded by a thick gaseous atmosphere and the three possible atmospheres were proposed that could satisfy the observations.

The first was that the atmosphere was accreted directly from the protoplanetary nebula during formation. In this instance, the atmosphere would likely retain much of the primordial composition of hydrogen and helium since the mass would be sufficient to keep it from escaping readily. The second was that the planet itself is composed mostly of ices of water, carbon dioxide, carbon monoxide and other compounds. If such a planet formed, sublimation could result in the formation of an atmosphere that would be unable to escape. Lastly, if a strong component of rocky material formed the planet, outgassings could produce an atmosphere of water steam from geysers, as well as carbon monoxide and carbon dioxide and other gasses.

The challenge for following astronomers would be to match the spectra of the atmosphere to one of these models, or possibly a new one. The new team is composed of Jacob Bean, Eliza Kempton, and Derek Homeier, working from the University of Göttingen and the University of California, Santa Cruz. Their spectra of the planet’s atmosphere was largely featureless, showing no strong absorption lines. This largely rules out the first of the cases in which the atmosphere is mostly hydrogen unless there is a thick layer of clouds obscuring the signal from it. However, the team notes that this finding is consistent with an atmosphere composed largely of vapors from ices. The authors are careful to note that “the planet would not harbor any liquid water due to the high temperatures present throughout its atmosphere.”

These findings don’t conclusively demonstrate that nature of the atmosphere, but narrow down the degeneracy to either a steam filled atmosphere or one with thick clouds and haze. Despite not completely narrowing down the possibilities, Bean notes that the application of transit spectroscopy to a super-Earth has “reached a real milestone on the road toward characterizing these worlds.” For further study, Bean suggests that “[f]ollow-up observations in longer wavelength infrared light are now needed to determine which of these atmospheres exists on GJ 1214b.”

Posted on by Jon Voisey

Missing Molecules in Exoplanet Atmospheres

Artist's View of Extrasolar Planet HD 189733b

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Every day, I wake up and flip through the titles and abstracts of recent articles posted to arXiv. With increasing regularity, papers pop up announcing the discovery of a new extra-solar planet. At this point, I keep scrolling. How many more hot Jupiters do you really want to hear about? If it’s a record setter in some way, I’ll read it. Another way I’ll pay attention is if there’s reports of detections of spectroscopic detection of components of the atmosphere. While a fistful of transiting planets have had spectral lines discovered, they’re still pretty rare and new discoveries will help constrain our understanding of how planets form.

The holy grail in this field would be to discover elemental signatures of molecules that don’t form naturally and are characteristic of life (as we know it). In 2008, a paper announced the first detection of CO2 in an exoplanet atmosphere (that of HD 189733b), which, although not exclusively, is one of the tracer molecules for life. While HD 189733b isn’t a candidate for searches for ET, it was still a notable first.

Then again, perhaps not. A new study casts doubt on the discovery as well as the report of various molecules in the atmospheres of another exoplanet.

Thus far there have been two methods by which astronomers have attempted to identify molecular species in the atmosphere of exoplanets. The first is by using starlight, filtered by the planet’s atmosphere to search for spectral lines that are only present during transit. The difficulty with this method is that, spreading the light out to detect the spectra weakens the signal, sometimes down to the very point that it’s lost in systematic noise from the telescope itself. The alternative is to use photometric observations, which look at the change in light in different color ranges, to characterize the molecules. Since the ranges are all lumped together, this can improve the signal, but this is a relatively new technique and statistical methodology for this technique is still shaky. Additionally, since only one filter can be used at a time, the observations must generally be taken on different transits, which allow the characteristics of the star to change due to star spots.

The 2008 study by Swain et al. that announced the presence of CO2 used the first of these methods. Their trouble started the following year when a followup study by Sing et al. failed to reproduce the results. In their paper, Sing’s team stated,”Either the planet’s transmission spectrum is variable, or residual systematic errors still plague the edges of the Swain et al. spectrum.”

The new study, by Gibson, Pont, and Aigrain (working from the Universities of Oxford and Exeter) suggests that the claims of Swain’s team were a result of the latter. They suggest that the signal is swamped with more noise than Swain et al. accounted for. This noise comes from the telescope itself (in this case Hubble since these observations would need to be made out of Earth’s atmosphere which would add its own spectral signature). Specifically, they report that since there’s changes in the state of the detector itself that are often hard to identify and correct for, Swain’s team underestimated the error, leading to a false positive. Gibson’s team was able to reproduce the results using Swain’s method, but when they applied a more complete method which didn’t assume that the detector could be calibrated so easily by using observations of the star outside the transit and on different Hubble orbits, the estimation of the errors increased significantly, swamping the signal Swain claimed to have observed.

Gibson’s team also reviewed the case of detections of molecules in the atmosphere of an extra solar planet around XO-1 (on which Tinetti et al. reported to have found methane, water, and CO2). In both cases, they again find that detections of were overstated and the ability to tease signal from the data was dependent on questionable methods.

This week seems to be a bad week for those hoping to find life on extra-solar planets. With this article casting doubt on our ability to detect molecules in distant atmospheres and the recent caution on the detection of Gliese 581g, one might worry about our ability to explore these new frontiers, but what this really underscores is the need to refine our techniques and keep taking deeper looks. This has been a frank reassessment of the current state of knowledge, but does not in any way claim to limit our future discoveries. Additionally, this is how science works; scientists review each others data and conclusions. So, looking on the bright side, science works, even if it’s not exactly telling us what we’d like to hear.

Posted on by Ryan Anderson

Extrasolar Volcanoes May Soon be Detectable

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We’ve all seen pictures of erupting terrestrial volcanoes from space, and even eruptions on Jupiter’s moon Io in the outer solar system, but would it be possible to detect an erupting volcano on an exoplanet? Astronomers say the answer is yes! (with a few caveats)

It’s going to be decades before telescopes will be able to resolve even the crudest surface features of rocky extrasolar planets, so don’t hold your breath for stunning photos of alien volcanoes outside our solar system. But astronomers have already been able to use spectroscopy to detect the composition of exoplanet atmospheres, and a group of theorists at the Harvard-Smithsonian Center for Astrophysics think a similar technique could detect the atmospheric signature of exo-eruptions.

By collecting spectra right before and right after the planet goes behind its star, astronomers can subtract out the star’s spectrum and isolate the signal from the planet’s atmosphere. Once this is done, they can look for evidence of molecules common in volcanic eruptions. Models suggest that sulfur dioxide is the best candidate for detection because volcanoes produce it in huge quantities and it lasts in a planet’s atmosphere for a long time.

Still, it won’t be easy.

“You would need something truly earthshaking, an eruption that dumped a lot of gases into the atmosphere,” said Smithsonian astronomer Lisa Kaltenegger. “Using the James Webb Space Telescope, we could spot an eruption 10 to 100 times the size of Pinatubo for the closest stars,” she added.

To be detected, exoplanet eruptions would have to be 10 to 100 times larger than the 1991 eruption of Mt. Pinatubo shown here. Image source: USGS

In 1991 Mount Pinatubo in the Philippines belched 17 million tons of sulfur dioxide into the stratosphere. Volcanic eruptions are ranked using the Volcanic Explosivity Index (VEI). Pinatubo ranked ‘colossal’ (VEI of 6) and the largest eruption in recorded history was the ‘super-colossal’ Tambora event in 1815. With a VEI of 7 it was about 10 times as large as Pinatubo. Even larger eruptions (more than 100 times larger than Pinatubo) on Earth are not unheard of: geologic evidence suggests that there have been 47 such eruptions in the past 36 million years, including the eruption of the Yellowstone caldera about 600,000 years ago.

The best candidates for detecting extrasolar volcanoes are super-earths orbiting nearby, dim stars, but the Kaltenegger and her colleagues found that volcanic gases on any earth-like planet up to 30 light years away might be detectable. Now they just have to wait until the James Webb Space Telescope is launched 2014 to test their prediction.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Exoplanet Weather Report

Trying to determine the behaviour of the atmosphere of a hot Jupiter – a gas giant so close to its star that it is either tidally locked or caught in a slow orbital resonance – is tricky, given that we have no precedents here in our solar system. But it is possible to explore in detail what exoplanet atmospheres might be like, based on solar system examples.

For example, there’s Venus – which, although not tidally locked, has such a slow rotation (once every 243 Earth days) that its dynamics virtually match those of a tidally locked planet.

Interestingly, Venus’ upper atmosphere super-rotates, meaning it circulates in the same direction as the planet’s rotation but much faster – in Venus’ case, at sixty times the speed of the planet’s rotation. It’s likely that these winds are driven by the large temperature gradient that exists between the day and night sides of the planet.

Conversely Earth, with its rapid rotation, has much less potential difference between its day and night side temperatures – so that its weather systems are more strongly influenced by the actual rotation of the planet and also by the temperature gradient between equator and pole. The nett result is lots of circular weather systems with their direction determined by the Coriolis effect – counter-clockwise in the northern hemisphere and clockwise in the southern.

And of course we do have gas giants, even if they aren’t hot. Being so far from the Sun, dayside-nightside and equator-pole temperature gradients have little influence on our gas giants’ atmospheric circulation. The most significant issues are each planet’s rotation speed and each planet’s size.

Jupiter and Saturn’s larger radius exceeds their Rhines scale forcing the bulk flow of their atmospheres to break up into distinct bands with turbulent eddies between them. However, the smaller radius of Uranus and Neptune allows the bulk of the atmosphere to circulate as an unbroken whole, only breaking into two smaller bands at each pole.

The 'Rhines Scale' applied to solar system gas giants predicts that atmospheric circulation on large radius planets (Jupiter and Saturn) fragments into distinct bands, but doesn't on smaller radius planets (Uranus and Neptune). Credit: Showman et al 2010.

Partly because it’s cooler, but mostly because it’s smaller, Neptune’s atmosphere has much less turbulent flow than Jupiter – which goes some way to explaining why it has the fastest stratospheric wind speeds in the solar system.

All these factors are useful in trying to determine how the atmosphere of a hot Jupiter might behave. Being so close to their star, it’s likely these planets will be partly or fully tidally locked – so the main driver for atmospheric circulation will be, like Venus, the dayside-nightside temperature gradient . So a super-rotating stratosphere, circulating many times faster than the inner parts of the planet, is plausible.

From there, modelling suggests that the combination of fast wind speed and slow rotation means the Rhines scale will become bigger than a Jupiter-sized planetary radius , so there will be less turbulent flow and the upper atmosphere might circulate as one, without breaking up into the multiple bands we see on Jupiter.

Anyway, that’s my take on an interesting 50 page arXiv article with lots of (to me) bewildering formulae, but also lots of comprehensible narrative and diagrams. The article consolidates current thinking and lays a sound foundation for making sense of future observational data – both hallmarks of a nicely crafted ‘lit review’.