Galactic Habitability and Sgr A*

Yesterday I looked at evidence for oxygen in a galaxy so distant that we are seeing it as it was a mere 500 million years after the Big Bang. It’s an intriguing find, because that means there was an even earlier generation of stars that lived and died, seeding the cosmos with elements heavier than hydrogen and helium. It’s hard to imagine the vast tracts of time since populated with stars and, inevitably, planets without speculating on where and when life developed.

But as we continue to speculate, we should also look at the factors that could shape emerging life in galaxies like our own. Tying in neatly with yesterday’s post comes a paper from Amedeo Balbi (Università degli Studi di Roma “Tor Vergata”), working with colleague Francesco Tombesi. The authors are interested in questions of habitability not in terms of habitable zones in stellar systems but rather habitable zones in entire galaxies. For we know that at the center of our Milky Way lurks the supermassive black hole Sgr A*, whose effects must be considered.

Such black holes are known to produce vast amounts of ionizing radiation in the highly visible form of quasars or active galactic nuclei (AGN). The atmospheric loss and biological damage inflicted on a rocky planet as it is exposed to intense X-ray and extreme ultraviolet radiation can be extreme, and such conditions would have marked our own galaxy’s AGN phase.

The concern here is to examine how ionizing radiation can impact habitability by exposing planetary surfaces to high-energy fluxes, while also degrading planetary atmospheres. We’ve looked at these issues now and again on this site, with particular regard to red dwarf stars, the most common class of star in the galaxy but also a type prone to flare activity particularly when young. We now consider whether there are regions in our galaxy that would be less likely to be habitable because of the effects of Sgr A*, which was not always as quiet as it is now.

Image: Centaurus A is one of the active galactic nuclei closest to Earth. It emits strong radio emission and produces a relativistic jet. Credit: ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray).

AGNs represent a class of galaxies that appear in a wide range of shapes and spectral features. Some of that spectral variation may, according to one theory, involve our viewing angle and the obscuring effects of dust. The peak of Sgr A*’s active phase is thought to have occurred less than 8 billion years ago and to have lasted between 107 and 109 years. The paper examines how this activity would affect the habitability of the Milky Way.

Image: An artist impression of the quasar ULAS J1120+0641. Credit: ESO/M. Kornmesser.

The researchers compared the XUV flux at various distances from galactic center to the dosage that would prove lethal for organisms on Earth, producing ‘critical fluxes’ for complex life as well as for prokaryotes (some radiation-resistant terrestrial prokaryotes can survive high radiation doses). How far from Sgr A* would a planet have to be to be exposed to a critical flux?

The issue is complicated by the possible response of local organisms, which might evolve to cope with increased radiation under varying environmental conditions, and it is also true that radiation doses lower than lethality could spur biological mutations. But given what they assume to be plausible values for a lethal absorbed dose of ionizing radiation, the authors believe that complex life would have been in jeopardy during Sgr A*’s peak active phase at distances as large as 10 kpc [32,600 light years] from galactic center. By comparison, the Sun lies about 8 kpc from the center of the Milky Way, having formed well after the Sgr A* peak.

From the paper:

This may not have prevented the appearance of life per se, since prokaryotes could have survived to higher fluxes. However, we point out that the biological effect of the ionizing radiation from Sgr A* would have been in addition to that of any other source of ionizing radiation, for example from the host star, and would add to the loss of a large fraction of the atmosphere. Even if some organisms might have survived by developing radiation-resistance or finding protected niches, the global effect on the biosphere would have been significant.

Let me circle back around to the question of atmosphere loss, also critical in assessing life’s chances in the era of Sgr A* peak activity. Assuming that the torus of the AGN would have been co-aligned with the galactic plane, the authors produce the chart below, showing the total amount of atmospheric mass lost at the end of the AGN phase of Sgr A* by a planet with the same density as Earth, as a function of the distance from the galactic center.

Image: This is Figure 1 from the paper. Caption: The total mass lost at the end of the AGN phase of Sgr A* by a terrestrial planet at distance D from the galactic center, in units of the atmosphere mass of present day Earth. Each curve was computed assuming a value for the efficiency of hydrodynamic escape of either ??=?0.1 or ??=?0.6. An optical depth ??=?1 corresponds to locations close to the galactic plane (maximum attenuation by the AGN torus) while ??=?0 corresponds to high galactic latitudes (no attenuation). Credit: Amedeo and Tombesi.

The conclusion is stark: Rocky planets in the galactic bulge would have been exposed to enough XUV radiation during peak Sgr A* activity to lose a significant fraction of their atmosphere. The mass loss for distances from the galactic plane of 0.5 kpc or less could be comparable to the atmosphere of Earth today. It would take significant volcanism and outgassing to regenerate an atmosphere sufficiently to repair such a loss.

…our results imply that the inner region of the Milky Way might have remained uninhabitable until the end of the AGN phase of its central black hole, and possibly thereafter. This has important consequences in assessing the likelihood of ancient life in the Galaxy, and should be taken into account in future studies of the Galactic habitable zone. It also suggests further investigations on the relation between supermassive black holes in galactic cores and planetary habitability.

The paper is Balbi and Tombesi, “The habitability of the Milky Way during the active phase of its central supermassive black hole,” Scientific Reports 7, article #: 16626 (2017). Full text.


Star Formation at ‘Cosmic Dawn’

When life first arose in the universe is a question to which we have no answer. A key problem here is that without knowing how rare — or common — life’s emergence is, we can’t draw conclusions about where (or when) to find it. One thing that is accessible to us, though, is information about when stars began the process of producing the elements beyond hydrogen and helium that are constituents of our own living systems. And on that score, we have interesting news from an international team of scientists about extremely old galaxies.

Led by Takuya Hashimoto and Akio Inoue (Osaka Sangyo University), the researchers have gone to work on a galaxy known as MACS1149-JD1, using data acquired from the Atacama Large Millimetre/Submillimetre Array (ALMA) and the European Southern Observatory’s Very Large Telescope (VLT). The team’s paper in Nature confirms that the galaxy is some 13.28 billion light years away. Thus we see it as it appeared when the universe was about 500 million years old. Another galaxy, GN-z11, has been measured at 13.4 billion light years using data from the Hubble Space Telescope.

But the Hubble data offer a less precise measurement than Hashimoto and Inoue found, because the distance to MACS1149-JD1 is based on two independent emission lines from atoms of hydrogen and oxygen, and therein lies a tale. This is the most distant known source of oxygen ever detected. It implies that this galaxy contains stars that formed using elements produced in a still earlier generation of stars whose death produced heavier elements. Here’s what second author Nicolas Laporte (University College London) has to say on this:

“This is an exciting discovery as this galaxy is seen at a time when the Universe was only 500 million years old and yet it already has a population of mature stars. We are therefore able to use this galaxy to probe into an earlier, completely uncharted, period of cosmic history.”

Image: Galaxy MACS1149-JD1 located 13.28 billion light-years away imaged with the NASA/ESA Hubble Space Telescope. The first zoom shows how the galaxy was observed with the ESO VLT centred on a rectangular slit. The final zoom shows the Hubble image with contours of ionized oxygen detected by ALMA. Credit: ALMA (ESO/NAOJ/NRAO), NASA/ESA Hubble Space Telescope, Hashimoto et al.

But let’s pause for a moment to remind ourselves of how tricky it is to calculate distances for objects at the edge of the visible universe. Here I’m quoting from the National Astronomical Observatory of Japan’s website on the matter:

…the distances to astronomical object beyond about a couple billion light-years are estimated using redshift — a change in the wavelength of light from the object. The relation between the redshift and the distance depends on the history of the Universe: how fast (or slow) the Universe has been expanding. This is determined by models (based on physics) and certain characteristic parameters. These parameters are still being determined, so their estimated values change from year to year. Also, there are different ways to define distances (co-moving distances, luminosity distances, …), which give different values. Because of the complexity, sometimes different astronomers find different distances for the same object.

With that in mind, we can proceed to Laporte, who detected hydrogen emissions from MACS1149-JD1 using the Very Large Telescope, producing an independent confirmation of the distance Hashimoto and Inoue had deduced from their ALMA data (both using, in NAOJ’s words, ‘cosmological parameters given in the most recent published results by the major collaborations’). In the ALMA work, the signal of ionized oxygen had been stretched from the infrared down to microwave wavelengths by the continuing expansion of spacetime.

Both Laporte and the Hashimoto/Inoue team have run up a history of detecting the most distant known sources, with Laporte responsible for an earlier detection of oxygen at 13.2 billion light years. What we wind up with the new paper is evidence for star formation at an even earlier period, some 250 million years after the Big Bang, meaning that galaxies existed at times earlier than we can currently detect them. Co-author Richard Ellis (University College London) notes the implications for life:

“Determining when cosmic dawn occurred is akin to the `Holy Grail’ of cosmology and galaxy formation. With MACS1149-JD1, we have managed to probe history beyond the limits of when we can actually detect galaxies with current facilities. There is renewed optimism we are getting closer and closer to witnessing directly the birth of starlight. Since we are all made of processed stellar material, this is really finding our own origins.”

Are these likewise the origins of other living things in the cosmos? My assumption is that the answer is yes, but without confirmatory data, that thought can be no more than a speculation.

The paper is Hashimoto et al., “The onset of star formation 250 million years after the Big Bang,” Nature 557 (2018), 392-395 (abstract).


Pluto: A Cometary Formation Model

The ongoing work of mining New Horizons’ abundant data from the outer system continues at a brisk pace. But missions occur in context, and we also have discoveries made at comet 67P/Churyumov-Gerasimenko by the European Space Agency’s Rosetta probe to bring to bear. The question that occupies Christopher Glein and Hunter Waite (both at SwRI) is how to explain the chemistry New Horizons found at Pluto and what it can tell us about Pluto’s formation.

At the heart of their new paper in Icarus is the question of Pluto’s molecular nitrogen (N2), which plays a role on that world similar to methane on Titan, water on Earth and CO2 on Mars. All are volatiles, meaning they can move between gaseous and condensed forms at the temperature of the planet in question. We’ve learned that solid N2 is the most abundant surface ice visible to spectroscopy on Pluto, as witness the spectacular example of Sputnik Planitia.

Image: NASA’s New Horizons spacecraft captured this image of Sputnik Planitia — a glacial expanse rich in nitrogen, carbon monoxide and methane ices — that forms the left lobe of a heart-shaped feature on Pluto’s surface. SwRI scientists studied the dwarf planet’s nitrogen and carbon monoxide composition to develop a new theory for its formation. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Anyone glued to their screen during New Horizons’ 2015 flyby will recall the surprise generated by Sputnik Planitia’s youthful terrain, now believed to be the result of the flow of solid N2. Scientists also found that nitrogen can sublimate at Pluto’s surface, so we have a volatile cycle that causes surface pitting and frosts, one that likewise accounts for the existence of Pluto’s atmosphere. Given the importance of nitrogen, the issue of its origin looms large because of what it can tell us about Pluto’s formation, and by extension that of other outer system objects.

Here the crossover with the Rosetta data is helpful. What Glein and Waite are suggesting is that Pluto may owe its existence to comets, accumulating the N2 observed by New Horizons through accretion. Says Glein, with a nod to the nitrogen-rich ice of Sputnik Planitia:

“We’ve developed what we call ‘the giant comet’ cosmochemical model of Pluto formation. We found an intriguing consistency between the estimated amount of nitrogen inside the glacier and the amount that would be expected if Pluto was formed by the agglomeration of roughly a billion comets or other Kuiper Belt objects similar in chemical composition to 67P, the comet explored by Rosetta.”

Image: New Horizons not only showed humanity what Pluto looks like, but also provided information on the composition of Pluto’s atmosphere and surface. These maps — assembled using data from the Ralph instrument — indicate regions rich in methane (CH4), nitrogen (N2), carbon monoxide (CO) and water (H2O) ices. Sputnik Planitia shows an especially strong signature of nitrogen near the equator. SwRI scientists combined these data with Rosetta’s comet 67P data to develop a proposed “giant comet” model for Pluto formation. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Whether Pluto formed from the accretion of cometary ices or from materials with a chemical composition closer to that of the solar nebula remains an open question, with the paper addressing both possibilities and finding both to be consistent with the data. Tangled up with the question is the issue of carbon monoxide (CO) and the low CO/N2 ratios observed at Pluto. The problem is that these ratios should be much higher than what we find in Pluto’s atmosphere. How to explain the missing carbon monoxide?

There are implications here for what may still lie beneath the icy world’s surface. From the paper:

…we have performed aqueous geochemical calculations showing the great thermodynamic instability of CO dissolved in cold (liquid) water, even for a restricted metastable equilibrium system… The destruction of CO to formate or carbonate species… would be strongly favored if Pluto has or had a subsurface ocean. This mechanism can be applied to the cometary model, but not to the solar model as the CO/H2O ratio is too large in the latter. Hence, the cometary model seems preferable, with more options to reconcile the CO/N2 ratio.

And implications for Pluto’s continuing surface activity are also present:

We note that the burial and aqueous destruction hypotheses for missing CO are not necessarily mutually exclusive. A major implication of these processes is that the observed composition of Pluto cannot be completely primitive, even if its N2 is indeed primordial. This resonates with the dynamic geology seen by New Horizons (Moore et al., 2016).

Numerous questions remain to be answered, but the authors propose an evolutionary scenario that begins with Pluto accumulating cometary nitrogen and CO, with subsequent interactions accounting for loss of the CO and outgassing of N2. This takes into account the surface accumulation of the latter at Sputnik Planitia and cometary ‘resupply’ of a small amount of CO that mixes with surface nitrogen. This is one of various possible scenarios, but it is consistent with the data the paper examines and points to future work.

Among many questions remaining, the issue of how much N2 might be inside Pluto looms large, as does its distribution in the crust, rocky core and possible liquid water ocean. We’d also like to know whether the abundance of N2 found at comet 67P is representative of the cometary population at large and of larger icy Kuiper Belt Objects.

And among ten questions posed by the authors in their conclusion, this one is significant: Why do we lack any detection of CO2 on Pluto while finding it on Triton, and is there a consistent way of resolving the question with regard to the lack of CO on both bodies? This gets us into the broad issue of the mechanisms that provide volatiles to worlds like these. Check the paper’s conclusion for a useful wrap-up of future directions to be taken with the New Horizons data, and the areas where new data will be required.

The paper is Glein and Waite, “Primordial N2 provides a cosmochemical explanation for the existence of Sputnik Planitia, Pluto,” Icarus Vol. 313 (October, 2018), pp. 79-92 (abstract / preprint).


Administrative Item

Some of you may have noticed a blip in the comments moderation over the past 24 hours. I think all messages have now come through, but a software upgrade on my server is the culprit. Things seem to have gone back to normal now.

On an unrelated matter, I won’t be able to get off a post today or tomorrow. On Tuesday, I’ll have some interesting information about Breakthrough Starshot. [I had originally promised this for Monday, having forgotten about the US holiday].


TESS: The View into the Galactic Plane

I want to be sure to get the first image from TESS, the Transiting Exoplanet Survey Satellite, into Centauri Dreams, given the importance of the mission and the high hopes riding on it as the next step in exoplanet exploration. Now we move from the Kepler statistical survey methodology to a look at bright, nearby stars, and plenty of them. TESS will cover an area of sky far larger than the amount of sky we see in this image, which looks out along the plane of the galaxy from a perspective that matches southern skies on Earth.

Image: This test image from one of the four cameras aboard the Transiting Exoplanet Survey Satellite (TESS) captures a swath of the southern sky along the plane of our galaxy. TESS is expected to cover more than 400 times the amount of sky shown in this image when using all four of its cameras during science operations. Credits: NASA/MIT/TESS.

Showing some 200,000 stars, the image is centered on the southern constellation Centaurus, with a bit of the Coalsack Nebula at the upper right and, if you look along the bottom edge, the bright star Beta Centauri readily visible. Here I add the usual caution that Beta Centauri has nothing to do with Alpha Centauri — it is a separate place and interesting in its own right. About 400 light years out, this is, like Alpha Centauri, a triple system, and with Alpha Centauri, it serves as one of the pointer stars to the lovely asterism known as the Southern Cross.

While a triple system, Beta Centauri’s component stars are nothing like the G-class Centauri A, K-class Centauri B and M-dwarf Proxima Centauri. At Beta Centauri we have a close binary consisting of two B-class stars orbiting each other over a period of 357 days, both stars thought to be variables now evolving off the main sequence. Orbiting the binary is Beta Centauri B, another B-class star with an orbital period of between 125 and 220 years [see comments to this post for the revision I’ve made to the orbital period of this star].

This first TESS image is a test of the spacecraft’s four cameras, with a science-quality image expected to be released in June. Meanwhile, the spacecraft is easing into its highly elliptical orbit, with a final thruster burn scheduled for May 30. Science operations should commence in mid-June once orbital adjustments and camera calibrations are completed.

Image: An illustration of TESS as it passed the Moon during its lunar flyby. This provided a gravitational boost that placed TESS on course for its final working orbit. Credit: NASA’s Goddard Space Flight Center.

This first story on an operational TESS gives me echoes of New Horizons, which we followed here from launch to Pluto and now on to MU69 and whatever KBO may be next. Some people caution against anthropomorphizing machines, a human tendency when we deal with our pets, applying human perspectives to creatures considerably different than ourselves. To me the distinction is meaningless. We apply our passions and values to the things that matter to us, and New Horizons and TESS fit that bill for me. I recognize that spacecraft are not ‘creatures’ but my own values of commitment and purpose ride with these machines that can feel neither.