Centauri Dreams

With my time-out period over (more about this next week), I want to get back into gear with the help of Ramses Ramirez, a specialist on planetary habitability whose work has now taken him to Japan. Born and raised in New York City, Ramses tells me he is much at home in his new position as a research scientist at the Earth-Life Science Institute (ELSI) in Tokyo, where opportunities for scientific collaboration abound and the chance to learn a new language beckons. We’ve looked at Ramses’ papers a number of times in these pages, and I was delighted when he offered this description of his work to our readership. A student of James Kasting, he received his Ph.D. from Penn State in 2014 and went on to postdoc work with Lisa Kaltenegger at Cornell’s Carl Sagan Institute. A fascination with astrobiology and the issues involved in defining habitable zones continues to be a primary focus. Ramses’ new paper ponders whether we are best served by looking for life similar to Earth’s because this is what we know, or whether there is a broader strategy, one that probes all the assumptions in our ideas of the classical habitable zone.

By Ramses Ramirez

I am glad to have the opportunity to formally introduce myself and give a summary of the recent work on planetary habitability and the habitable zone. I define the habitable zone (HZ) as the circular region around a star(s) where standing bodies of liquid water could exist on the surface of a rocky planet [1]. The inclusion of the phrase “standing bodies of water” excludes dry worlds that may exhibit small outpourings of seasonal surface water (e.g. possibly Mars). Defined this way, the HZ is properly focused for detecting worlds that have large surface bodies of water (e.g. seas, big lakes, oceans) that are in direct contact with the atmosphere. If life is present on such a world, potential atmospheric biosignatures could be detected with current technology. However, in the absence of such large water bodies, life would not be detectable even if it were present. Likewise, although life may be possible within the seas of a Europa or Enceladus exoplanetary analogue, the global ice layer covering their oceans would prevent the detection of such subsurface life. Such observational issues keep the HZ within an orbital region that is placed somewhat closer to the star.

When I first started my Ph.D. in 2010 under my mentor Professor James F. Kasting, I did not know the exact topic that I would be working on at first, but with my interest in the search for extraterrestrial life, I knew that I wanted to work on planetary habitability. Fortunately, I got involved as one of the two lead authors on the 2013 HZ paper [2], where we updated the seminal Kasting et al. [3] HZ limits [2,3]. Although I was fascinated by the HZ concept as a navigational tool to find potentially habitable planets, my investigations soon led me to realize that the HZ definition described in these earlier works, what I dub the “classical HZ”, would be insufficient for capturing the diversity of such planets. From that point on, it became my personal mission to turn the HZ into an even more capable navigational tool.

The classical HZ assumes that CO₂ and H₂O are the key greenhouse gases on potentially habitable planets, following the carbonate-silicate cycle on the Earth, which is thought to regulate CO₂ between the atmosphere, surface, and the interior (e.g. [3]). Given that the concept of a universal carbonate-silicate cycle on habitable exoplanets is itself far from proven (e.g., [4]), I always thought that this assumption was needlessly restrictive and geocentric. After all, HZ planets with atmospheres consisting of different gas mixtures can also support standing bodies of liquid water and potentially be habitable. Reduced greenhouse gases, like H₂ and CH₄, have been considered as major atmospheric constituents on both early Earth and early Mars (e.g., [5,6]). It has even been suggested that planets with primordial hydrogen envelopes may be habitable in some circumstances [7]. If hypothetical planets consisting of dense 10-bar CO₂ atmospheres near the outer edge of the classical HZ are potentially habitable (e.g. [2][3]), then why not worlds composed of these other atmospheric constituents (Figure 1)?

Figure 1: The classical HZ extended to A-stars (blue) with CO₂-CH₄ (green) and CO₂-H₂ (red) outer edge extensions for stars of stellar effective temperatures between 2,600 and 10,000 K (reproduced from Ramirez [1]).

Moreover, the classical HZ really targets potentially habitable planets orbiting main-sequence stars. However, this approach ignores the importance of the temporal evolution of the HZ, particularly a star’s pre-main-sequence phase. It is during this early stage that many M-stars are bright and luminous enough to completely desiccate any planets that are now thought to be located within the main-sequence HZ (e.g. [8][9][10]), like Proxima Centauri-b and many of the TRAPPIST-1 planets, unless such worlds are able to accrete enough water to offset losses (e.g., [11][12]) (Figure 2).

Figure 2: The pre-main-sequence HZ for a late (M8) star. A planet that forms at ~0.03 AU undergoes a runaway greenhouse state for ~100 Myr before finally entering the HZ, settling near the outer edge after ~1 billion years (reproduced from Ramirez [1]).

In my view, we are currently ill-equipped to infer the atmospheric conditions that are most suitable for extraterrestrial life. All we know is that life did somehow arise on this planet. It is therefore pretentious to take our one poorly-understood example and assume that alien life must follow a similar trajectory. Some might argue that we should focus on finding life similar to Earth’s because it is familiar and would be simpler to find. However, this argument from the principle of mediocrity is not convincing to me because we have no idea how common or rare our particular brand of life is until we are able to find a second instance of life’s occurrence elsewhere.

It is then illogical to rely on any one iteration of the HZ to find potentially habitable planets. Our inadequate understanding of biology, the origin of life, and planetary processes require that all of these working hypotheses, including the assumptions used in the classical HZ, be tested by observations. Ideas that are later found to be unsupported by nature can then be rejected whereas those that are confirmed should be refined. Only then will we begin to truly understand life’s possibilities and make more informed – and possibly more restrictive – design decisions for subsequent missions. But only proper observations (and possibly better theoretical constraints) can tell us this, and not adherence to some ambiguous and untested geocentric philosophy. Finding extraterrestrial life will be one of the hardest endeavors undertaken by our species. To meet the challenge, we need to start with a thorough and openminded search for potentially habitable planets.

Thus, as some Centauri Dreams readers may already know, part of my work (in collaboration with Lisa Kaltenegger) in recent years has focused on improving our understanding of both the spatial [13][14] and temporal evolution of the HZ [8][15]. These papers have already been discussed in previous Centauri Dreams posts. However, in addition to our own work, other researchers and colleagues have also proposed their own revisions and extensions to different aspects of the HZ, further increasing its utility as a tool for finding life that is both similar and dissimilar to Earth’s. This gradual but steady evolution in how we think about the HZ has come to a good point (I believe) for a review paper to come along such as this one.

I hope that your readers will like my new summary of recent advances (linked below) in our understanding of the HZ. I see this work as both a primer for the student and layman as well as a guide for space missions. It includes many recommendations and suggestions for how the classical HZ, in conjunction with newer HZ formulations, can be used to maximize our chances of finding extraterrestrial life. For example, I do not think that a proper assessment of planetary habitability can be made without properly assessing a planet’s pre-main-sequence habitability. A-stars should also be considered as potential hosts for life-bearing planets. Most importantly, we should employ the various HZ formulations and rubrics to rank which planets are most likely to host life. Finally, my paper clarifies many common misconceptions about the HZ concept and explains why the search must necessarily be limited to finding surface liquid water (at least for now).

The review paper is “A more comprehensive habitable zone for finding life on other planets,” published in Geosciences 8(8), 280 (28 July 2018). Full text available here. [PG: Expect a close look at this paper in coming weeks on Centauri Dreams].

References

1) Ramirez, R.M. 2018. “A more comprehensive habitable zone for finding life on other planets.” Geosciences 8(8), 280.

2) Kopparapu, R., Ramirez, R.M. (co-primary author), Kasting, J., et al., 2013. Habitable zones around main-sequence stars: New Estimates. ApJ, 765, 2, 131

3) Kasting, James F., Daniel P. Whitmire, and Ray T. Reynolds. “Habitable zones around main sequence stars.” Icarus 101.1 (1993): 108-128.

4) Bean, Jacob L., Dorian S. Abbot, and Eliza M-R. Kempton. “A statistical comparative planetology approach to the hunt for habitable exoplanets and life beyond the solar system.” The Astrophysical Journal Letters 841.2 (2017): L24.

5) Wordsworth, Robin, and Raymond Pierrehumbert. “Hydrogen-nitrogen greenhouse warming in Earth’s early atmosphere.” Science 339.6115 (2013): 64-67.

6) Ramirez, R.M., Kopparapu, R., Zugger, M., Robinson, T.D.,Freedman, R., Kasting, J.F., 2014. Warming early Mars with CO2 and H2. Nat. Geosc., 7, 59 – 63

7) Pierrehumbert, Raymond, and Eric Gaidos. “Hydrogen greenhouse planets beyond the habitable zone.” The Astrophysical Journal Letters 734.1 (2011): L13.

8) Ramirez, R.M., Kaltenegger, L., 2014. Habitable Zones of Pre-Main-Sequence Stars. The Astrophysical Journal Letters, 797, 2, L25

9) Luger, Rodrigo, and Rory Barnes. “Extreme water loss and abiotic O2 buildup on planets throughout the habitable zones of M dwarfs.” Astrobiology 15.2 (2015): 119-143.

10) Tian, Feng, and Shigeru Ida. “Water contents of Earth-mass planets around M dwarfs.” Nature Geoscience 8.3 (2015): 177.

11) Levi, Amit, Dimitar Sasselov, and Morris Podolak. “The Abundance of Atmospheric CO2 in Ocean Exoplanets: a Novel CO2 Deposition Mechanism.” The Astrophysical Journal 838.1 (2017): 24.

12) Ramirez, R.M. and Levi, A. 2018. The ice cap zone: a unique habitable zone for ocean worlds. The Monthly Notices of the Royal Astronomical Society, 477, 4, 4627- 4640

13) Ramirez, R.M., Kaltenegger, L., 2017. A volcanic hydrogen habitable zone. The Astrophysical Journal Letters, 837, 1

14) Ramirez, R.M., Kaltenegger, L. 2018. A methane extension to the classical habitable zone. The Astrophysical Journal 858, 2

15) Ramirez, R.M., Kaltenegger, L., 2016. Habitable Zones of Post-Main Sequence Stars. The Astrophysical Journal, 823, 6, 14pp

A cluster of stars sharing a common origin, now gravitationally unbound, is referred to as a stellar association. I’ve written before about how useful some of these groupings can be. In the form of so-called moving groups — a stellar association that is still somewhat coherent — they help us identify stars of similar age, an aid as we discover new objects. Now we have word of an object called 2MASS 0249 c, found in the Beta Pictoris moving group, that has striking similarities to the most famous member of that group, Beta Pictoris b.

2MASS 0249 c, like Beta Pictoris b, was found by direct imaging, meaning we’re actually looking at the object under discussion in the image below. The two objects are all but identical in mass, brightness and spectrum. Images from the Canada-France-Hawaii Telescope (CFHT) showed an object moving at a large distance from its host, which turned out to be a pair of closely spaced brown dwarfs.

Follow-up observations with the Keck instrument allowed that determination, while spectroscopy at the NASA Infrared Telescope Facility and the Astrophysical Research Consortium 3.5-meter Telescope at Apache Point Telescope completed the data.

“To date, exoplanets found by direct imaging have basically been individuals, each distinct from the other in their appearance and age. Finding two exoplanets with almost identical appearances and yet having formed so differently opens a new window for understanding these objects,” said Michael Liu, astronomer at the University of Hawai`i Institute for Astronomy, and a collaborator on this work.

Image: Image of the 2MASS 0249 system taken with Canada-France-Hawaii Telescope’s infrared camera WIRCam. 2MASS 0249 c is located 2000 astronomical units from its host brown dwarfs, which are unresolved in this image. The area of sky covered by this image is approximately one thousandth the area of the full moon. Credits: T. Dupuy, M. Liu.

The difference in formation scenarios that Liu talks about is instructive. Here we have two host stars, one of them being a binary system of brown dwarfs, that formed in the same stellar nursery, as per their membership in the moving group. But while Beta Pictoris b, a gas giant of about 13 Jupiter masses, orbits a star 10 times brighter than the Sun, 2MASS 0249 c orbits a brown dwarf pair 2000 times fainter. At a distance of 9 AU, Beta Pictoris b is relatively close to its star, while 2MASS 0249 c is a whopping 2000 AU from its brown dwarf hosts.

These distances imply different formation scenarios. Beta Pictoris b likely formed from the accumulation of dust grains in the circumstellar disk surrounding its star. 2MASS 0249 c could not have done so, given that the two brown dwarfs it orbits would not have had sufficient disk material to produce it. That implies the new planet took form through the gravitational collapse of a gas cloud found in the original stellar birth cluster.

“2MASS 0249 c and beta Pictoris b show us that nature has more than one way to make very similar looking exoplanets,” says Kaitlin Kratter, astronomer at the University of Arizona and a collaborator on this work. “They’re both considered exoplanets, but 2MASS 0249 c illustrates that such a simple classification can obscure a complicated reality.”

Image: The infrared spectra of 2MASS 0249 c (top) and beta Pictoris b (bottom) are similar, as expected for two objects of comparable mass that formed in the same stellar nursery. Unlike 2MASS 0249 c, beta Pictoris b orbits much closer to its massive host star and is embedded in a bright circumstellar disk. Credit: T. Dupuy, ESO/A.-M. Lagrange et al.

What we get out of all this is opportunity. The formation of gas giants is a key phase in the emergence of new planetary systems, and being able to use direct imaging to study such worlds means we can probe their atmospheres directly, examining the composition, surface temperature, chemistry and other physical properties of the exoplanet. Moreover, direct imaging is most effective when working with planets far from their star, as this object is. If we are after insights into gas giant formation, the different formation pathways for Beta Pictoris b and 2MASS 0249 c may provide evidence both orbital and spectral. The paper notes:

As directly imaged objects, ? Pic b and 2MASS J0249?0557 c provide a new opportunity to test atmospheric compositions and angular momentum evolution for a close-in planet and a very wide companion that share a common mass and age and that formed from the same material.

And on the issue of spectra:

If different formation mechanisms produced these objects, then their spectra could contain evidence of their divergent pasts. As noted above, we suspect that 2MASS J0249?0557 c arose from a star-formation-like process of global, top-down gravitational collapse in the same way as the freefloating object 2MASS J2208+2921. On the other hand, ? Pic b bears architectural resemblance to planetary systems and thus may have formed via core accretion. Core accretion models and observations of solar system gas giants show substantial metal enrichment (e.g., Stevenson 1982; Bolton et al. 2017). Thus, if ? Pic b is a scaled-up gas giant (?13 MJup), then we may expect to see substantial metal enrichment in its atmosphere.

What an interesting find 2MASS 0249c turns out to be. Is it a planet or actually a brown dwarf? One thing is for sure: The discovery puts a spotlight on the boundary between planet and brown dwarf, both in terms of composition and in terms of formation history. Maybe it’s best to fall back on how today’s paper describes the object: a ‘planetary-mass companion.’ Now we can go to work, via direct imaging, on issues like variability, rotation and atmospheric composition. How these data vary across different formation histories should occupy astronomers for some time.

The paper is Dupuy et al., “The Hawaii Infrared Parallax Program. III. 2MASS J0249-0557 c: A Wide Planetary-mass Companion to a Low-mass Binary in the beta Pic Moving Group,” accepted at the Astronomical Journal (preprint).

Red dwarfs have a lot of things going for them when it comes to finding possibly habitable planets. A planet of Earth size in the HZ will produce a substantial transit signal because of the small size of the star (‘transit depth’ refers to the amount of the star’s light that is blocked by the planet), and the tight orbit the planet must follow increases the geometric probability of observing a transit. But planets that do not transit are also more readily detected because of the large size of the planet compared to the star, gravitational interactions producing a strong radial velocity signature, which is what we have in the case of Ross 128b.

About 11 light years from Earth, the planet was culled out of more than a decade of radial velocity data in 2017 using the European Southern Observatory’s HARPS spectrograph (High Accuracy Radial velocity Planet Searcher) at the La Silla Observatory in Chile. The location of the planet near the inner edge of its star’s habitable zone excited interest, as did the fact that Ross 128 is much less subject to flares of ultraviolet and X-ray radiation than our nearest neighbor, Proxima Centauri, which also hosts a planet in a potentially habitable orbit.

Image: Artist’s impression of the exoplanet Ross 128b. Credit: ESO.

What we know about Ross 128b is that it orbits 20 times closer to its star than the Earth orbits the Sun, but receives only 1.38 times more irradiation than the Earth, with an equilibrium temperature estimated anywhere between -60 degrees Celsius and 20°C, the host star being small and relatively cool. But bear in mind that what we get from radial velocity is a minimum mass, because we don’t know at what angle this system presents itself in our sky. Now a team led by Diogo Souto (Observatório Nacional, Brazil) is attempting to deduce more about the planet’s composition using an unusual method: Analyzing the composition of the host star.

If we learn the chemical abundances found in the star Ross 128, the thinking goes, we should be able to make reasonable estimates about the composition of any planets that orbit it. Souto and team are presenting new techniques for making these measurements, using data from the Sloan Digital Sky Survey’s APOGEE spectroscope. Measuring the star’s near-infrared light, where Ross 128 shines the brightest, the researchers have been able to derive abundances for carbon, oxygen, magnesium, aluminum, potassium, calcium, titanium and iron.

“The ability of APOGEE to measure near-infrared light, where Ross 128 is brightest, was key for this study,” says co-author Johanna Teske (Carnegie Institution for Science). “It allowed us to address some fundamental questions about Ross 128 b’s `Earth-like-ness.’”

APOGEE is the Apache Point Galactic Evolution Experiment, an investigation using high-resolution spectroscopy to probe the dust that obscures the inner Milky Way. The project surveyed 100,000 red giant stars across the galactic bulge, but also observed M-dwarfs in the neighborhood of the Sun as a secondary study. Tightening up our knowledge of stellar parameters, the paper notes, offers an indirect route to studying exoplanet composition.

The assumption in this work is that the chemistry of a host star influences the contents of the disk from which planets form around it, which in turn affects the interior structure of any planet. Thus we can hope to tell from the amount of magnesium, iron and silicon available something about the exoplanet. This is the first detailed abundance analysis for Ross 128, and it shows that the star has iron levels similar to the Sun. The silicon level could not be measured, but the ratio of iron to magnesium points to a large core for the planet, larger than Earth’s.

Souto and team believe that knowledge of Ross 128b’s minimum mass (from the radial velocity data), coupled with their data on stellar abundances, can provide a broad estimate of the planet’s radius, a key factor because it would allow a calculation of its density. From the paper:

While both mass and radius are not available for Ross 128b, we can estimate its radius given its observed minimum mass and assuming the stellar composition of the host star is a proxy for that of the planet. We calculate the range of radii possible for Ross 128b using the ExoPlex software package (Unterborn et al. 2018) for all masses above the minimum mass of Ross 128b (1.35M_?; Bonfils et al. 2017). Models were run assuming a two-layer model with a liquid core and silicate mantle (no atmosphere). We increase the input mass until a likely radius of 1.5R_? was achieved, roughly the point where planets are not expected be gas-rich mini-Neptunes as opposed to rock and iron-dominated super-Earths…

Measurements of the temperature of Ross 128 coupled with the estimated radius of the exoplanet and its inferred composition allow the team to calculate Ross 128b’s albedo, the amount of light reflecting off its surface. These estimates allow the possibility of a temperate climate, taking into account the insolation flux (energy received from the host star) and equilibrium temperature. “Our results,” the authors write, “support the claim of Bonfils et al. (2017) that Ross 128b is a temperate exoplanet in the inner edge of the habitable zone.”

But the paper urges caution in the interpretation:

However, this is not to say that Ross 128b is a “Exo-Earth.” Geologic factors unexplored in Bonfils et al. (2017) such as the planet’s likelihood to produce continental crust, the weathering rates of key nutrients into ocean basins or the presence of a long-term magnetic field could produce a planet decidedly not at all “Earth-like” or habitable due to differences in its composition and thermal history. Furthermore, other aspects of the M-dwarf’s stellar activity and its effect on the retention of any atmosphere and potential habitability should be studied, although we find no evidence of activity in the Ross 128 spectra.

Indeed. The number of variables affecting ‘habitability’ is striking. So let’s say this: We have a planet for which mass-radius modeling based on the composition of its host star indicates a mixture of rock and iron, the relative amounts of each being set by the ratio between iron and magnesium. The derived values for insolation and equilibrium temperature are not inconsistent with previous studies indicating a temperate planet at the inner edge of its star’s habitable zone.

The work hinges on modeling of an exoplanet based on a deeper analysis of its host star than has previously been available for an M-dwarf. Tuning up such modeling will demand further data, in particular applying these methods to the host stars of transiting worlds (think TRAPPIST-1) to test their accuracy and reliability in characterizing planets we cannot see.

The paper is Souto et al., “Stellar and Planetary Characterization of the Ross 128 Exoplanetary System from APOGEE Spectra,” Astrophysical Journal Letters Vol. 860, No. 1 (13 June 2018). Abstract / preprint.