Centauri Dreams

These days we have a keen interest in small red dwarf stars (M-dwarfs) not only because they’re ideal for study, with deep transits of worlds in their habitable zones and the prospect of future analysis of their atmospheres, but also because they are so plentiful. Comprising perhaps 80 percent of all stars, they may well be home to the great majority of planets in the galaxy. And while they are common, they’re also long-lived, so that life would have plenty of opportunity to develop.

Now we have word of new work using both the Transiting Exoplanet Survey Satellite (TESS) and the Spitzer Space Telescope. TESS is, of course, a transit hunter, looking for the telltale dips in light from a parent star when a planet passes in front of it. The planet in question is LHS 3844b, about 48.6 light years out, and discovered by TESS in 2018. Follow-up observations in the infrared with Spitzer have detected light from the surface of this newly discovered world, allowing study of its atmosphere and composition. Note: This is not direct imaging; see below for more on the techniques used.

LHS 3844b orbits its star in 11 hours, making it almost certainly tidally locked; i.e., with one side always facing the star. The Spitzer data show that the dayside here reaches 770 degrees Celsius, while the nightside temperature is consistent with 0 Kelvin. In other words, the researchers could detect no heat being transferred from one side to the other, a process we would expect in the presence of an atmosphere.

Heat transfer is a mechanism that could ameliorate the effects of tidal lock, spreading warmth to the dark side and moderating global temperatures, but it takes an atmosphere to do that. We learn, then, that LHS 3844b is an object something like the Moon, or at any rate, a large version of it. Laura Kreidberg (Harvard-Smithsonian Center for Astrophysics), lead author of the paper that appears in Nature, says that this planet “…matches beautifully with our model of a bare rock with no atmosphere.” The scientist continues:

“We’ve got lots of theories about how planetary atmospheres fare around M dwarfs, but we haven’t been able to study them empirically. Now, with LHS 3844b, we have a terrestrial planet outside our solar system where for the first time we can determine observationally that an atmosphere is not present.”

Image: This artist’s illustration depicts the exoplanet LHS 3844b, which is 1.3 times the mass of Earth and orbits an M dwarf star. The planet’s surface may be covered mostly in dark lava rock, with no apparent atmosphere, according to observations by NASA’s Spitzer Space Telescope. Credit: NASA/JPL-Caltech/R. Hurt (IPAC).

This is painstaking analysis indeed, drawing on phase curve data from the planet’s transits. Phase curves are a combination of reflected light and thermal emission from the planet. Unable to resolve the planet from the host star, astronomers must work with their combined light, and observe light variations of exquisite subtlety as planets go through phase changes as they orbit. A phase curve, then, is the time-dependent change in the brightness of a planet as seen from Earth during one orbital period.

Image: Detecting Light from Exoplanet LHS 3844b. Credit: NASA/JPL-Caltech/L. Kreidberg (CfA | Harvard & Smithsonian).

Learning about the atmosphere (or lack thereof) of a small rocky world — LHS 3844b has a radius 1.3 times that of Earth — is therefore something of a coup, and bodes well for future discovery. The authors infer from the planet’s reflectivity (albedo) that it is covered with basalt, much like the mare of the Moon, which is probably an indication of volcanic activity in the distant past. From the paper:

We modeled the emission spectra of several rocky surfaces and compared with the measured planet-to-star flux… We considered multiple geologically plausible planetary surface types, including primary crusts that form from solidification of a magma ocean (ultramafic and feldspathic), secondary crust that forms from volcanic eruptions (basaltic), and a tertiary crust that forms from tectonic re-processing (granitoid). Governed by the reflectivity in the visible and the near-infrared and the emissivity in the mid-infrared, the surface types have distinct emission spectra. The measured planet-to-star flux for LHS 3844b is most consistent with a basaltic composition. Such a surface is comparable to the lunar mare and Mercury, and could result from widespread extrusive volcanism.

But this is a world much larger than the Moon, so what happened to its atmosphere? M-dwarf flare activity is thought to erode early planetary atmospheres, especially given how closely worlds like this orbit their star. The researchers rule out an atmosphere of over 10 bars (Earth’s atmospheric pressure at sea level is about 1 bar), and largely rule out one between 1 and 10 bars. They believe stellar winds and flares are the culprit. Modeling atmospheric escape over time, they assess how an early atmosphere dissolves within a magma ocean during planet formation or photolyzes into hydrogen and oxygen because of the intense bombardment of X-rays and UV from flares.

Thus a thick atmosphere is ruled out by the data, while stellar winds could account for further erosion of a thin atmosphere, all leading to the conclusion that LHS 3844b is a bare rock unless a thin atmosphere is replenished over time. Should we assume that hot terrestrial planets orbiting well inside the habitable zone of M-dwarfs are all devoid of atmospheres? Perhaps, but these stars may still be of astrobiological interest:

The results presented here motivate similar studies for less-irradiated planets orbiting small stars. Cooler planets are less susceptible to atmospheric escape and erosion, and may provide a friendlier environment for the evolution of life. In coming years this hypothesis can be tested, thanks to the infrared wavelength coverage of the James Webb Space Telescope and the influx of planet detections expected from current and future surveys.

The paper is Kreidberg et al., “Absence of a thick atmosphere on the terrestrial exoplanet LHS 3844b,” Nature 19 August 2019 (abstract / preprint)

The faint glow of a directly imaged planet will one day have much to tell us, once we’ve acquired equipment like the next generation of extremely large telescopes (ELTs), with their apertures measuring in the tens of meters. Discovering the makeup of planetary atmospheres is an obvious deep dive for biosignatures, but there is another. Biofluorescence, a kind of reflective glow from life under stress, could be detectable in some conditions at astronomical distances.

New work on the matter is now available from Jack O’Malley-James and Lisa Kaltenegger, at Cornell University’s Carl Sagan Institute. The duo have been on the trail of biofluorescence for some time now, and in fact their paper in Monthly Notices of the Royal Astronomical Society picks up on a 2018 foray into biosignatures involving the phenomenon (citation below). Here the question is detectability in the context of biofluorescence as a protective mechanism, an ‘upshift’ of damaging ultraviolet into longer, safer wavelengths.

“On Earth, there are some undersea corals that use biofluorescence to render the sun’s harmful ultraviolet radiation into harmless visible wavelengths, creating a beautiful radiance,” says Kaltenegger.” Maybe such life forms can exist on other worlds too, leaving us a telltale sign to spot them.”

Image: An example of coral fluorescence. Coral fluorescent proteins absorb near-UV and blue light and re-emit it at longer wavelengths. Credit: Available under Creative Commons CC0 1.0 Universal Public Domain Dedication.

Biofluorescence in vegetation is an effect that is detectable from Earth orbit. Here the effect is comparatively small, accounting for 1-2 percent of the vegetation reflection signal, but of course it is also widespread given the coverage of vegetation over much of Earth’s surface. The phenomenon is also seen in corals, which produce a higher degree of fluorescence. Earth levels of biofluorescence are clearly too small to act as useful biosignatures for exoplanets, but higher levels may well occur elsewhere.

That’s because our early work on the atmospheres of Earth-sized planets will delve into systems around small M-dwarf stars. Such worlds are plentiful and the Transiting Exoplanet Survey Satellite (TESS) is expected to add to our inventory of habitable zone examples nearby. Even now, we have interesting targets: Proxima Centauri b, Ross 128b, TRAPPIST-1e, -f, -g, LHS 1140-b, for example, all of which orbit M-dwarf stars. And M-dwarf stars are known to flare.

This, in fact, is one of the cases originally made against life in such systems, for extreme X-Ray and UV radiation would create a challenging environment for even the simplest lifeforms. M-dwarfs vary in terms of the amount of radiation they produce; indeed, a planet around an inactive M-dwarf receives a lower dose of ultraviolet than Earth. But planets around active stars, particularly given the fact that the habitable zone is so close to the small star, receive much higher flux, and such flaring remains active for longer on M-dwarfs than on stars like the Sun.

Life could conceivably flourish underground on such worlds, or within oceans, but even on Earth, the authors note, we see biological responses like protective pigments and DNA repair pathways that are ways of mitigating radiation damage. The corals mentioned above use biofluorescence to reduce the risk of damage to symbiotic algae, absorbing blue and ultraviolet photons and re-emitting them at longer wavelengths.

Corals that display this phenomenon cover a mere 0.2 percent of the ocean floor, making for only a tiny change in our planet’s visible flux. But the situation could be different in actively flaring M-dwarf systems. To study the matter, O’Malley-James and Kaltenegger begin with a biofluorescent surface biosphere in a shallow, transparent ocean, adjusting the variables to simulate different ocean conditions. They vary fluorescent protein absorption and emission to produce values for reflected and emitted light. The assumption is that life evolving in conditions of extreme UV flux will produce ever more efficient fluorescence, in terms of absorbed vs, emitted photons (at maximum efficiency, all photons are absorbed and re-emitted).

The authors calculate the UV flux for different classes of M-dwarfs and quantify the outgoing emissions of common pigments during fluorescence. The model also includes atmospheric effects with varying cloud coverage, generating spectra and colors for hypothetical planetary conditions. False positives from mineral fluorescence are considered, as are signals produced by surface vegetation, with different fractions of surface coverings and biofluorescent life.

But let’s cut to the chase. A biosphere otherwise hidden from us could be revealed through the temporary glow resulting from the flare of an M-dwarf. From the paper:

Depending on the efficiency of the fluorescence, biofluorescence can increase the visible flux of a planet at the peak emission wavelength by over an order of magnitude during a flare event. For comparison, the change in brightness at peak emission wavelengths caused by biofluorescence could increase the visible flux of an Earth-like planet by two orders of magnitude for a widespread biofluorescent biosphere and clear skies, with low-cloud scenarios being more likely for eroded atmospheres. In an M star system, the reflected visible flux from a planet will be low due to the host star’s low flux at these wavelengths; however, the proposed biofluorescent flux is dependent on the host star’s UV flux, resulting in additional visible flux that is independent of the low stellar flux at visible wavelengths. This suggests that exoplanets in the HZ of active M stars are interesting targets in the search for signs of life beyond Earth.

Video: Lisa Kaltenegger, director of Cornell University’s Carl Sagan Institute, explains why studying bioluminescence on Earth can guide the way humans search for life on other planets.

If biofluorescence can evolve on the planets of active M-dwarf stars, it may turn out that high ultraviolet flux could be the key to reveal its existence. We need to quantify such effects because our ground- and space-based assets in the hunt for biosignatures are going to be homing in on the most readily studied planets first, and that means nearby worlds around red dwarfs. What O’Malley-James and Kaltenegger are doing is charting one possible signature that we may or may not find, but one which we now know to include in our investigative toolkit.

The paper is O’Malley-James and Kaltenegger, “Biofluorescent Worlds – II. Biological fluorescence induced by stellar UV flares, a new temporal biosignature,” Monthly Notices of the Royal Astronomical Society 13 August 2019 (abstract/full text). The earlier paper on this issue is O’Malley-James and Kaltenegger, “Biofluorescent Worlds: Biological fluorescence as a temporal biosignature for flare stars worlds,” accepted at MNRAS (preprint).

So much rides on the successful launch and deployment of the James Webb Space Telescope that I never want to take its capabilities for granted. But assuming that we do see JWST safely orbiting the L2 Lagrange point, the massive instrument will stay in alignment with Earth as it moves around the Sun. allowing its sunshield to protect it from sunlight and solar heating.

Thus deployed, JWST may be able to give us information more quickly than we had thought possible about the intriguing system at TRAPPIST-1. In fact, according to new work out of the University of Washington’s Virtual Planetary Laboratory, we might within a single year be able to detect the presence of atmospheres for all seven of the TRAPPIST-1 planets in 10 or fewer transits, if their atmospheres turn out to be cloud-free. Right now, we have no way of knowing whether any of these worlds have atmospheres at all. A thick, global cloud pattern like that of Venus would take longer, perhaps 30 transits, to detect, but is definitely in range.

“There is a big question in the field right now whether these planets even have atmospheres, especially the innermost planets,” says Jacob Lustig-Yaeger, a UW doctoral student who is lead author of the paper on this work. “Once we have confirmed that there are atmospheres, then what can we learn about each planet’s atmosphere — the molecules that make it up?”

Image: New research from UW astronomers models how telescopes such as the James Webb Space Telescope will be able to study the planets of the intriguing TRAPPIST-1 system. Credit: NASA.

Working with Lustig-Yaeger are UW’s Victoria Meadows, principal investigator for the Virtual Planetary Laboratory, and doctoral student Andrew Lincowski. The latter should be a familiar name if you’ve been following TRAPPIST-1 studies, because back in November of 2018 he was lead author on a paper on climate models for this fascinating system (see Modeling Climates at TRAPPIST-1).

We’ll now be hoping to follow up that work with early JWST data. Briefly, Lincowski and team pointed to the extremely hot and bright early history of TRAPPIST-1, a tiny M-dwarf 39 light years out with a radius not much bigger than Jupiter (although with considerably more mass — the star is about 9 percent the mass of the Sun). These early conditions could produce planetary evolution much like Venus, with evaporating oceans and dense, uninhabitable atmospheres. The Lincowski paper, though, did point to TRAPPIST-1 e as a potential ocean world.

These findings were in the context of a system among whose seven transiting worlds are three — e, f and g — that are positioned near or in the habitable zone, where liquid water might exist on the surface. Now we have Lustig-Yaeger and company modeling our early JWST capabilities. The paper finds that beyond the presence of an atmosphere, we may be able to draw further conclusions, particularly with regard to the evolution of what gas envelopes we find.

Although oxygen as a biosignature may not be detectable for the potentially habitable TRAPPIST-1 planets, oxygen as a remnant of pre-main-sequence water loss may be easily detected or ruled out… the 1.06 and 1.27 µm O₂-O₂ CIA [collisionally-induced absorption] features are key discriminants of a planet that has an oxygen abundance greatly exceeding biogenic oxygen production on Earth and may therefore indicate a planet that has undergone vigorous water photolysis and subsequent loss during the protracted super-luminous pre-main-sequence phase faced by late M dwarfs,,,

Such features could be detected fairly quickly:

… in as few as 7-9, 15, 8, 49-67, 55-82, 79-100, and 62-89 transits of TRAPPIST-1b, c, d, e, f, g, and h, respectively, should they possess such an atmosphere. These quoted number of transits may be sufficient to rule out the existence of oxygen-dominated atmospheres in the TRAPPIST-1 system. Additional evidence of ocean loss could be provided by detection of isotope fractionation, which may also be possible in as few as 11 transits with JWST.

Moreover, the authors find that water detection could help to pare down various evolutionary scenarios on these worlds, particularly for TRAPPIST-1 b, c and d, assuming atmospheres high in oxygen content that have not been completely desiccated by the star’s early history. Thus we are probing planetary evolution, but assessments of habitability are going to be tricky, and it seems clear that we will need to turn such analysis over to future direct-imaging missions.

On balance, we are talking about getting useful results with a fairly low number of transits. JWST’s onboard Near-Infrared Spectrograph will use transmission spectroscopy — where the star’s light passes through a planet’s atmosphere to reveal its spectral ‘fingerprint’ — to detect the presence of an atmosphere via the absorption of CO₂. Such analysis can likewise either detect or rule out oxygen-dominated atmospheres, while constraining the extent of water loss through measurements of H₂O abundance. All of this provides fodder for other, still evolving observing strategies using the JWST instrument package that can begin the characterization of these compelling worlds.

The paper is Lustig-Yaeger et al., “The Detectability and Characterization of the TRAPPIST-1 Exoplanet Atmospheres with JWST,” Astronomical Journal Vol. 158, No. 1 (21 June 2019). Abstract / preprint. The Lincowski paper referenced above is “Evolved Climates and Observational Discriminants for the TRAPPIST-1 Planetary System,” Astrophysical Journal Vol. 867, No. 1 (1 November 2018). Abstract / Preprint.