Centauri Dreams

With 250 times more X-ray radiation than Earth receives and high levels of ultraviolet, would Proxima b, that tantalizing, Earth-sized world around the nearest star, have any chance for habitability? The answer, according to Jack O’Malley-James and Lisa Kaltenegger (Cornell University) is yes, and in fact, the duo argue that life under these conditions could deploy a number of possible strategies for dealing with the radiation influx. Their conclusions appear in a new paper in Monthly Notices of the Royal Astronomical Society.

Kaltenegger is director of Cornell’s Carl Sagan Institute, where O’Malley-James serves as a research associate. Modeling surface environments on four exoplanets that are prone to frequent flares — Proxima-b, TRAPPIST-1e, Ross-128b and LHS-1140b — Kaltenegger and O’Malley-James examined different atmospheric solutions that could suppress UV damage in living cells.

Thin atmospheres and a lack of ozone protection fail to block UV radiation well, no surprise there, and such atmospheres do not measure up favorably when compared to atmospheres like that of the Earth today. But go back four billion years and we find that the modeled planets receive radiation in the UV significantly lower than what the Earth experienced in that era of its development. Earth was at that time uninhabitable by human standards — had any humans been available — but life had indeed emerged and continued to thrive. Thus the authors write that UV radiation “…should not be a limiting factor for the habitability of planets orbiting M stars.”

Image: The intense radiation environments around nearby M stars could favor habitable worlds resembling younger versions of Earth. Credit: Jack O’Malley-James/Cornell University.

The extremophile Deinococcus radiodurans is key to this study, for it is one of the most radiation-resistant organisms known. By varying the UV wavelengths, the scientists assessed the mortality rates of the organism, in which it becomes clear that some wavelengths of UV are more damaging to biological molecules than others. From the paper:

…we use this as a benchmark against which to compare the habitability of the different radiation models. This action spectrum compares the effectiveness of different wavelengths of UV radiation at inducing a 90 per?cent mortality rate. It highlights which wavelengths have the most damaging irradiation for biological molecules: for example, the action spectrum in Fig. 4 shows that a dosage of UV radiation at 360?nm would need to be three orders of magnitude higher than a dosage of radiation at 260?nm to produce similar mortality rates in a population of this organism.

Image: This is Figure 4 from the paper. Caption: Relative biological effectiveness of UV surface radiation on Proxima-b. (A) The biological effectiveness of UV on DNA and the radiation-resistant microorganism D. radiodurans (Voet et al. 1963; Diffey 1991) quantifies the relative effectiveness of different wavelengths of UV radiation to cause DNA destruction or, for D. radiodurans, mortality, which increases with decreasing wavelength. Biological effectiveness of UV damage for (B) oxygenic atmospheres and (C) anoxic atmosphere models shown as convolution of the surface UV flux and action spectrum over wavelength (solid line shows flaring, dashed line quiescent star), compared to present-day Earth (red solid) and early Earth (3.9 billion years ago) (red dashed). Credit: Lisa Kaltenegger/Jack O’Malley-James/Cornell University.

We can’t rule out organisms below ground or living in water or rock, not to mention such survival characteristics as biofluorescence or protective pigments. We know of microorganisms that can tolerate full solar UV in space exposure experiments, using protective cells or pigments as effective UV screens. Biofluorescence offers protection against radiation because UV can be upshifted to longer wavelengths that produce less harm. The authors think protective biofluorescence would be at its most useful during the intense UV flux of flares, although a constant level of high UV might produce continuous fluorescence.

Here we have a potential biosignature, cited by the authors in a previous paper:

Because biofluorescence is independent of the visible flux of the host star and only dependent on the UV flux of the star, emitted biofluorescence can increase the visible flux of a planet orbiting an active M-star by several orders of magnitude (O’Malley-James & Kaltenegger 2018) during a flare.

We may get our first look at such atmospheres by observing ozone, which is potentially detectable by the James Webb Space Telescope. On the other hand, a high-enough level of UV could also produce a biosphere below ground that would present, if any, only the weakest of biosignatures. Even so, the authors conclude that nearby planets around M-dwarfs like those studied here are serious candidates for biosignature examination by future observatories.

While a multitude of factors ultimately determine an individual planet’s habitability our results demonstrate that high UV radiation levels may not be a limiting factor. The compositions of the atmospheres of our nearest habitable exoplanets are currently unknown; however, if the atmospheres of these worlds resemble the composition of Earth’s atmosphere through geological time, UV surface radiation would not be a limiting factor to the ability of these planets to host life. Even for planets with eroded or anoxic atmospheres orbiting active, flaring M stars the surface UV radiation in our models remains below that of the early Earth for all cases modelled. Therefore, rather than ruling these worlds out in our search for life, they provide an intriguing environment for the search for life and even for searching for alternative biosignatures that could exist under high-UV surface conditions.

The paper is O’Malley-James & Kaltenegger, “Lessons from early Earth: UV surface radiation should not limit the habitability of active M star systems,” Monthly Notices of the Royal Astronomical Society Vol. 485 Issue 4 (June 2019), pp. 5598-5603 (full text).

An iron and nickel-rich planetesimal is apparently all that survives of a planet following the death of its star, SDSS J122859.93+104032.9. We are talking about an object in an orbit around a white dwarf so tight that it completes a revolution every two hours. Significantly, spectroscopic methods were used to make the identification, the first time a solid body has been found around a white dwarf with spectroscopy. Variations in emitted light were used to identify the gases generated by the planetesimal, with data from the Gran Telescopio Canarias in La Palma.

Lead author Christopher Manser (University of Warwick) notes the advantages of the method the team developed to study a white dwarf 400 light years away:

“Our discovery is only the second solid planetesimal found in a tight orbit around a white dwarf, with the previous one found because debris passing in front of the star blocked some of its light — that is the “transit method” widely used to discover exoplanets around Sun-like stars. To find such transits, the geometry under which we view them has to be very finely tuned, which means that each system observed for several hours mostly leads to nothing. The spectroscopic method we developed in this research can detect close-in planetesimals without the need for a specific alignment.”

Image: A planetary fragment orbits the star SDSS J122859.93+104032.9, leaving a tail of gas in its wake. Credit & copyright: University of Warwick/Mark Garlick.

This is an extreme environment, the white dwarf in question being surrounded by a debris disk through which the object passes in its orbit. The star itself is about 70 percent of the mass of the Sun and, like all white dwarfs, this one — roughly the size of Earth — is quite dense, a survivor of the star’s red giant phase. An object moving this close to the white dwarf will be under extreme gravitational stress; the gravity of SDSS J122859.93+104032.9 is fully 100,000 times that of the Earth. The fact that the team could identify a planetesimal deep within the gravitational well indicates it must be an object of great density, probably made up of iron and nickel.

On where the object came from, the paper offers this intriguing possibility:

This object may be the differentiated iron core of a larger body that has been stripped of its crust and mantle by the tidal forces of the white dwarf. The outer layers of such a body would be less dense and would disrupt at greater semimajor axes and longer periods than those required for core disruption. This disrupted material would then form a disc of dusty debris around SDSS J1228+1040, leaving a stripped corelike planetesimal orbiting within it.

Manser’s colleague and co-author Boris Gaensicke adds if the assumption that we are dealing with a planetary core is correct, then the original body would have been at least hundreds of kilometers in diameter, because it is only at this size that planets begin to differentiate, with heavier elements sinking to form a metal core. It could, of course, have been much larger.

Thus the survival of a planetesimal here, actually orbiting within the original radius of its star, suggests a large object ultimately shredded by gravitational forces. We are glimpsing what our own Solar System may resemble in 5 to 6 billion years, when it will be a white dwarf orbited by the outer planets along with asteroids and comets. Our star’s expansion into a red giant will savage the inner system, perhaps leaving debris like what we see around SDSS J122859.93+104032.9. Bear in mind, too, that the vast majority of the stars known to host planets will end their lives as white dwarfs, so we are looking at a common destiny.

The debris disk of the white dwarf is rich in magnesium, iron, silicon and oxygen, and it is within that disk that the scientists found gas streaming from the evidently solid body. The object appears to be about a kilometer in size but could be as large as a few hundred kilometers in diameter. Whether it is the source of the gas or simply the cause of the gaseous ‘tail’ as it collides with debris in the disk is not yet known. Learning more will involve studying other debris disks similar to SDSS J122859.93+104032.9 (eight gaseous white dwarf debris discs are currently known), where the spectroscopic method will perhaps find other instances of planetesimals orbiting near or within the parent star’s debris disk.

The paper is Manser et al., “A Planetesimal Orbiting Within the Debris Disc Around a White Dwarf Star,” Science April 4 2019 (abstract).

A method for enhanced exoplanet investigation takes center stage today as we look at the GRAVITY instrument, a near-infrared tool aided by adaptive optics that brings new precision to exoplanet imaging. In operation at the European Southern Observatory’s Very Large Telescope Interferometer (VLTI) at Paranal Observatory in Chile, GRAVITY works with the combined light of multiple telescopes to produce what would otherwise take a single telescope with a mirror diameter of 100 meters to equal. The early demonstrator target is exoplanet HR 8799e.

The method at work is interferometry, and here we are applying it to a ‘super Jupiter,’ more massive and much younger (at 30 million years) than any planet in our Solar System. The GRAVITY observations of this target mark the first time that optical interferometry has been used to study an exoplanet at this level of precision, producing a highly detailed spectrum. The planet is part of a 5-planet system some 130 light years away, all 5 of the planets being gas giants between 5 and 10 times the mass of Jupiter.

Image: This wide-field image shows the surroundings of the young star HR 8799 in the constellation of Pegasus. This picture was created from material forming part of the Digitized Sky Survey 2. Credit: ESO/Digitized Sky Survey 2. Acknowledgement: Davide de Martin.

The high resolution images that resulted from this work show what we can expect from optical interferometry going forward. We now know the distance between HR8799e and its star with 10 times the accuracy of previous estimates, which in turn helps to refine the planet’s orbit, one that appears to be slightly inclined compared to the orbital plane of its four companions.

That high-grade spectrum has spoken volumes about the composition of the planet’s atmosphere, says team leader Sylvestre Lacour (Observatoire de Paris and the Max Planck Institute for Extraterrestrial Physics):

“Our analysis showed that HR8799e has an atmosphere containing far more carbon monoxide than methane — something not expected from equilibrium chemistry. We can best explain this surprising result with high vertical winds within the atmosphere preventing the carbon monoxide from reacting with hydrogen to form methane.”

Also present here are clouds of iron and silicate dust, all suggestive of violent storms, as convection causes the dust to rise and then descend into the interior. We’re seeing a giant planet in its turbulent infancy. And what an impressive demonstration of interferometry’s ability to separate star and, in this case, a very close planet, with a result that the European Southern Observatory considers much cleaner than what could be achieved with a coronagraph that would mask out the light of the star.

Image: Exoplanet HR 8799e has been analyzed spectroscopically separate from the parent star HR 8799 using the new technique (artistic impression). Credit: © ESO/Luis Calçada.

We’ll be using analyses of planetary atmospheres to look for biosignatures one day on cooler and more clement worlds. In the interim, astronomers plan to continue the investigation of the HR8799e system, allowing so complete an analysis of the planet’s orbit that it will be used to reveal the mutual gravitational interactions of the giant planets as well as the influence of the central star. That, in turn, takes us to accurate estimates of the planetary masses.

And these excerpts from the paper are notable. In the first, ‘Kmag’ refers to ‘K magnitude,’ the magnitude of the star about which the extrasolar planet orbits as viewed through a specific filter at near-infrared wavelengths, in this case between 2.0 and 2.4 ?m. GRAVITY operates within the K band:

The interferometric technique brings unique possibilities to characterize exoplanets. With the technique described here, any planet with K ? 19, ?Kmag ? 11, and separation ? 100 mas is, in theory, observable with GRAVITY. The numbers are still to be refined, but it would mean that GRAVITY could observe most of the known imaged planets, and maybe in the near future planets detected by radial velocity [italics mine].

And this:

…the idea that an interferometer can resolve the surface of exoplanets, giving radius and resolving clouds patchiness, is now becoming more plausible. However, it would require an interferometer with baselines on the order of 10 km. This could be a goal for ESO after ELT construction.

The paper is Lacour et al., “First direct detection of an exoplanet by optical interferometry,” in Astronomy and Astrophysics, Vol. 623 (March 2019), L11 (abstract / preprint).

Picking up on TESS (Transiting Exoplanet Survey Satellite), one of whose discoveries we examined yesterday, comes news of a document called the “TESS Habitable Zone Star Catalog.” The work of Cornell astronomers in collaboration with colleagues at Lehigh and Vanderbilt, the paper has just been published in Astrophysical Journal Letters (citation below), where we find 1,822 stars where TESS may find rocky terrestrial planets.

The listed 1,822 are nearby stars, bright, cool dwarfs, with temperatures roughly between 2,700 and 5,000 Kelvin, with a TESS magnitude brighter than 12 and reliable data from the Gaia Data Release 2 as to distance. Here TESS can detect 2 transits of planets that receive stellar irradiation similar to Earth’s, during the 2-year prime mission. 408 of these stars would allow TESS to detect transiting planets down to Earth size during one transit. The catalog is fine-tuned to the TESS instrumentation and mission parameters, the stars selected because they offer sufficient observing time to be able make these detections.

From the paper:

What distinguishes this catalog from previous work like HabCat (Turnbull & Tartar 2003), DASSC (Kaltenegger et al 2010) and CELESTA (Chandler, McDonald, & Kane 2016) is that the stars included here are specifically selected to have sufficient observation time by TESS for transit detection out to the Earth-equivalent orbital distance. We also use Gaia DR2 data, which allows us to exclude giant stars from the star sample and provides reliable distances for our full star sample. All the stars have been included in the TESS exoplanet Candidate Target List, ensuring that they will also have 2-minute cadence observation (provided they do not fall in TESS camera pixel gaps), providing a specific catalog for the TESS mission of stars where planets in the Habitable Zone can be detected by TESS. This data will be available to the community in the ongoing public TESS data releases.

In case you’re wondering, 137 stars in the catalog are within the continuous viewing zone of the James Webb Space Telescope, which will be able to observe them to characterize planetary atmospheres and search for biosignatures. Many more will be followed up after any TESS planet identifications by ground-based extremely large telescopes currently under construction.

Image: The TESS search space compared to that of the Kepler Mission. Credit: Zach Berta-Thompson.

The idea, then, is to help us shape our target lists for TESS by pointing to the most likely places of discovery. Meanwhile, Elisa Quintana (NASA GSFC) has been thinking about planets we can’t yet detect but which may indeed be present, using Kepler data as massaged by a mathematical model that has implications for TESS and future mission datasets. The difference is that in Quintana’s case, these are systems where we already know planets exist. The question: What other planets might yet be found in the same systems?

After all, we have to wonder what our methods may have missed. The Kepler mission has led us to believe that most stars in the galaxy have planetary companions, but around even relatively close stars, we may be seeing a subset of what’s actually there. Using the transit method, which Kepler employed to such brilliant effect, we only see the planets that move across the face of the star as seen from Earth. There could be others in the same system that do not.

Think about how rare a transit of Venus is. Even from our vantage so close to the planet, we see Venus cross the Sun only in pairs of transits eight years apart, separated by gaps of over a century. Indeed, the last transit of Venus of the 21st Century has already taken place (5,6 June, 2012); we have to wait until December of 2117 for the next. The orbit of Venus is responsible for the rarity of the phenomenon; it’s inclined by 3.4° relative to the Earth’s orbit. Exoplanetary systems are presumably not immune to such variation.

Quintana has been working as mentor with an 18 year-old high school student named Ana Humphrey, who developed the model to predict possible planets in such systems. Out of Humphrey’s work, which has garnered a a $250,000 prize in the Regeneron Science Talent Search, we learn that there may be as many as 560 ‘hidden’ planets in exoplanet systems identified by Kepler. Says Humphrey:

“I was completely fascinated by this idea of finding new planets using mass, based on data that we already had. I think it just shows that even if your data collection is complete, there’s always new questions that can be asked and can be answered.”

Image: Ana Humphrey won a $250,000 prize for calculating the potential for finding more planets outside our solar system. Credit: NASA GSFC.

Indeed, as Quintana points out, systems like Kepler-186 show a large gap that exists between the four planets close in to the star and the outer planet. Another world the size of Earth could be there on an orbit inclined enough that we would not see it. Extend this over the range of multi-planet systems found thus far and there is ample room for additional discovery. Humphrey’s model manipulates the possible space between the hypothetical planet and its neighbors, to see what worlds of varying mass could be present without disrupting their orbits.

This could come in handy for TESS, which as we saw yesterday, is already producing planetary discoveries like TOI-197. Applying the new model to the exoplanet database being assembled by TESS would allow both it and future missions to predict systems in which hidden planets might be found. Such systems might then be studied both by transits and other methods.

In examining such questions, Quintana and Humphrey are simply extending a time-honored method of planetary discovery, one that led Johann Gottfried Galle, working with calculations from Urbain Le Verrier, to discover Neptune in 1846 (and yes, Neptune was observed before this but was not known to be a planet). The mathematical calculations that produced Neptune as planet captured the imagination of François Arago, who said that Le Verrier had discovered a planet “with the point of his pen.” Thus does one world grow out of another — it was data on Uranus and the irregularities of its orbit that led to our learning the true nature of Neptune.

Remarkably, Triton was discovered a mere 17 days after the discovery of Neptune, another case of data cascade. Applying the same concept to exoplanets has been a natural progression. We can actually see only a few such worlds through direct imaging. Fine-tuning our models to fit the methods and instruments at hand maximizes the opportunity to enlarge our catalog.

The paper is Kaltenegger et al., “TESS Habitable Zone Star Catalog,” Astrophysical Journal Letters Vol. 874, No. 1 (26 March 2019). Abstract / preprint.