Centauri Dreams

Given our intense scrutiny of planets around other stars, I find it interesting how little we know even now about the history of our own Sun, and its varying effects on habitability. A chapter in an upcoming (wildly overpriced) Elsevier title called The Archean Earth is informative on the matter, especially insofar as it illuminates which issues most affect habitability and how the values for these vary over time. It’s also a fascinating look at changing conditions on Venus, Earth and Mars.

We know a great deal about the three worlds from our local and planetary explorations, but all too little when it comes to explaining the evolution of their atmospheres and interior structures. But it’s important to dig into all this because as Stephen Kane, director of The Planetary Research Laboratory at UC-Riverside and colleagues point out, we seem to be looking at the end state of habitability on both Mars and Venus, meaning that our explorations of these worlds should yield insights into managing our own future. Powerful lessons are likely to emerge, some obvious, some obscure.

The chapter title is “Our Solar System Neighborhood: Three Diverging Tales of Planetary Habitability and Windows to Earth’s Past and Future” (citation below), from which this:

Resolving the numerous remaining questions regarding the conditions and properties of our local inventory of rocky worlds is… critical for informing planetary models that show how surface conditions can reach equilibrium states that are either temperate and habitable, or hostile with thick and/or eroded atmospheres.

Given the challenges of working at the Venusian surface, Mars is particularly valuable as a case study, a world that has evolved through a period of habitability (is this true of Venus?) and undergone interior changes including cooling, not to mention loss of atmosphere. The more we learn about the evolution of rocky worlds, the more we will be able to apply these discoveries to planets around other stars. We’re in search of models, in other words, that show how planets can reach equilibrium as habitable or hostile.

The Archean eon (roughly 4 to 2.5 billion years in extent) takes us through the formation of landmasses, Earth’s early ‘reducing’ atmosphere (light on oxygen but rich in methane, ammonia, hydrogen carbon dioxide) and all the way to the emergence of life. So we have plenty to work with, but for today I want to focus on one aspect of habitability that we haven’t discussed all that often, the changes in the Sun since Earth’s formation. Our G-class star had to progress through a protostar stage lasting, perhaps, 100,000 years or more, then a T-tauri phase likely lasting 10 million years. There’s a lot going on here, including shedding angular momentum as the young Sun expels mass, slowing its rotation rate and producing a powerful solar wind.

The Sun’s arrival on the main sequence follows, and we’re immediately involved in a singularly vexing astronomical puzzle. The consensus seems to be that the early main sequence Sun was only about 70 percent as bright as the star we see today. That being the case, how do we account for the fact that water appears early in Earth’s history in liquid form on the surface? The expectation of a frozen world is a natural one, yet liquid water at the surface seems to have been available for some four billion years.

Image: The landscape during the early Archean could have looked like this: small continents on a planet mostly covered by oceans, illuminated by the much fainter young Sun, and with a Moon that appears larger in the sky because of its smaller distance to Earth. Credit: Potsdam Institute for Climate Impact Research.

Infant stars in the T-tauri phase are quite bright, getting their energy primarily from gravitational contraction. When they reach the main sequence and hydrostatic equilibrium sets in, they are considerably dimmer. The Sun enters the main sequence undergoing stable hydrogen fusion, which drives its luminosity. The notion that the Sun was dimmer early in the main sequence derives from models showing a solar core that is less dense and contains less helium. As core density increases as hydrogen is converted to helium, the resultant heating accounts for the steady brightening since.

Carl Sagan was an early voice in exploring the ‘faint young Sun’ problem in relation to liquid water and the matter is still controversial, although a number of possibilities exist. The authors believe the logical way to explain why a fainter star could provide conditions for liquid water on the surface is through the presence of greenhouse gases like methane and carbon dioxide, although the influence of methane is problematic unless, as in recent papers, scientists invoke planetary impacts or a primitive photosynthetic biosphere that amplifies the methane cycle. Whatever the case, mechanisms to regulate habitability are clearly crucial. From the chapter:

It is remarkable that Earth seems to have become habitable within its first few hundreds of millions of years and maintained that state seemingly without interruption to the present. This persistence is a testimony to the power of feedbacks that drive and regulate climate evolution with notable success on Earth in comparison with our Solar System neighbors Venus and Mars. Most important is the silicate weathering negative feedback, whereby rising temperatures coinciding with a warming Sun are offset by increasing rates of CO2 consumption through continental weathering, which is enhanced under warmer, wetter conditions. And we can thank diverse plate tectonic processes and their roles in such carbon cycling for this sustained thermostatic capacity over much of our history.

But let’s return to that very early Sun emerging onto the main sequence. Factors affecting a young star’s luminosity include the rotation rate of the star, and here we have little information (the Sun’s rotational rate in this era is uncertain, although some evidence suggests that it was a slow rotator). But the earliest planets could have been pummeled by radiation in this early state for tens or even hundreds of millions of years, which would have had a dire effect on any atmospheres they had formed. Note this:

…the intensity of the emission depends on the rotation rates of the stars, which are highly variable (from observation of stars in <500 Myr old stellar clusters). “Fast rotating” young stars can reach about a hundred times the rotation rate of the modern Sun, whereas moderate rotators could rotate at about ten times that rate and slow rotators would reach a few times the Sun’s rotation rate (Tu et al., 2015). Corresponding luminosities for young stars in the EUV and X-ray range of the spectrum can reach on the order of a 1000 times present mean solar luminosity at similar wavelengths for fast rotators, a few 100 times for moderate rotators and between a few tens to a hundred times the reference for slow rotators.

We’ve looked before at both the solar wind and solar flares as factors influencing planetary atmospheres and interacting with their magnetic fields, perhaps stripping atmospheres entirely in the case of M-class planets orbiting tightly around their primary. Atmospheric escape takes two forms, the first being thermal loss, where atoms are accelerated to the point where they reach escape velocity. Intense radiation is one but not the only way of doing this, but the effect on an atmosphere can be dire.

Other processes that can adversely affect an atmosphere include ions being accelerated along magnetic field lines by the solar wind or photochemical reactions. You would think that these effects would vary with distance from the star, but as the authors note, the loss estimates for Mars, Venus and Earth are relatively similar, and this despite the major differences in the makeup of the atmospheres on these planets and their magnetic field situation. Mars lacks a protective magnetic field today but seems to have had one in the past. Venus likewise lacks an intrinsic magnetic field like that produced on Earth through its convective dynamo, although interactions between the solar wind and the CO2-rich atmosphere do produce a weak magnetic effect.

Image: Diagramming a solar flare encountering Earth’s magnetosphere. Credit: Jing Liu (Shandong University, China and National Center for Atmospheric Research in the U.S).

As to the Sun, variations in the solar magnetic field over time are assumed given that a faster spinning star would have produced higher levels of magnetic activity. Today’s solar cycle, eleven years long, is the result of the magnetic reversal cycle of the Sun. The twists and tangles of magnetic field lines during this cycle cause sunspot activity to peak and then decline as the north and south magnetic poles reverse. The Sun’s magnetic field strengthens and weakens during the cycle, with effects apparent in the solar wind itself and in spectacular events like coronal mass ejections.

If we back out to the largest timescales, the variations in the Sun’s magnetic field indicate a much stronger field early in the Sun’s evolution, coupled with a (presumed) faster stellar rotation rate. From the standpoint of habitability, that tells us that ultraviolet and X-ray flux along with the solar wind has been decreasing with time. We’re probably talking about orders of magnitude difference in UV and X-ray radiation, but the authors point out that the intensity of emission depends upon the rotation rate of stars, and there is much we still have to learn about both this and luminosity.

The high degree of uncertainty on the Sun’s rotation rate and temperature affect our estimates of the ultraviolet at X-ray emissions our star would have produced, and thus the impact of this flux on the atmospheres of the terrestrial planets. Enlarging our field of view to exoplanets, it’s clear that the more we can piece together about the Sun’s early history, the more we can learn about the atmospheres and habitability of planets around the various spectral types of stars. Kane and team see the Sun as a template for planetary evolution, which makes heliophysics crucial for the study of astrobiology.

This study, then, homes in on the factors that need to be clarified as both heliophysics and exoplanetary science move forward. Even the closest planet to us presents formidable challenges, and the authors’ discussion of Venus is well worth your time. Here we have a world whose young surface – relatively uniform in the range of 250 to 1000 million years old – makes digging into its more remote past all the more difficult. Evidence can only be indirect at this stage of planetary exploration. We’re lucky to have Mars within reach as we widen the study of planetary formation to worlds other than our own.

The chapter in the Elsevier book is Kane et al., “Our Solar System Neighborhood: Three Diverging Tales of Planetary Habitability and Windows to Earth’s Past and Future,” available as a preprint. Two Sagan papers on the faint young Sun paradox are notable: Sagan & Mullen, “Earth and Mars: Evolution of Atmospheres and Surface Temperatures”, Science 177 (1972), 52 (abstract) and Sagan & Chyba, “The early faint sun paradox: Organic shielding of ultraviolet-labile greenhouse gases,” Science 276 (1997), 1217 (abstract). You might also want to look at Ozaki et al., “Effects of primitive photosynthesis on Earth’s early climate system,” Nature Geoscience 11 (2018), 55-59 (abstract), which offers up a methane solution for the faint young Sun.

Over the weekend I learned about Joseph Haydn’s Symphony No. 47, unusual in that it offers up some of its treasures in perfect symmetry. Dubbed ‘The Palindrome,’ the symphony’s third movement, Minuetto e Trio, is crafted to play identically whether attacked normally – moving forward through the score – or backwards. You can check this out for yourself in this YouTube video, or on this non-auditory reference.

The pleasure of unexpected symmetry is profound, and when seen through the eyes of our spacecraft, can be startling. Consider the storied star Vega. We see this system from our perspective at a very low inclination angle relative to its rotational axis, as if we were looking down from above the star’s pole. This face-on perspective is profoundly interesting when examined through our space-borne astronomical assets. In the image below, we get two views of Vega’s disk, from Hubble and then JWST.

Image: The disk around Vega as seen by Hubble (left) and Webb (right). Hubble detects reflected light from dust the size of smoke particles largely in a halo on the periphery of the 160-billion-kilometer-wide disk. Webb resolves the glow of warm dust in a disk halo, at 37 billion kilometers out. The outer disk (analogous to the solar system’s Kuiper Belt) extends from 11 billion kilometers to 24 billion kilometers. The inner disk extends from the inner edge of the outer disk down to close proximity to the star. Credit: NASA, ESA, CSA, STScI, S. Wolff (University of Arizona), K. Su (University of Arizona), A. Gáspár (University of Arizona).

We don’t, of course, have the perfect symmetry of the Haydn minuet and trio here, but do notice how smooth this disk appears, and particularly take note of the lack of any embedded planets, the sort of large objects we observe in formation around stars like Beta Pictoris. This bright star, a summer object in Lyra for those of us in the northern hemisphere, was well studied in the infrared by astronomers using the Spitzer Space Telescope, and now we have the clearest view ever, the subject of two upcoming papers in The Astrophysical Journal. Co-author Andras Gáspár (Steward Observatory, University of Arizona, calls the Vega disk “ridiculously smooth.”

The view shows layered structure, with Hubble detecting dust on the outer regions of the disk and Webb’s infrared view resolving the warmer dust in the inner disk, the disk in this region being sand-sized as compared to the outer halo, whose particles are of the consistency of smoke. These are useful observations because they offer information on dust movement in circumstellar disks, and also point out the stark difference among the planetary disks thus far observed. The hot, A-class Vega at 450 million years old remains a relatively young star and we would expect interactions among debris in the disk to keep replenishing it, a process that continues in our own much older system.

But where are the planets in formation? Lead author Kate Su (University of Arizona), says that the lack of them forces astronomers to rethink the way they’ve looked at forming planetary systems. It’s possible to tease out a gap at about 60 AU, but everywhere else the disk appears smooth, which rules out planets larger than Neptune in outer orbits. Adds Su:

“We’re seeing in detail how much variety there is among circumstellar disks, and how that variety is tied into the underlying planetary systems. We’re finding a lot out about the planetary systems — even when we can’t see what might be hidden planets. There’s still a lot of unknowns in the planet-formation process, and I think these new observations of Vega are going to help constrain models of planet formation.”

In the second of the two papers, lead author Schuyler Wolff (University of Arizona) and colleagues home in on the star Fomalhaut by way of comparison, this being a star that is a close twin to Vega in terms of luminosity, distance and age. As the paper notes, “[T]he Vega disk morphology differs significantly from Fomalhaut,” going on to say:

Vega’s fellow archetypical disk, Fomalhaut, has been extensively studied in scattered light (Kalas et al. 2005; Gáspár & Rieke 2020) and shows a narrow cold belt from 130-150 au (possibly confined by an undetected planet e.g., Boley et al. 2012) with a scattered light halo extending outwards from the belt. The parent planetesimal belt is clearly detected with ALMA (MacGregor et al. 2017), with an estimated dust mass of 0.015±0.010M⊕. Fomalhaut has also recently been the target of groundbreaking JWST observations (Gáspár et al. 2023) showing the same narrow cold belt (albeit with a slight radial offset indicative of grain size stratification) and halo.

Indeed, Fomalhaut appears to be building a planet within its debris disk, and possibly more than one, these objects acting as ‘shepherds’ shaping the disk. Any such worlds in the Vega system are clearly much smaller, if indeed they exist. Vega’s dust distribution is broad, whereas at Fomalhaut the outer debris ring is the dominant source of light available for our observation. Three nested debris belts appear at Fomalhaut.

Nothing better than a new astronomical puzzle, especially when it challenges our notions of planet formation. You may remember Vega’s role in Heinlein’s Have Spacesuit Will Travel (1958), where its civilization acts as a kind of overseer for Earth, and perhaps the ‘Vegan Tyranny’ from James Blish’s Cities in Flight series. For that matter, Vega has an important commercial role in Asimov (the source of ‘Vegan tobacco’ in the Foundation books), appears prominently in Jack Vance’s work, and is of course of great interest to Ellie Arroway in Carl Sagan’s book and film Contact. How enjoyable that it now throws a new mystery at us.

The papers are Su et al., “Imaging of the Vega Debris System using JWST/MIRI,” accepted at The Astrophysical Journal (preprint) and Wolff et al., “Deep Search for a scattered light dust halo around Vega with the Hubble Space Telescope,” accepted at The Astrophysical Journal (preprint).