Title: Habitable Zone and Atmosphere Retention Distance (HaZARD)
Authors: Gwenaël Van Looveren, Sudeshna Boro Saikia, Oliver Herbort, Simon Schleich, Manuel Güdel, Colin Johnstone, and Kristina Kislyakova
First Author’s Institution: Department of Astrophysics, University of Vienna, Vienna, Austria
Status: Published in Astronomy and Astrophysics [open access]
The search for habitable exoplanets is one of the most exciting frontiers in modern science and could, in time, provide us with an answer to the age-old question of whether we are alone in the Universe. As technology improves, the dream of finding planets capable of supporting life has become more tangible than ever before. The James Webb Space Telescope (JWST), with its advanced capabilities, is at the forefront of this quest, offering unprecedented insights into the atmospheres of exoplanets and bringing us closer than ever to understanding their potential to host life.
While life elsewhere in the Universe could, in principle, be radically different from anything on Earth, the focus for JWST remains on planets that share similarities with our own– small rocky worlds existing in what is known as the habitable (or Goldilocks) zone. Here they are neither too close nor too far from their host star but rather at exactly the right distance that liquid water may be present on the surface. (And water is important for life!)
However, a prerequisite for liquid water on the surface is an atmosphere. Much like how excessive sunlight can cause sunburns on our skin due to ultraviolet radiation, the magnetic activity of a star, in the form of high-energy X-ray and ultraviolet (XUV) radiation and stellar winds, can erode a planet’s atmosphere over time. Thus, a spot in the habitable zone does not guarantee that a rocky planet still has an atmosphere, even if it used to have one in the past. It is this issue that brings us to today’s paper.
Balance your gains and losses
Many of the rocky planets found so far have been around low-mass stars. A number of these have been observed on close orbits around M-dwarfs (i.e. stars with a mass of 60 percent of the Sun or less). This is not surprising as these stars are among the most common in the Galaxy. Additionally, scientists detect planets orbiting around stars by either observing periodic dips in the star’s brightness or by measuring the tug of the planets as they orbit. For a given planet’s size, smaller stars mean bigger dips in brightness and if the planet is more comparable in mass to the star it will make it wobble that much more.
However, the short orbital distances from their host star mean that the planetary atmospheres are subject to increased mass loss. In this paper, the authors use a model for planetary atmospheres that takes into account heating from the host star’s XUV spectrum, cooling due to infrared emission, and thermal conduction. They use this model to calculate a lower limit for the mass loss rate due to Jeans escape and then define a minimum orbital distance where the atmosphere of an Earth-mass planet, consisting of varying amounts of carbon dioxide (CO2) and nitrogen (N2), would survive over hundreds of millions of years. They call this distance the atmospheric retention distance, or ARD.
Why carbon dioxide and nitrogen? While any primary atmosphere that the planet formed with is very likely lost quickly in the star’s more active youth, the constituents for another can hide in the mantle and be outgassed over time, producing another atmosphere. (Much like how children lose their first set of teeth before developing their adult teeth.) During the early planetary life, outgassing through a global magma ocean, and in particular, its solidification, can lead to the buildup of a secondary atmosphere. In fact, rocky planets can outgas enough gas to make a whole new atmosphere over longer timescales just through volcanism alone if the atmosphere has been lost previously. As an example the current outgassing rate of all volcanism on Earth is around 300 million tonnes of CO2 each year (which is incidentally around the same amount of plastic waste produced globally or the approximate weight of every human on Earth). While this at present is kept in balance by weathering and lost to space by solar winds, it does represent enough CO2 to fill the entire atmosphere in 10 million years! As for the other gas nitrogen, delivery to the atmosphere can also occur through volcanism in small amounts, usually less than 1% of all outgassed material. (For present-day Earth, the nitrogen cycle is inextricably tied to the biosphere, A.K.A. life!)
These atmospheric compositions also represent common atmospheric compositions found around the rocky bodies in the Solar System and, at least in the case of Earth, we do have some evidence for an atmosphere as far back as geological records can take us.. This outgassing may sustain an atmosphere on rocky planets for billions of years before the planet becomes geologically dead.
Beyond the atmosphere retention distance
In Figure 1, the authors compare the ARD to the extent of the habitable zone for the Solar System. Above the coloured lines–each representing a different atmospheric composition– the mass loss is less than the outgassing. Below the lines, the outgassing cannot keep pace and the entire mass of the Earth’s atmosphere is lost in less than 10 million years, signifying that the planet fails to retain a (significant) atmosphere.
The authors note that although the Earth falls outside the ARD for any type of atmosphere for the first 25 million years and likely experienced a high mass-loss rate for the next 100 million years after, this period coincided with a time when the outgassing rate was probably higher as the crust solidified, possibly re-melted during the theorised Moon-forming impact, and then solidified again. As the age of the Sun increases, the more volatile nitrogen-dominated is able to survive at the Earth-Sun distance of 1 astronomical unit (AU).
Figure 1: The figure shows the distance at which an atmosphere of different compositions (coloured lines) can be retained for a planet of similar mass to Earth orbiting a sun-like star. Moving upwards on the vertical axis indicates a greater distance to the Sun. Above the lines, the atmosphere is retained as the distance becomes great enough that the atmosphere isn’t lost faster than it can be replenished. At smaller distances, the escape rate is greater than the outgassing rate which makes it unlikely that the planet can retain a significant atmosphere. At greater distances, however, the planet gets to keep its atmosphere over long timescales. The green shaded region represents the habitable zone, while the two horizontal dashed lines represent Venus (bottom) and Earth (top). Figure 2 in the paper.
The authors performed this analysis–of changing the composition of the CO2 and N2 in the atmosphere– for various stellar masses, and the results are shown in Figure 2. Here, the authors note that especially young stars below 0.4 solar masses with their increased XUV luminosity tend to push the ARD way beyond the habitable zone. In fact, for stars below 0.3 solar masses, the mass-loss rate remains orders of magnitudes (at least 10 times) greater than the outgassing for more than 2 billion years, leading them to speculate whether any close-orbiting planets would ever stand a chance of retaining any atmosphere at all.
Figure 2: The habitable zone and atmospheric retention distance (ADR) are shown for different masses of stars and at different ages. As the star ages and settles down, the ARD (coloured lines) shifts. The region to the right of the lines represents the distance from the star at which a planetary atmosphere can be retained. Figure 3 in the paper.
To bring us back to where it all began, the authors use the last panel of Figure 2 as a best-case scenario and add the currently scheduled stellar-planet targets for JWST. As can be seen in Figure 3, none of the planets lie beyond the ARD, even though several are located in their host star’s habitable zone. This doesn’t completely rule out the possibility of an atmosphere around these planets, but the models suggest that it is unlikely that they have atmospheres.
Figure 3: This figure shows the habitable zone (green) and atmosphere retention distance (ARD) for different stars at the age of 5 billion years (same as panel 5 in Figure 2). Each symbol represents scheduled planetary targets for the James Webb Space Telescope. All targets, including the ones in the habitable zone such as TRAPPIST-1 f, g and h, are too close for them to retain even a nearly pure CO2 atmosphere as predicted by the authors. Figure 4 in the paper.
Since there is a certain degree of uncertainty, the authors are adamant that their results can only be used to help inform on the likelihood of different atmospheres surviving. The Jeans escape represents a minimum level, and there are several other processes that can add to the mass loss rate for a planetary atmosphere. At low masses, flares are more common, and these might significantly change the mass loss rate of the atmosphere. At the same time, outgassing is highly dependent on the geology of the planet and not the only way material that may be used to build an atmosphere can be delivered.
The authors do, however, try to set a lower limit so that their models might be able to exclude certain combinations of stellar mass, orbital distance, and atmospheric compositions that are very unlikely to be stable in the long term. With the scheduled observations from JWST due soon, the detection (or lack thereof) of a planetary atmosphere around one of these rocky worlds could provide valuable insights into the durability of exoplanet atmospheres. This, in turn, could guide future searches for potential signs of life.
Astrobite edited by Mckenzie Ferrari
Featured image credit: NASA, ESA, Joseph Olmsted (STScI)
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