Motivation
The development of the human mind begins in wonder, and few things elicit wonder as readily as the beauty of the night sky. It is deeply regretful that access to the stars has become a luxury, and protecting dark skies where they remain should be as much of a priority as protecting beautiful monuments down on Earth. It is not only the beauty of the stars that makes them wondrous, however. We now know that almost all stars are worlds of their own, exo-Suns surrounded by their own extra-solar systems. One cannot help but wonder if one of these worlds are looking back at us. This possibility is one motivation for why we have been exploring what it takes to make a habitable planet and how likely such habitable planets are in our Galaxy.
A second motivation is our desire to understand our own history, and therefore the origins of our own planet, including the physical and chemical processes that conspired to make the Earth a habitable planet, and eventually an inhabited one. Combining these two puzzles – the likelihood of habitable planets outside of the Solar System and the origin of the Earth – enables us to better understand who we are as a planetary species, and how we fit into the galactic ecosystem.
I do not think it is by chance that grasping at these scientific truths has been a source of revelation of the beauty of our Galaxy. Through the development of new telescopes operating at wavelengths both shorter and longer than the human eye can see, engineers and astronomers have together unveiled the previously unseen beauty of interstellar clouds, protostars and planet-forming disks where new solar systems are currently assembling. These images seem to universally attract us, and through them astronomers are perhaps providing a source of peace, and a small compensation for the night sky that so many of us have lost. This paper is about how we use such astronomical observations to explore how to make a habitable planet.
What is a Habitable Planet?
There is no consensus on how to define a habitable planet [e.g. 7]. Most definitions of habitability, however, presume life to be water-based, as it is here on Earth. It is difficult to imagine the development of a complex enough chemistry to generate an origins of life without having access to a solvent, a liquid within which chemical reactions can take place and more complex chemical products can accumulate. Water is a unique solvent in its ability to dissolve a large range of inorganic and organic compounds. Therefore, while acknowledging that water is not the only conceivable solvent for organic chemistry, in this contribution we will consider access to water one of the criteria of habitability. This criterion has two parts: 1) that the planet has the right temperature to maintain liquid water, and 2) that there is liquid water present. In the Solar System this currently applies to the Earth and some of the moons in the outer Solar System, while in the past it likely applied to Venus and Mars as well.
A second criterion for planetary habitability is access to organic and inorganic building blocks of biomolecules, i.e., the dissolution of such building blocks in the planet’s water reservoir. This criterion presumes that life in the Universe builds on organic chemistry, which is motivated by the chemical complexity enabled by carbon bond formation. There is evidence in the Solar System record [e.g. 2] that several planets and moons accreted a considerable organic reservoir, and would fulfill this criteria as well.
A third, and more controversial, criterion is that only planets with access to dry continents and reasonably transparent atmospheres are hospitable to origins of life. These criteria are based on origins of life scenarios that make use of UV chemistry in surface water [10], and would reduce the set of habitable planets to those that are truly Earth-like, that is, rocky planets with some, but not too much water, orbiting UV-luminous stars like our own. It would exclude so-called water worlds, moons and planets with subterranean oceans, and planets around cool stars that do not emit substantial amounts of UV. In this paper we are mostly agnostic to the particular origins of life scenario, and will focus on how often planets form with access to water and organics.
Planet Formation
The formation of a planetary system begins with the collapse of an interstellar cloud that consists of gas and dust [13]. Most of the collapsing cloud goes into forming a protostar. Due to preservation of angular momentum, the collapse also produces a rotating disk around the young star. In the disk, planet formation begins by the coagulation of small dust grains to form pebbles and boulders. These then combine to form planetesimals, comet and asteroid-sized bodies, which collide to build up the planet core. The composition of the dust grains therefore determines the chemical composition of the solid part of a planet. These solids are mainly composed of metals and rock-like material in the inner part of the disk, which is hot due to the proximity to the star, and of a combination of metals, rock and ice in the outer cooler disk. Earth-like planets form from dry dust grains, while gas and ice giants, and comets form from ice-rich grains. Once the planet core is formed, the planet can obtain water and organics through accretion from the disk gas, outgassing from the rocky core, and delivery from comet and volatile-rich asteroid impacts. This all presumes that the disk does indeed contain water and organics, which we have not yet established.
During the past few years we have been able to obtain images of planetary systems in the making. Such images reveal young stars surrounded by disks of dust and gas in which planets are assembling. While we can rarely detect the planets directly, the forming planets leave their clear marks on the disk in the form of dark lanes where they have accreted the dust and gas in their orbits. Importantly for this paper, planetary systems are currently being made, which means that it is, in theory, possible to answer how often they are being made conducive to habitability. In other words, do planets typically form with access to water?
Water in Planet-Forming Disks
The conceptually most straightforward path to explore whether planets typically form in the vicinity of water would be to use one of our telescopes to observe water in planet-forming disks. This is, however, technically challenging due the presence of water in the Earth’s atmosphere. Instead, much of our evidence for water in these disks is indirect and takes into account the environment within which these planet-forming disks assemble.
Astronomical observations of interstellar clouds and protostars have revealed that water is one of the most common molecules present during the early stages of solar system formation. This should perhaps not be surprising since it is made out of hydrogen, the most abundant element in the Universe, and oxygen, the third most abundant element. Still, it is far from obvious that the exotic chemistry that characterizes the very cold and low-density interstellar clouds should conspire to produce this familiar molecule. It turns out that cold gas-phase chemistry is quite inefficient at producing water, and it is only because of the presence of interstellar grains and the associated grain-surface chemistry that large amounts of water are produced at the onset of star formation [14].
There are several pieces of evidence that this water becomes incorporated into the disk and further into forming planets, and that Earth’s water is interstellar. In the Solar System, all water contains an excess of heavy water compared to what is expected from the cosmological abundance of deuterium. This kind of deuterium enrichment is a tell-tale sign of low-temperature (typically <30 K) water formation in interstellar clouds [5]. The inference that the Solar System planets formed in a water-rich environment due to inheritance of water from the molecular cloud phase makes it exceedingly likely that other disks also inherited water from their birth cloud and therefore that planets generally form in water-rich environments.
This inference is confirmed by the Spitzer Space Telescope’s observations of water vapor in the innermost regions of many planet-forming disks [4, 11], close to where habitable planets may form. Furthermore, water ice has also been detected towards a handful of disks, with the special geometry required for ice absorption spectroscopy [1]. The typical water abundance in these disks is currently not well constrained, but the newly deployed James Webb Space Telescope (JWST) has the sensitivity and instrumentation to achieve exactly this, and we are currently eagerly analyzing the first data from JWST.
In conclusion, all evidence is currently pointing towards that planets form in water-rich environments and hence likely form with substantial water inventories. This is good news, since for a planet to be habitable it needs sufficient amounts of water to sustain a water-based chemistry. However, it is probably preferable to not have so much water that there is no dry land, i.e., to avoid it being a water world. This entails that an Earth-like planet needs to primarily form from dry dust grains that, at most, contain small amounts of water – on Earth the ocean contributes less than a permille of the total Earth mass. Whether this fine-tuning of water delivery is common or not remains to be seen, and can probably only be demonstrated by direct observations of the atmospheres of Earth-like planets.
Organic Chemistry During Planet Formation
Given that Earth-like planets regularly have access to water during their formation, do they also acquire the right kinds of organic and inorganic material to be chemically habitable? Molecular clouds do contain many of the organic molecules that are considered precursors to biomolecules, including nitriles, alcohols, aldehydes, and organic acids [e.g. 6]. There is some evidence from Solar System studies that these organics, analogous to water, survive disk formation. Our strongest evidence for the nature of the organic environment within which planets assemble comes from direct astronomical observations of organics in protoplanetary disks, however. Infrared spectroscopy has detected hot organic gas in the innermost disk regions, revealing a disk gas rich in nitriles and acetylene, just interior or terrestrial planet formation [4, 12]. It is currently unclear if this organic gas is also present in the terrestrial planet-forming region and therefore whether terrestrial planets accrete primary atmospheres that are rich in reactive organic molecules, and future observations with JWST and other telescopes are needed to explore this. If confirmed, this would imply that young Earth-like planets generally obtain a substantial organic inventory from birth.
A second source of organics for terrestrial planets is delivery from impacting planetesimals from the outer disk, where organic solids are abundant due to either inheritance from the interstellar medium or organic chemistry in the disk. The outer disk organic chemistry can be observed at millimeter wavelengths, which probes rotational transitions from colder molecules. These observations have demonstrated that there is indeed survival of interstellar organics and that we therefore should expect planetesimals assembling in outer disk regions to generally be rich in the same organics (generally oxygen rich) that are common in the interstellar medium [3].
Inheritance is not the only contributor to the organic inventory in the outer comet-forming parts of disks, however. Millimeter observations have also revealed large abundances of nitriles and other reduced forms of carbon that are implicated in origins of life chemistry [9]. These form through gas and grain surface chemistry in disks once most of the oxygen has become locked up in water ice and oxygen-rich organic ices. This oxygen-poor organic chemistry can become directly swept up in the atmospheres of planets forming in the outer parts of disks, but more importantly for the formation of habitable planets, they can freeze-out on grains and become incorporated in comets. These comets can then deliver a combination of oxygen-rich organic acids and alcohols, and oxygen-poor nitriles and carbon chains to young terrestrial planets, seeding their surfaces with a range of prebiotically interesting organics [8].
Concluding Remarks
The key ingredients to make a habitable planet are water and organics, and based on astronomical observations both are commonly available to planets as they are forming. We should therefore expect to find that many extra-solar planetary systems contain chemically habitable planets. Whether all or any have gone from habitable to inhabited is a much bigger question that hopefully future astrophysical, chemical, and biological research will answer; even if potentially habitable planets are common, planets where life has been realized may be quite rare. Our own habitable planet made that step from habitable to inhabited about 3.5-4 billion years ago, resulting in an incredibly beautiful world, and if there is anything that can compete with the stars in elicit wonder, it is turning our gaze back on the Earth and realize how contingent its beauty is, and how many things had to go right to make this habitable planet of ours.
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