By Alex Masegian, Fall 2020.
We are in the midst of a golden age of exoplanet science. Since the discovery of the first exoplanets in 1995, the field has grown in leaps and bounds, with the number of confirmed exoplanets nearly doubling every 27 months [2]. Of particular interest to astronomers are habitable exoplanets, or planets orbiting stars other than our Sun that may be able to host life. Identifying these habitable exoplanets could significantly narrow our search for extraterrestrial life; unfortunately, defining what makes a planet habitable is not a straightforward process. Typically, a planet is considered habitable if it has Earth-like features and is orbiting within its star’s habitable zone, a region of space that is at just the right distance from the star to be conducive to life. However, recent studies have suggested that this Earth-centric definition of habitability is flawed [3, 6]. Not only might it be possible for exoplanets that don’t fit either of the previously-mentioned criteria to host life, but there may exist exoplanets that are even more suited to hosting life than Earth: “superhabitable” exoplanets.
In order for an exoplanet to be considered superhabitable, it must be able to support both a greater quantity and greater diversity of life than Earth. It may be surprising, therefore, that an exoplanet can be classified as superhabitable with planetary parameters that are only slightly different from those of Earth. Take, for example, the amount of water that a planet has. Water is one of the most important building blocks for life as we know it, acting as the primary solvent for the vast majority of life processes [1]. A higher water content is beneficial because it allows for increased biodiversity. For example, the tropical rainforests of Earth cover only about 6% of the planet, but they are home to more than half of all known species [4]. The distribution of a planet’s available water is just as important: biological diversity and biomass are highest in shallow-water areas, so a highly regular distribution of land and water that creates many shallow pools would be ideal for life to thrive. An exoplanet with slightly more water and more regularly-distributed landmasses than Earth, therefore, could quite easily be superhabitable.
Of course, having more regularly-distributed land is only one way to increase the number of places that can host extremely diverse ecosystems. The other possible solution is even simpler: why not just have more land? Terrestrial planets can only grow so big, but a planet that is bigger than Earth would offer more surface area for life to develop on. If the land is distributed properly, the planet would be able to host even more of the aforementioned shallow pools that are great for fostering biodiversity. Larger planets also tend to have larger amounts of interior heating, allowing them to host life for a longer period of time than smaller planets that may lose their internal heat before life can fully develop. However, we must be careful not to classify every terrestrial planet bigger than Earth as superhabitable; planets that are larger than about five Earth radii are less likely to have an active plate tectonics system, which could be a limiting factor to the development of life. Here on Earth, plate tectonics are the primary mechanism that regulates atmospheric CO2 levels and keeps the planet warm. A planet without active plate tectonics would likely be too cold for life to form [3]. Based on this knowledge, scientists currently estimate that a planet slightly bigger than Earth, with a radius of around 1.6 Earth radii, may be ideal for the formation and maintenance of life in a way that aligns with the current concept of superhabitability. These planets would have more surface area and thus more room for diverse life to develop while also maintaining the vital plate tectonics systems that allow life to thrive [6].
Factors beyond the characteristics of planets themselves can also affect habitability, with the most notable influence being the characteristics of the star that a planet is orbiting. Our Sun is what is known as a G dwarf star, a class of star that lives for about ten billion years. Considering that it took about seven billion years for our current civilization to emerge on Earth, it is possible that life on planets orbiting other G stars may simply run out of time before it can surpass Earth in terms of biomass and biodiversity. As a result, it seems more ideal for habitable planets to orbit stars that are cooler, smaller, and fainter than our Sun. The longer lifespans of these stars would provide a more stable environment for the evolution of life, thus making it more likely that exoplanets in orbit around them could be classified as superhabitable [3]. Based on this, scientists have concluded that K dwarf stars, which are a step down from our Sun in terms of size and temperature and have lifespans of about 30 billion years, may be ideal targets in the search for superhabitable exoplanets. [6]
For the most part, the characteristics of superhabitable planets are still theoretical. Exoplanets are small, far away, and very faint, which often makes directly observing them extremely difficult. Because of this, there are currently no confirmed superhabitable exoplanets, though this will likely change in the next few decades as new instruments for studying exoplanets are launched. The James Webb Space Telescope, for example, will be equipped with the instrumentation necessary to study the atmospheric composition of nearby exoplanets in detail [5], which will likely lead to a wealth of data for superhabitability studies. In the meantime, astrophysicists have already made significant progress on identifying candidates for superhabitable exoplanets by examining the data that we do have access to. Heller and Armstrong suggested in 2014 that Alpha Centauri B, one of the closest stars to our Solar System, may be a viable candidate for hosting superhabitable exoplanets due to its status as a K dwarf star and the recent identification of an Earth-like exoplanet orbiting it [3]. A recent study published in October 2020 went slightly further by presenting a sample of 24 candidate superhabitable exoplanets that fit the criteria of being less than two Earth radii in diameter and orbiting in the habitable zone of a K dwarf star [6]. In order to definitively identify superhabitable exoplanets, in-depth follow-up observations of these candidates (and others!) will be necessary. At the current rate that the field of exoplanet science is progressing, however, this information may soon be within our grasp.
Although we have yet to conclusively identify superhabitable exoplanets, the recent work in this area highlights the importance of shifting how we define habitability, especially with regard to the search for extraterrestrial life. Earth may not be the exemplar when it comes to defining what makes a planet habitable; there are a variety of factors, such as the ones discussed in this article and many more besides, that could result in greater biomass and biodiversity on one of the countless alien worlds that exist throughout the galaxy (and likely throughout the universe). Though several of the characteristics of superhabitable exoplanets are similar to characteristics already possessed by Earth, they differ in important ways. Earth could turn out to be a marginally habitable world in the grand scheme of things, which would completely overturn our current perception of habitability. Though this hypothesis will remain unfounded until our instrumentation advances enough to directly measure definitive indicators of superhabitability, it is an important consideration that astronomers and the general public alike should keep in mind as the search for extraterrestrial life continues.
[1] Ball, P. (2017). “Water is an active matrix of life for cell and molecular biology.” Proceedings of the National Academy of Sciences of the USA, no. 114 (51). https://doi.org/10.1073/pnas.1703781114.
[2] Batalha, N. (2014). “Exploring exoplanet populations with NASA’s Kepler Mission.” Proceedings of the National Academy of Sciences of the USA, no. 111 (35). https://doi.org/10.1073/pnas.1304196111.
[3] Heller, R. and Armstrong, J. (2014). “Superhabitable Worlds.” Astrobiology, no. 14 (1). http://dx.doi.org/10.1089/ast.2013.1088.
[4] Johnson, H. (2015, May 11). “Resource Library Encyclopedic Entry: Rainforest.” National Geographic. https://www.nationalgeographic.org/encyclopedia/rain-forest/.
[5] Pulliam, C. (2020, February 5). “NASA’s Webb Will Seek Atmospheres around Potentially Habitable Exoplanets.” NASA. https://www.nasa.gov/feature/goddard/2020/nasa-s-webb-will-seek-atmospheres-around-potentially-habitable-exoplanets.
[6] Schulze-Makuch, D. et al. (2020). “In Search for a Planet Better than Earth: Top Contenders for a Superhabitable World.” Astrobiology, no. 20 (12). https://doi.org/10.1089/ast.2019.2161.