7 unnecessary assumptions about life in the Universe

Here in 2024, thousands of planets are known beyond Earth.

What do planets outside our solar system, or exoplanets, look like? A variety of possibilities are shown in this illustration. Scientists discovered the first exoplanets in the 1990s. As of 2024, the tally stands at over 5,000 confirmed exoplanets. None are known to be inhabited, but a few raise tantalizing possibilities: largely among the Earth-sized planets, but not so much among the larger ones.

Although many have biologically-friendly ingredients, only Earth is known to harbor life.

Although NASA’s Perseverance rover landed on Mars in early 2021, it took over 400 Martian sols for Perseverance to encounter and photograph the parachute that allowed it to gently set down on the red planet’s surface. In this region, photographed in April of 2022, the numerous organic compounds have been found in the Martian soils where liquid water must once have been abundant. Organics, however, do not necessarily mean life, and the instrument suite aboard Perseverance is insufficient to draw such a conclusion. Either a follow-on, or sample return, mission will be needed to learn more.

However, many properties that Earth possesses may be unnecessary for life’s emergence.

Our notion of a habitable zone is defined by the propensity of an Earth-sized planet with an Earth-like atmosphere at that particular distance from its parent star to have the capacity for liquid water, without a cover of ice, on its surface. Although this describes the conditions that Earth possesses, it is unknown whether this is a requirement, or even a preference, of life. Many worlds assumed to be good candidates for life will likely be uninhabited; others not presently considered will likely surprise us down the line.

Here are seven common assumptions that we should be eager to challenge.

The Moon and clouds over the Pacific Ocean, as photographed by Frank Borman and James A. Lovell during the Gemini 7 mission. Earth, around our Sun, has the right conditions for life. We frequently assume that a large Moon, which stabilizes Earth’s axial tilt, is helpful or even necessary for life. But this assumption may be unfounded.

1.) Having a large Moon supports life.

About 50 million years after Earth formed, it was struck by a large, Mars-sized object named Theia. The aftermath of the collision superheated the Earth and kicked up an enormous amount of debris, a large fraction of which wound up forming the Moon. The remainder either escaped the Earth-Moon system or fell back onto one of the two bodies. The Moon’s large mass and close proximity to Earth stabilizes our rotational axis.

Formed from a giant impact, our Moon stabilizes our axial tilt.

The Earth presently rotates on its axis, which is inclined to the Sun at 23.44 degrees. Over time, the Earth’s axial tilt varies only slightly: from 22.1 to 24.5 degrees, as compared to planets like Mars which vary by more than 10 times as much. Our Moon stabilizes our axial tilt, but whether this is necessary or even beneficial to life has not been properly determined.

However, an unstable rotational axis may be neutral toward biological activity.

The Hawaiian islands, like most island arcs that form on Earth, initially arose as a mantle plume delivered material up to Earth’s surface by rising through the crust. Over time, the lava builds up to poke above Earth’s oceanic surface, and then, as the plate slides over so that the forming, growing mountain is no longer over the same hot-spot, a new island begins to form. Once a mountain has moved off of its hotspot, it can only erode, not grow any further. This provides strong evidence for Earth’s active lid tectonics; a property not presently seen on other planets in our Solar System.

2.) Plate tectonics are desirable.

This image shows a volcanic feature on Io, imaged up close by the Galileo spacecraft during 7 close encounters from 1995-2003. The yellowish color here is due to sulfur, while active lava flows can be seen unambiguously even from space. Tidally induced tectonics can theoretically support life just as well as plate tectonics can.

Strong tidal forces could release internal planetary energy just as easily.

The animation depicts a mapping of the positions of known near-Earth objects (NEOs) at points in time over the past 20 years and finishes with a map of all known asteroids as of January 2018. Despite how crowded a diagram such as this appears, the space between asteroids, on average, is enormous when compared to their actual sizes. The impact rate on Earth is dramatically increased, not decreased, by the presence of Jupiter.

3.) Nearby gas giants protect habitable planets.

At approximately 10:40 PM UT, an impact event occurred on Jupiter on September 13, 2021, which appeared as a brief, transient flash of white light near Jupiter’s equator. This marks the 9th impact on Jupiter observed since 2009. Although Jupiter receives more impacts than any other planet in our Solar System, it doesn’t protect Earth, but rather increases our planet’s collision rate by approximately a factor of 3.5 over a scenario where Jupiter didn’t exist.

Nearby, massive planets increase, not decrease, the impact rates on neighboring worlds.

Here in our own Solar System, a single star anchors the system, where inner, rocky planets, an intermediate-distance asteroid belt, and then more distant gas giant planets eventually give way to the Kuiper belt and Oort cloud. Only around stars that have formed with a large enough fraction of heavy elements from the lives and deaths of previous generations of stars can rocky worlds, the only home for life that we know of, come into existence.

4.) Earth’s late cosmic emergence supports life arising on it.

The existence of complex, carbon-based molecules in star forming regions is interesting, but isn’t anthropically demanded. Here, glycolaldehydes, an example of simple sugars, are illustrated in a location corresponding to where they were detected in an interstellar gas cloud: offset from the region presently forming new stars the fastest. Interstellar molecules are common, with many of them being complex and long-chained.

Life seemingly requires rocky planets with organic molecules.

If life began with a random peptide that could metabolize nutrients/energy from its environment, replication could then ensue from peptide-nucleic acid coevolution. Here, DNA-peptide coevolution is illustrated, but it could work with RNA or even PNA as the nucleic acid instead. Asserting that a “divine spark” is needed for life to arise is a classic “God-of-the-gaps” argument, but asserting that we know exactly how life arose from non-life is also a fallacy. These conditions, including rocky planets with these molecules present on their surfaces, likely existed within the first 1-2 billion years of the Big Bang.

Such systems emerged merely 1-2 billion years after the Big Bang.

This color-coded map shows the heavy element abundances of more than 6 million stars within the Milky Way. Stars in red, orange, and yellow are all rich enough in heavy elements that they should have planets; green and cyan-coded stars should only rarely have planets, and stars coded blue or violet should have absolutely no planets at all around them. Note that the central plane of the galactic disk, extending all the way into the galactic core, has the potential for habitable, rocky planets. but that stars facing away from the galactic center (far left and right) are much lower in heavy element abundance.

5.) Life is disfavored near the galactic center.

This image shows the magnetized galactic center, with various features highlighted, as imaged by the SOFIA/HAWC+ FIREPLACE survey team. The giant bubble at the left of the image is some 30 light-years wide, several times larger than any other supernova-blown bubble ever discovered. This violence-rich environment is likely the only part of the galaxy too energetic for life to sustain itself.

Only the innermost few light-years should be uninhabitable.

The European Space Agency’s space-based Gaia mission has mapped out the three-dimensional positions and locations of more than one billion stars in our Milky Way galaxy: the most of all-time. Looking toward the center of the Milky Way, Gaia reveals both light-blocking and luminous features that are scientifically and visually fascinating.

Long-term, stable conditions persist almost everywhere.

The (modern) Morgan–Keenan spectral classification system, with the temperature range of each star class shown above it, in kelvin. The overwhelming majority of stars today are M-class stars, with only 1 known O- or B-class star within 25 parsecs. Our Sun is a G-class star, along with about 5-10% of total stars. However, in the early Universe, almost all of the stars were O- or B-class stars, with an average mass 25 times greater than average stars today.

6.) A Sun-like star is required.

This graphic compares a Sun-like star with a red dwarf, a typical brown dwarf, an ultra-cool brown dwarf, and a planet like Jupiter. Only about 5% of all stars are like the Sun or more massive; K-type stars represent 15% of all stars, while red dwarfs represent 75-80% of all stars. Brown dwarfs, although they are failed stars, may be just as common as red dwarfs are.

Lower mass stars far outnumber Sun-like ones.

This glimpse into the stars found in the densest region of the Orion Nebula, near the heart of the Trapezium Cluster, shows a modern glimpse inside a star-forming region of the Milky Way. However, star-formation properties vary over cosmic time, from galaxy to galaxy, at different radii from the galactic center, etc. All of these properties and more must be reckoned with to compare the Sun with the overall population of stars within the Universe. Note that our Sun, born 4.6 billion years ago, is younger than 85% of all stars.

Once flaring settles down, K-type and M-type stars should support habitability.

Looking at the same region of space in three different wavelengths of light, a short-wavelength infrared view, a long-wavelength infrared view, and a narrowband view at a wavelength of 1.87 microns, reveals many different features within the same region of the Orion Nebula. The bright, glowing features at long wavelengths of light indicate large amounts of modestly cool neutral matter, pointing to star-formation still being ongoing in those regions. Actively star-forming regions create not only singlet stellar systems like our own, but also binary, trinary, and even richer multi-star systems as well.

7.) Metal-poor stars are uninhabitable.

These charts show the estimated star-formation rate density as a function of redshift and metallicity of the stars that form. Although there are substantial uncertainties, it can be safely concluded that somewhere between only about 3% and 20% of all stars have a heavy element content that’s greater than or equal to our Sun’s, with most estimates falling between just 4-10%. However, most stars with at least ~25% of the Sun’s heavy element content possess planets.

Stars with only a fraction of the Sun’s heavy elements house rocky, potentially life-supporting planets.

This view from the James Webb Space Telescope (JWST) of the protoplanetary disk, or proplyd, Orion 294-606 showcases not only how magnificent JWST is at imaging objects like this, but also how distant stellar systems truly are from one another, even within the star-forming regions where they’re created. This newly-forming object is due to a collapsing gas cloud and will someday become a star, but is not yet one. Stars only need a small fraction of the heavy elements that the Sun possesses in order to form planets.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.