Three lines of evidence point to the idea of complex, multicellular alien life being a wild goose chase. But are we clever enough to know?
- Everyone wants to know if there is alien life in the universe, but Earth may give us clues that if it exists it may not be the civilization-building kind.
- Most of Earth's history shows life that is single-celled. That doesn't mean it was simple, though. Stunning molecular machines were being evolved by those tiny critters.
- What's in a planet's atmosphere may also determine what evolution can produce. Is there a habitable zone for complex life that's much smaller than what's allowed for microbes?
Protozoa—a term for a group of single-celled eukaryotes—and green algae in wastewater, viewed under the microscope.
Credit: sinhyu via Adobe Stock<p>Another way the story of life on Earth might not get repeated elsewhere in the cosmos relates to the composition of planetary atmospheres. Our world did not begin with its oxygen-rich air. Instead, oxygen didn't show up until almost two billion years after the planet formed and one billion years after life appeared. Earth's original atmosphere was, most likely, a mix of nitrogen and CO2. Remarkably it was life that pumped the oxygen into the air as a byproduct of a novel form of photosynthesis invented by a novel kind of single-celled organism, the nucleus-bearing eukaryotes. The appearance of oxygen in Earth's air was not just a curiosity for evolution. Life soon figured out how to use the newly abundant element and, it turns out, oxygen-based biochemistry was supercharged compared to what came before. With more energy available, evolution could build ever larger and more complex critters.</p><p>Oxygen may also be unique in allowing the kinds of metabolisms in multicellular life (especially ours) needed for making fast and fast-thinking animals. Astrobiologist <a href="http://faculty.washington.edu/dcatling/Catling2008CatalystMag.pdf" target="_blank" rel="noopener noreferrer">David Catling</a> has argued that only oxygen has the right kind of chemistry that would allow for animals to form on any world.</p><p>Atmospheres may play another role in what can and can't happen in the evolution of life. In 1959, <a href="https://astro.uchicago.edu/alumni/su-shu-huang-1949.php" target="_blank" rel="noopener noreferrer">Su-Shu Huang</a> proposed that each star would be surrounded by a "<a href="https://www.nasa.gov/ames/kepler/habitable-zones-of-different-stars" target="_blank" rel="noopener noreferrer">habitable zone</a>" of orbits where a planet would have temperatures neither too hot nor too cold to keep life from forming (i.e. liquid water could exist on the planet's surface). Since then, the habitable zone has become a staple of astrobiological studies. Astronomers now know that the outer part of the habitable zone will be dominated by worlds with lots of greenhouse gases like CO<em>2</em>. A planet in a location like Mars, for example, would require a thick CO2 blanket to keep its surface above freezing. But all that CO2 could present its own problems for life. Almost all forms of animal life on Earth, including sea creatures, die when placed in CO2-rich environments. This has led astronomer <a href="https://eschwiet.github.io/" target="_blank" rel="noopener noreferrer">Eddie Schwieterman</a> and colleagues to propose a <a href="https://iopscience.iop.org/article/10.3847/1538-4357/ab1d52" target="_blank" rel="noopener noreferrer">habitable zone for complex life</a>: A band of orbits where planets can stay warm without requiring heavy CO2 atmospheres. According to Schwieterman, animal life of the kind we know would only be able to form in this much thinner band of orbits. </p>
A unique star system where exoplanets orbit their star backwards located by researchers.
- Astrophysicists find a very rare system with two exoplanets orbiting their star backwards.
- The star system K2-290 is 897 light years away.
- In our Solar System, all the planets revolve in the same direction as the rotation of the Sun.
A protoplanetary disc was twisted almost 180° before planet formation.
Illustration: Christoffer Grønne.
One galaxy may have eaten one of its slightly smaller, more primitive neighbors.
A new study found the possible reason why some dwarf galaxies appear to not have dark matter.
- A new paper presents a possible reason for why some dwarf galaxies appear to be missing dark matter.
- The researchers at the University of California, Riverside ran cosmological simulations to find the answers.
- They discovered some galaxies were stripped of dark matter through extreme tidal loss.
Laura Sales (seated, left) and her research group of students, including Jessica Doppel (seated, right).
Credit: UCR/Stan Lim
Artist Seán Doran recently created more than eight hours of high-definition video using images captured by Japan's SELENE lunar orbiter.
- In 2007, Japan's SELENE lunar orbiter, better known as Kaguya, became the first orbiter to capture high-definition images of the moon.
- Kaguya's images helped scientists create a highly detailed topography of the lunar surface.
- Artist Seán Doran synthesized and polished the Kaguya images to simulate what it's like to orbit the moon in real time.