Astronomers now regularly discover planets orbiting stars outside the solar system – they are called exoplanets. But in the summer of 2022, teams working on NASA’s Transiting Exoplanet Survey satellite found a few particularly interesting planets orbiting in the habitable zones of their parent stars.
A planet is 30% larger than Earth and revolves around its star in less than three days. The other is 70% larger than Earth and could harbor a deep ocean. Both of these exoplanets are super-Earths – more massive than Earth but smaller than ice giants like Uranus and Neptune.
I am an astronomy professor who studies galactic nuclei, distant galaxies, astrobiology and exoplanets. I am closely searching for planets that could harbor life.
Earth is still the only place in the universe that scientists know is home to life. It would seem logical to focus the search for life on Earth clones – planets with properties close to those of Earth. But research has shown that astronomers’ best chance of finding life on another planet is probably on a super-Earth similar to those recently discovered.
Common and easy to find
Most super-Earths orbit cool dwarf stars, which are lower in mass and much longer-lived than the Sun. There are hundreds of cold dwarf stars for every star like the Sun, and scientists have found super-Earths orbiting around 40% of the cold dwarfs they have observed. Using this number, astronomers estimate that there are tens of billions of super-Earths in habitable zones where liquid water can exist in the Milky Way alone. Since all life on Earth uses water, water is considered essential for habitability.
According to current projections, about a third of all exoplanets are super-Earths, making them the most common type of exoplanet in the Milky Way. The closest is only six light years from Earth. We could even say that our solar system is atypical since it does not have a planet whose mass is between that of the Earth and that of Neptune.
Another reason super-Earths are ideal targets in the search for life is that they are much easier to detect and study than Earth-sized planets. Astronomers use two methods to detect exoplanets. One looks for the gravitational effect of a planet on its parent star, and the other looks for a brief attenuation of a star’s light as the planet passes in front of it. Both of these detection methods are easier with a larger planet.
Super-Earths are super-habitable
More than 300 years ago, the German philosopher Gottfried Wilhelm Leibniz claimed that the Earth was “the best of all possible worlds”. Leibniz’s argument aimed to answer the question of why evil exists, but modern astrobiologists have explored a similar question by asking what makes a planet hospitable to life. It turns out that Earth isn’t the best of all possible worlds.
Due to the Earth’s tectonic activity and changes in the Sun’s brightness, the climate has changed over time from searing ocean heat to planetary freezing cold. Earth has been uninhabitable for humans and other larger creatures for most of its 4.5 billion year history. The simulations suggest that Earth’s long-term habitability was not inevitable, but was a matter of luck. Humans are literally lucky to be alive.
Researchers have compiled a list of attributes that make a planet very suitable for life. Larger planets are more likely to be geologically active, a feature that scientists believe promotes biological evolution. Thus, the most habitable planet would have about twice the mass of Earth and be between 20% and 30% larger in volume. It is also said to have oceans shallow enough for light to stimulate life down to the seabed and an average temperature of 77 degrees Fahrenheit (25 degrees Celsius). It would have a thicker atmosphere than Earth’s which would act as an insulating blanket. Finally, such a planet would orbit a star older than the Sun to give life more time to develop, and it would have a strong magnetic field that protects it from cosmic radiation. Scientists believe these attributes combined will make a planet super habitable.
By definition, super-Earths have many of the attributes of a super-habitable planet. To date, astronomers have discovered two dozen super-Earth exoplanets that are, if not the best of all possible worlds, theoretically more habitable than Earth.
Recently there has been an exciting addition to the inventory of habitable planets. Astronomers have started discovering exoplanets that have been ejected from their star systems, and there could be billions of them wandering the Milky Way. If a super-Earth is ejected from its star system and has a dense atmosphere and a watery surface, it could sustain life for tens of billions of years, far longer than life on Earth could persist before death. Sun.
Detecting life on super-Earths
To detect life on distant exoplanets, astronomers will search for biosignatures, byproducts of biology detectable in a planet’s atmosphere.
NASA’s James Webb Space Telescope was designed before astronomers discovered exoplanets, so the telescope is not optimized for exoplanet research. But it is able to do some of that science and is expected to target two potentially habitable super-Earths in its first year of operation. Another set of super-Earths with massive oceans discovered in recent years, as well as the planets discovered this summer, are also hot targets for James Webb.
But the best chance of finding signs of life in exoplanet atmospheres will come with the next generation of giant Earth-based telescopes: the 39-meter Extremely Large Telescope, the Thirty-meter Telescope, and the 24.5-meter Giant Magellanic Telescope. . These telescopes are all under construction and should start collecting data by the end of the decade.
Astronomers know that the ingredients for life exist, but habitable does not mean inhabited. Until researchers find evidence of life elsewhere, it’s possible that life on Earth was a one-time accident. While there are many reasons why a habitable world might not have signs of life, if over the next few years astronomers look at these super-habitable super-Earths and find nothing, humanity could be forced to conclude that the universe is a lonely place.
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