Astronomers estimate that there are tens of billions of “super-Earths” in the Milky Way136
News on October 3, there are tens of billions of “super-Earths” in the Milky Way alone, which are more common and more habitable for human beings than the Earth we live in now.
Astronomers often discover planets orbiting stars outside our solar system and refer to them as “exoplanets.” In the summer of 2022, NASA’s team responsible for the Transiting Exoplanet Survey Satellite discovered several particularly interesting exoplanets orbiting the habitable zone of their parent star.
One of the planets is 30% larger than Earth and orbits the star in less than three days. Another planet, 70% larger than Earth, may have a deep ocean on its surface. These two exoplanets are super-Earths, larger than Earth but smaller than giant planets such as Uranus and Neptune.
Earth is still the only place where life is currently known to exist in the universe, so focusing the search for life on Earth-like planets seems a logical choice. The research suggests that the most likely targets for astronomers to find life on other planets are super-Earths.
Most super-Earths orbit cold dwarf stars with lower mass and much longer lifespans than the Sun. Scientists have discovered that super-Earths orbit 40 percent of the cool dwarf stars they have observed. Using this calculation, astronomers estimate that in the Milky Way alone, there are tens of billions of super-Earths in the habitable zone, and they may have liquid water on them. Since all life on Earth requires water, water is considered a key factor for a habitable planet.
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 nearest exoplanet is only 6 light-years from Earth. As you can see, our solar system is so unusual because it doesn’t have a super-Earth with a mass between Earth and Neptune.
Another reason super-Earths are ideal targets in the search for life is that they are easier to find and study than Earth-sized planets. Astronomers detect exoplanets in two ways. One is the gravitational effect of the planet on its parent star, and the other is the brief dimming of the star’s light as the planet passes in front of the parent star. Both detection methods are easier for larger planets.
Modern astrobiologists are exploring why planets are habitable for life. It turns out that Earth isn’t the best of all worlds where life could exist. For most of its 4.5 billion-year history, Earth was uninhabitable by humans and other larger creatures. Simulations show that the long-term habitability of the earth is not inevitable, but there is a great chance. It can even be said that humans are really lucky to be alive.
The researchers listed a series of properties that make planets ideal for life. Larger planets are more geologically active, a feature that scientists believe will facilitate biological evolution. Therefore, the most habitable planets are roughly twice the mass of Earth and 20 to 30 percent larger than Earth. It will also have oceans shallow enough for light to reach life on the seafloor, with an average temperature of 25 degrees Celsius.
Additionally, it will have a thicker atmosphere than Earth to act as an “insulation blanket”. In the end, such a planet would orbit a star older than the sun to allow life to develop longer, and it would have a magnetic field strong enough to withstand cosmic radiation. Scientists believe that combining these properties would make a planet super habitable.
By this definition, a super-Earth has many of the properties of a super-habitable planet. So far, astronomers have discovered 24 super-Earth exoplanets that are theoretically more habitable than Earth, if not the best of all possible worlds.
Recently, an exciting new discovery has been added to the list of habitable planets. Astronomers have begun to discover exoplanets thrown from star systems, and there may be billions of such planets roaming the Milky Way. If a super-Earth was ejected from its star system, with a dense atmosphere and water, it could sustain life for tens of billions of years, far longer than life on Earth could last before the sun died.
To detect life on distant exoplanets, astronomers will look for biosignatures that can be detected in planetary atmospheres, a byproduct of biology.
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 plans to target two potentially habitable super-Earths in its first year of operation. Another group of super-Earths with huge oceans discovered in the past few years, as well as planets discovered this summer, are also the targets of the James Webb Telescope.
But the best chance of finding signs of life in exoplanet atmospheres will come from the next generation of giant ground-based telescopes, including the Extremely Large Telescope with a mirror diameter of 39 meters, the Thirty Meter Telescope, and the 25.4-meter Giant Magellan Telescope. The telescopes are all under construction and are set to begin collecting data by 2030.
Astronomers know that super-Earths have life-supporting elements, but habitability doesn’t mean they’re inhabited. Until researchers find evidence of life elsewhere, life on Earth may just be a unique accident. While there are many reasons why habitable planets would not show signs of life, if astronomers look at these super-habitable super-Earths and find nothing in the next few years, humanity may be forced to conclude that admitting “The universe is a lonely place”.