Key Takeaways About the Most Earth-Like Planets
- Although astronomers have directly imaged several dozen gas giants on the scale of Jupiter, Earth-sized planets are too small and dim to image directly.
- That doesn't mean there's a shortage of rocky, Earth-sized exoplanets. There are many of them—it's just that most of them orbit M dwarf stars, also known as red dwarfs, which have a narrower habitable zone and emit more harmful radiation than G-type stars like the Sun, according to NASA.
- A notable exception is Kepler 452b, which lies in the habitable zone of a G-type star almost identical to ours.
Over the past 30 years, astronomers have discovered more than 6,000 planets outside our solar system, according to NASA.
With each new discovery, the main question arises: could it contain extraterrestrial life? And since Earth is the only planet confirmed to have this, scientists are especially keen to find rocky balls similar to the ones we call home.
But even as the pace of discovery accelerates, bringing more exoplanets to our attention each year, experts say there are still few promising candidates. Stephen Kane, a planetary astrophysicist at the University of California, Riverside, counters the hype about potentially habitable worlds with a dose of sober realism:
If by “Earth-like” we mean a planet the size of our own orbiting a star like our Sun, then “to be honest, we haven’t found anything like it at all.”
Read more: Strange lemon-shaped exoplanet discovered orbiting bizarre star
Are there exoplanets similar to Earth?
Part of the problem is that we have so little information about the exoplanets we have discovered; our knowledge is largely limited to measurements of size, mass and orbit, with detailed atmospheric information conspicuously lacking.
Although astronomers have directly imaged several dozen gas giants on the scale of Jupiter, Earth-sized planets are too small and dim to be imaged directly, according to NASA.
“We don't know what they look like, not even one pixel at a time,” Kane says. “Everything we know comes from indirect observations.”
Rocky exoplanets orbiting red dwarfs
That doesn't mean there's a shortage of rocky, Earth-sized exoplanets. There are many of them—it's just that most of them orbit M dwarf stars, also known as red dwarfs, which have a narrower habitable zone and emit more harmful radiation than G-type stars like the Sun, according to NASA. From what scientists have learned so far, red dwarfs tend to strip their planets of their protective atmosphere.
For example, according to NASA, TRAPPIST-1 systemwhich lies about 39 light-years away in the constellation Aquarius, is often cited as a strong candidate for extraterrestrial life, with seven planets in the habitable zone, all of which appear rocky based on their mass. But the star is a volatile red dwarf, and observations from James Webb Space Telescope suggest that at least some of the planets are likely to lack an atmosphere, according to a study conducted in Astrophysical Journal.
The same goes for a system of three exoplanets orbiting the star Teagarden, 12 light-years away in the constellation Aries—they're the perfect size and distance from their star, but the fact that it's a red dwarf doesn't bode well, according to the Infrared Processing and Analysis Center (IPAC) at Caltech. Unfortunately, red dwarfs are by far the most common type of star in the Universe, meaning that most exoplanets may exist in biologically unfavorable conditions.
Which planet is most similar to Earth and can people live there?
A notable exception is Kepler 452b, which lies in the habitable zone of a G-type star almost identical to ours.
There's a catch, however: the planet itself is about 1.6 times the size of Earth, meaning it's just above the threshold at which planets tend to transition from the terrestrial state to the gaseous state. Clearly, for a world to truly resemble ours, many parameters need to be aligned, Kane says, and “very rarely do we get it all.”
How astronomers discover exoplanets
According to NASA, most exoplanets are discovered by the transit method, which involves waiting for a planet to pass between its star and Earth. During this transit, no measurable amount of light can reach Earth, allowing scientists to judge the planet's size and orbital distance. But for this to work, the planet and its star must be correctly aligned with Earth – otherwise there will be no transit.
The Kepler space telescope, which NASA says has discovered more exoplanets than any other, observed 170,000 stars over nine years, from 2009 to 2018. No doubt most, if not all, of these stars have planets, but Kepler only counted about 2,700, simply because the vast majority of their orbits are oriented incorrectly. Statistically speaking, Kane says, if there is a habitable exoplanet in our cosmic environment, it almost certainly won't be transiting.
He argues that the search for exoplanets must change course. There are other ways to learn about distant worlds. For example, each pulls only slightly on its host star, causing a change in the star's “radial velocity”—that is, a slight wobble, according to the Planetary Society. By measuring a star's wobble, we can determine how many planets it has, as well as their mass and orbital distance.
Could we one day find a habitable exoplanet?
And then there's the Holy Grail: direct imagery. Until now it was impossible to take pictures of the rocky exoplanetspartly because they are too small, and partly because the brightness of their stars makes them functionally invisible. But a new generation of telescopes could soon overcome these problems, giving us our first real look at an Earth-like exoplanet.
First up is NASA's Nancy Grace-Roman Space Telescope, scheduled to launch in 2027. It will be equipped with a coronagraph, an instrument designed to block out overwhelming light from stars and allow faint exoplanets to be seen for the first time.
At the same time, NASA is planning well for the future. The Habitable Worlds Observatory, or HabWorlds, tentatively scheduled to launch in the late 2030s or early 2040s, will be the first telescope built specifically to probe space for alien biosignatures. Featuring the largest mirror ever sent into space, it will be able to detect molecules in the atmospheres of exoplanets that could indicate the presence of life.
Of course, when HabWorlds finally comes online, it's unlikely that anyone will be talking about TRAPPIST, Teegarden or Kepler 452b. The development and implementation of this ambitious mission will take more than a decade (if it ever comes to fruition), by which time astronomers will have identified new targets, some of which we have yet to discover.
“We just need to be patient,” Kane says. “The last 20 years have been incredible. Let's see where we are in 20 years.”
Read more: JWST helps explore the atmosphere of an exoplanet 40 light-years away and in the Goldilocks zone
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