Earth-like planets seem to orbit about half of all stars like our Sun. Find out why knowing this takes us closer to finding life in space.
I can recall sitting by a fire at my aunt and uncle’s lakeside cottage and gazing up at the clear night sky. Even then, I realized that people have done the same thing for thousands of years.
Apart from countless stars’ stark beauty, the night sky inspires us to ask if there are other worlds out there. We can imagine someone on some Earth-like planet looking at their sky and wondering if we exist.
Space exploration benefits us in many ways. It contributes to science, encourages new product development, and enables technologies like GPS and weather satellites.
Main Interest in Space Travel Is to Find Out Who’s Out There
Everyone loves a hero’s journey story, and astronauts provide that for us. For all that, if we’re being honest, our main interest in space travel is to find out who’s out there.
The first report of a planet outside of our solar system came in 1991. Professor Andrew Lynne published a paper in the Journal Nature declaring that he’d uncovered an exoplanet–orbiting a pulsar no less.
He was honoured with an invitation to address the annual American Astronomical Society Conference in Atlanta. He shocked his audience by announcing that he had discovered a mistake in his calculations three weeks before the meeting. He freely admitted that his so-called “discovery” was a blunder.
“The Most Honorable Thing I’ve Ever Seen”
Professor Lynne received a standing ovation. Astrophysicist John Bahcall called it “the most honourable thing I’ve ever seen.” We can all learn from this example of intellectual humility.
In the years that followed, scientists have successfully discovered other worlds, including many Earth-like planets. Astronomers first found a planet orbiting a star like our Sun in 1995. They called it Dimidium.
Since then, scientists have identified over 4,000 planets in our galaxy. NASA’s Kepler Space Telescope alone has found over 2,800 of them.
Scientists Have Identified Over 4,000 Planets in Our Galaxy
Kepler has finally run out of fuel and shut down. Its mission was to look for Earth-like planets in our region of the Milky Way galaxy. A team of researchers from NASA has reviewed the nine years’ worth of data gathered from the Kepler mission.
They looked at stars with temperatures close to that of our Sun. Their analysis concluded that our galaxy contains a bare minimum of 300 million Earth-like planets orbiting Sun-like stars. They feel that their estimate is conservative.
The 300 million figure is based on a highly cautious interpretation. In this judicious scenario, only about 7% of stars similar to ours hosted Earth-like planets.
Half of All Sun-Like Stars Have Earth-Like Planets
The more likely interpretation suggests that at least half of all Sun-like stars have Earth-like planets orbiting them. If we threw caution to the wind, there’s a scenario where this might be true 75% of the time.
Some of these planets aren’t very far away in galactic terms. Four of them are less than thirty light-years away. There’s one that’s “only” about twenty light-years from here.
One way to detect planets around distant stars is called the “transit method.” When an object passes in front of a star from our perspective, it dims that star’s light very slightly.
Earth-Like Planet With a Civilization Shading that Star?
Telescopes like Kepler can detect that transit effect. Is an Earth-like planet with a civilization shading that star? That’s what we really want to know, but it’s still beyond our grasp.
Steve Bryson is a researcher at NASA’s Ames Research Center. He explained, “Kepler already told us there were billions of planets, but now we know a good chunk of those planets might be rocky and habitable. It’s extremely exciting that we calculated these worlds are this common with such high confidence and precision.”
In the past, astrobiologists have pointed to potentially life-supporting worlds based on the “Goldilocks zone.” If a planet was not too close, but not too far away, from its star, they considered it to be “just right” to be habitable.
New Study Applies a More Refined Analysis
That’s a fairly broad approach, and it overlooks other vital considerations in defining Earth-like planets. This new study applies a more refined analysis.
“We always knew defining habitability simply in terms of a planet’s physical distance from a star, so that it’s not too hot or cold, left us making a lot of assumptions,” said co-author Ravi Kopparapu, from the Goddard Space Flight Center.
To overcome this shortcoming, the team combined the Kepler data with information gathered by the European Space Agency’s Gaia mission. The Gaia telescope had generated a database of measurements of the energy output of each star under study.
“Allowed Us to Look at Planets AND STARS In A New Way”
“Gaia’s data on stars allowed us to look at these planets and their stars in an entirely new way.” Professor Kopparapu explained. “Not every star is alike, and neither is every planet.”
Readers must wonder what’s behind that massive range between 7% and 75% in the estimates. Astrobiologists are only beginning to understand how planetary atmospheres affect the volume of light planets need to support surface liquid water.
So, although they’ve improved their parameters about how stars function, they have more to learn about the worlds they support. Still, even broadly estimating the number of Earth-like planets out there rewards our curiosity and helps scientists plan future missions.
“Extremely Valuable for the Design of Upcoming Missions”
As co-author Dr. Michelle Kunimoto of MIT explains, “Knowing how common different kinds of planets are is extremely valuable for the design of upcoming exoplanet-finding missions. Surveys aimed at small, potentially habitable planets around Sun-like stars will depend on results like these to maximize their chance of success.”
Bookshops are full of science fiction novels because we humans feel a profound need to tell stories about life on other worlds. By fuelling further study, these discoveries take us a baby step closer to having true stories to share about the nature of Earth-like planets.
We always have more to learn if we dare to know.
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