In the entire Universe, we know of only one planet capable of supporting life. That is the planet Earth. So, when we look for exoplanets that could support life, we look for a rocky exoplanet orbiting a Sun-like star at a distance that is neither too hot nor too cold for liquid water on the surface.
To calculate the likelihood of life elsewhere in the Milky Way, one must first determine how many exoplanets exist that fit this description.
Now, with years of exoplanet-hunting data in the bag, astronomers have made a new calculation and determined there could be as many as 6 billion Earth-like planets orbiting Sun-like stars in the Milky Way.
“My calculations place an upper limit of 0.18 Earth-like planets per G-type star,” said astronomer Michelle Kunimoto from the University of British Columbia (UBC) in Canada. (You may remember that Kunimoto discovered a whopping 17 exoplanets in Kepler data quite recently.)
“Estimating how common different kinds of planets are around different stars can provide important constraints on planet formation and evolution theories, and help optimise future missions dedicated to finding exoplanets.”
As technology advances, the number of planets discovered outside the Solar System grows by leaps and bounds. We’ve confirmed 5,235 exoplanets so far, and the number is growing.
But that’s a drop in the bucket when you consider how many planets there could be out there. The Milky Way galaxy contains an estimated 100 to 400 billion stars, with approximately 7% of them being G-type main-sequence stars like our Sun.
However, the majority of the exoplanets discovered so far are large gas or ice giants like Jupiter or Neptune. We study the effects of planets on their stars because it is extremely difficult for us to see planets directly due to the enormous distances involved. Smaller, rocky planets, such as Earth and Mars, are more difficult to detect because their effects are much smaller, with a lower signal-to-noise ratio.
So it’s quite possible that our galaxy contains many more Earth-like exoplanets than we’ve discovered so far. To account for these missing planets, the team used forward modeling to simulate data based on the model’s parameters, applying it to a catalog of 200,000 stars studied by the Kepler planet-hunting spacecraft.
“I started by simulating the full population of exoplanets around the stars Kepler searched,” expounded the researcher in UBC’s press release.
“I marked each planet as ‘detected’ or ‘missed’ depending on how likely it was my planet search algorithm would have found them. Then, I compared the detected planets to my actual catalogue of planets. If the simulation produced a close match, then the initial population was likely a good representation of the actual population of planets orbiting those stars.”
From this approach, Kunimoto and her UBC colleague, astronomer Jaymie Matthews, could estimate the number of Earth-like planets in the Milky Way. They defined these as between 0.75 and 1.5 times the mass of Earth, orbiting a G-type star at a distance between 0.99 and 1.7 astronomical units (AU, the distance between Earth and the Sun).
At the upper limit of the estimate of G-type stars in the galaxy – a figure that is also very hard to pin down – these calculations returned a maximum of 6 billion of such exoplanets.
While the scientists came up with an astounding number of hypothetical Earths, this does not necessarily imply the number of such planets exists or whether they have life similar to ours. However, this new estimate increases the likelihood that comparable worlds exist.
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