Astronomers Estimate There Are 6 Billion Earth-Like Planets in the Milky Way

Six billion Earth-like planets in the Milky Way? If true, that’s astounding. But the number needs some context.

The Milky Way has up 400 billion stars. So even if there are six billion Earth-like planets, they’re still spread far and wide throughout our vast galaxy.

A new study came up with the six billion number. The co-authors are Michelle Kunimoto and Jaymie Matthews, both from the University of British Columbia. The study’s title is “Searching the Entirety of Kepler Data. II. Occurrence Rate Estimates for FGK Stars.” It’s published in The Astronomical Journal.

An Earth-like world is one that’s rocky, roughly the same size as Earth, and that orbits a Sun-like, or G-Type, star. It also has to orbit that star in the habitable zone, which is a range of distance allowing for liquid water on the planet. It’s worth noting that the most common type of exoplanet we’ve detected is a Neptune-size planet far from the habitable zone.

Artist’s illustration of the habitable zone around different types of stars. Credit: NASA

“My calculations place an upper limit of 0.18 Earth-like planets per G-type star,” said co-author Kunimoto in a press release. “Estimating how common different kinds of planets are around different stars can provide important constraints on planet formation and evolution theories, and help optimize future missions dedicated to finding exoplanets.”

Previous work on the occurrence of Earth-like planets have come up with other numbers, from 0.02 potentially habitable Earth-like worlds per Sun-like star, up to greater than one per star.

“Our Milky Way has as many as 400 billion stars, with seven per cent of them being G-type,” said co-author Matthews. “That means less than six billion stars may have Earth-like planets in our Galaxy.”

The vast majority of the exoplanets we’ve discovered have been found using the transit timing method. Automated observatories like Kepler monitored stars for the telltale dip in brightness created by a planet passing in front of its star. But that method has an unavoidable bias.

Since a larger planet will cause a much more pronounced dip in starlight than a smaller planet, we’ve found many more large gas planets than we have smaller, rocky worlds. Kepler was also more likely to spot planets with shorter orbital periods. So we can’t just take Kepler data and extrapolate it to the entire Milky Way.

Sizes of verified planets just after a release of 715 confirmed planets from Kepler data in February 2014. Kepler’s results aren’t a true representation of exoplanet populations because it can find larger planets easier than it can find smaller ones. Credit: NASA

In their paper, the researchers write that “Finding Earth-size planets is challenging due to their small sizes and low transit signal-to-noise ratios (S/Ns), meaning planet detection pipelines have greater difficulty uncovering them than larger planets, and a higher risk of confusing them with transit-like noise in the data.”

To get over this sampling bias, Kunimoto used a technique known as ‘forward modelling’.

“I started by simulating the full population of exoplanets around the stars Kepler searched,” she explained. “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.”

Their study is based on a Kepler catalogue of about 200,000 stars, and precision radius measurements from the Gaia Data Release 2. They also took into account detection efficiency, and transit-like noise signals in the data. In the end, as the authors write, “For planets with sizes 0.75–1.5 R ? orbiting in a conservatively defined habitable zone (0.99–1.70 au) around G-type stars, we place an upper limit (84.1th percentile) of <0.18 planets per star.”

This figure from the study shows exoplanet occurrence rates around Sun-like G-Type stars. The y-axis shows planet radii, and the x-axis shows orbital periods. Each square is also color-coded by the legend on the right. Image Credit: Kunimoto and Matthews, 2020.

But coming up with that number was only part of the study. This new work also had something to say about what’s known as “the radius gap of planets.”

The radius gap is also known as the Fulton gap, after Benjamin Fulton, an astronomer and research scientist at the NASA Exoplanet Science Institute. It describes a phenomenon outlined in a 2017 paper by Fulton and a team of researchers.

For some reason, it’s very uncommon for an exoplanet with an orbital period of fewer than 100 days to have a radius between 1.5 and 2 times Earth’s.

The radius gap of exoplanets. For some reason, it’s very unusual to find an exoplanet with an orbit of less than 100 days, with a radius between 1.5 and 2 times that of Earth. Figure from THE CALIFORNIA-KEPLER SURVEY.
III. A GAP IN THE RADIUS DISTRIBUTION OF SMALL PLANETS. Image Credit: Fulton et al, 2017.

One explanation for this radius gap is photoevaporation. The closest planets are so close to their stars that they lose their atmospheres due to stellar high-energy radiation from their stars. But stars simmer down after 100 million years or so, so larger planets with thicker hydrogen/helium envelopes may still retain some of their envelopes by the time the high energy radiation from their star shuts down. Even if they retain a small percentage of their original H/He atmospheres, that’s enough to inflate their radii.

But Kunimoto and Matthews found something else.

They found that this radius gap actually occurs over a smaller range of orbital periods than previous work showed. The team’s results can “provide constraints on planet evolution models that explain the radius gap’s characteristics.”

A figure from Kunimoto’s and Matthews’ paper. The background grey is the Fulton gap, while the new data is in black. Image Credit: Kunimoto and Matthews, 2020.

One of the problems in this type of work is the term “habitable zone.” There’s no exact definition of the term, meaning it can be difficult to compare work between different teams of people. “A partial explanation for the lack of consistency between literature values lies in how authors define the “HZ,” the authors write.

Another problem is the definition of a rocky planet. “Another complicating factor is how authors define the size of a potentially habitable, rocky planet. Too small, and a planet will not be able to retain an atmosphere or support plate tectonics.”

In this work, the authors use a definition of habitable zone that’s becoming more common: from 0.99 to 1.70 astronomical units. They also use a lower radius limit of 0.75 Earth radii for a rocky planet, and 1.5 Earth radii for an upper limit. Other researchers are working with these same definitions.

Artist’s conception of HD 21749c, the first Earth-sized planet found by NASA’s Transiting Exoplanets Survey Satellite (TESS), as well as its sibling, HD 21749b, a warm sub-Neptune-sized world. Credit: Robin Dienel/Carnegie Institution for Science.

This won’t be the final work on exoplanet populations of Earth-like planets. We’re still in the infancy of exoplanet studies, and we’re only starting to get good at finding exoplanets, and reliably characterizing their sizes, type, and positions. As Kunimoto explained in the press release, this type of research will help us refine our understanding of exoplanet populations, and how to search for them.

But if there are 6 billion Earth-like planets in the Milky Way, expect to hear about more of them as time goes on. Missions like NASA’s TESS and the ESA’s CHEOPS are taking planet-finding to the next level. If there are other planets that are like Earth, they can’t hide forever.

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Evan Gough

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