Up to one-in-four stars could support Earth-like planets, capable of supporting life, suggests new research.
The question of how unique is our planet is one that is central to both our search for exoplanets–planets orbiting stars other than our own, and in the search for life elsewhere in the cosmos.
The key characteristics that would define a planet as being ‘Earth-like’ include occupying a similar distance to its star as we do to the sun and the size of the exoplanet. A new study published in the Astronomical Journal this month details a model which calculates the frequency of planets with these qualities.
What the team from Penn State University discovered is that, according to their model, approximately one-in-four stars supports an Earth-like planet. of course, that is only an estimate and according to uncertainties in the calculation, that figure could drop as low as one-in-thirty-three or rise as high as one-in-two.
Knowing the rate at which such Earth-like exoplanets– which are potentially habitable–is important to the design and planning of future astronomical missions. Especially those that seek to find nearby rocky planets around sun-like stars which could support life.
The team took advantage of the thousands of exoplanets discovered by NASA’s Kepler Space Telescope, which operated between 2009 and 2018 when its fuel was exhausted.
Eric B. Ford, professor of astronomy and astrophysics at Penn State and one of the leaders of the research team, says: “Kepler discovered planets with a wide variety of sizes, compositions and orbits.
“We want to use those discoveries to improve our understanding of planet formation and to plan future missions to search for planets that might be habitable.”
Kepler was able to pinpoint exoplanets by recording transit events which occur when a planet’s orbit passes between its star and the space telescope.
This transit results in the exoplanet screening some of the light emitted by its host star, which, in turn, results in the star appearing to dim. The amount of dimming caused by the transit and the duration of the event allowed astronomers to calculate the size of the exoplanet and its the distance from it to its parent star.
But there are drawbacks to this method, as Ford explains: “Simply counting exoplanets of a given size or orbital distance is misleading since it’s much harder to find small planets far from their star than to find large planets close to their star.”
The researchers designed their new model to combat this problem. Their method allows astronomers to infer the occurrence rates of planets of certain sizes and at certain orbital distances.
In order to do this, the new model involves the creation of simulated universes full of stars and planets. These simulated universes are then observed in order to determine how many planets would have been discovered by Kepler were it operational in said universe.
Danley Hsu, a graduate student at Penn State and the first author of the paper, explains: “We used the final catalogue of planets identified by Kepler and improved star properties from the European Space Agency’s Gaia spacecraft to build our simulations.
“By comparing the results to the planets catalogued by Kepler, we characterized the rate of planets per star and how that depends on planet size and orbital distance.”
This novel approach allowed the team to account for several effects that have not been included in previous studies.
The model could provide extra assistance for astronomers searching other solar systems and galaxies for bio-markers, found in the composition of exoplanet atmospheres.
Ford says: “Scientists are particularly interested in searching for biomarkers–molecules indicative of life–in the atmospheres of roughly Earth-size planets that orbit in the ‘habitable zone’ of Sun-like stars.
“The habitable zone is a range of orbital distances at which the planets could support liquid water on their surfaces. Searching for evidence of life on Earth-size planets in the habitable zone of sun-like stars will require a large new space mission.”
The size of a mission such as the one described by Ford would depend on just how abundant Earth-like planets are. The mission concepts that are currently being examined by organisations such as NASA differ in size and capability substantially.
Should Earth-like planets be rare, that implies the nearest of these exoplanets would be further away and thus require larger and more ambitious missions to locate and examine. Conversely, should such planets be common, a relatively small mission may be capable of studying their atmospheres.
Hsu explains: “While most of the stars that Kepler observed are typically thousands of light-years away from the Sun, Kepler observed a large enough sample of stars that we can perform a rigorous statistical analysis to estimate of the rate of Earth-size planets in the habitable zone of nearby sun-like stars.”
Based on their simulations, the researchers estimate that planets very close to Earth in size, from three-quarters to one-and-a-half times the size of earth, with orbital periods ranging from 237 to 500 days, occur around approximately one in four stars.
Importantly, their model quantifies the uncertainty in that estimate. They recommend that future planet-finding missions plan for a true rate that ranges from as low about one planet for every 33 stars to as high as nearly one planet for every two stars.
Ford concludes: “Knowing how often we should expect to find planets of a given size and orbital period is extremely helpful for optimizing surveys for exoplanets and the design of upcoming space missions to maximize their chance of success.”
Original research: https://iopscience.iop.org/article/10.3847/1538-3881/ab31ab