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There may be 300 million (or more) Earth-like planets in our Galaxy



The first planets discovered outside of our solar system were alien, alien worlds. These are giants like Jupiter, hotter than Venus, ripping off their suns within Mercury’s orbit. Since then, astronomers have discovered planets with winds of 10,000 miles per hour swept up in metallic storm clouds that create molten iron rain.

The variety of extremes is seductive, and also has a section beside the point.

What we are really after is more Earth. Planets have “moderate” conditions for life as we know them are rare in galaxies or do Goldilocks have a large family? As the planetary population grows, the evidence increasingly supports the latter.

This week, in an article led by 44 astronomers led by Steve Bryson of NASA Ames and accepted to be published on Astronomical magazineScientists have come up with a new estimate of the prevalence of Earth-like planets in the Milky Way.

The calculation, known as eta-Earth, is an attempt to estimate the number of Earth-sized rocky planets in the habitable zone of stars like the sun. Also known as the “Goldilocks”

;, a habitable zone is an area that is neither too hot nor too cold for liquid water to form on the planet’s surface. That is also where we think life is most likely to arise.

This is not the first eta-Earth estimate, but it is possibly the best one.

This group took its data from the complete list of alien planetary discoveries made by the Kepler space telescope between 2009 and 2018. Kepler surveyed more than 150,000 stars, found more than 4,000 planets. candidate exoplanet and confirmed more than 2,800. Scientists have also spent years examining Kepler data with a wide-toothed comb, correcting blisters and various artifacts to increase confidence that there are only real planets and has been confirmed.

The team also used data from the European Space Agency’s Gaia project – which mapped the positions and luminosity of one billion stars – to improve its estimate of habitable zones.

Previous studies have identified habitable areas with a fixed orbital distance range. This can be problematic because it does not explain the changing temperatures of the stars. Of course, the habitable zone of hotter stars will differ from the area around colder stars. That means you could miss out on some planets in the habitable zone and include others that shouldn’t be there.

Combining the Gaia and Kepler data, the team determined instead the habitable zone using the “set flux” —that is a preferred way of telling how sunlight actually hits the planet’s surface. crystal – instead of orbital distance. They say this makes for a more accurate estimate.

“We are measuring the actual temperature of the planet – whether it is actually in the habitable zone – for all the planets around all the stars in our sample,” Bryson said. . Evaluation of MIT Technology.

The paper provides a complex range of values ​​with associated confidence levels, error bars, and conservative and optimistic estimates. Cautiously, they predict an average of 37 to 60% of sun-like stars with Earth-like planets in the habitable zone. Optimistic, balloons averaged 88% of sun-like stars with a habitable regional planet.

Taking the paper’s most conservative lower limit, 7% of the galaxy’s estimated 4 billion sun-like stars may have an Earth-like planet in the habitable zone. That translates into population at least There are 300 million such planets in the Milky Way. With vertical drives and maps, it’s a habitable planet for every 26 people in the world. (Indeed, four such planets could be 30 light years away from the sun, the closest within 20 light years).

The key word here is potential.

This might be our best prediction at eta-Earth, but since all the dependent factors should be clear, it will likely be a moving target for many years to come. Although the computation narrows the bands of uncertainty, those bands are still quite wide. Perhaps the biggest contributor to this is the fact that researchers are extrapolating from a very small group of planets.

More observations reduce uncertainty even more. But even then, the actual conditions on these planets is something anyone can guess. They are too far away to be observed in detail (although the final launch of the James Webb space telescope may change that somewhat).

It’s also worth noting that while sun-like stars are meant as a starting point in our search for life – the only life we ​​know is right here, in the solar system. Ours – other stars are also attractive targets. Smaller, cooler, and older red dwarfs have more sun-like stars. An estimate by the University of California, Courtney Dresser of Berkeley, suggests that a quarter to half of these stars could contain planets in the habitable zone. Kepler’s successor, the Exoplanet Survey Satellite (TESS), will scan the vicinity of Earth, where about 75% of the stars are red dwarfs.

Other discoveries may also expand our range. Finding life in the clouds of Venus, Titan’s frozen methane lakes, or the ocean below Europa’s surface might extend what we call habitable. Maybe not always need liquid water on the surface. Perhaps we will add moons outside the solar system to the planets.

Whatever happens, the quest for life in the galaxy is about to get interesting. This study suggests that the most fundamental conditions for life could have happened hundreds of millions of times in our own galaxy alone. For a conservative estimate, that is still a whopping figure. And while it may narrow as we learn more, it looks like it will evolve.

As Carl Sagan said, “If it were just us, it would be a terrible waste of space.”

Image credit: Kepler illustration 186f with NASA Ames / JPL-Caltech / T. Pyle support


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