[ad_1]
The first planets discovered outside our solar system were strange and unknown worlds. These were giants like Jupiter, hotter than Venus, moving around their suns within the orbit of Mercury. Astronomers have since discovered planets in which winds of 10,000 miles per hour raise metallic clouds that rain molten iron.
The diversity of extremes is captivating and even a little off topic.
What we are really looking for are more Earths. Are planets with the “right” conditions for life as we know it rare in the galaxy or does Goldilocks have a large family? As the planetary census has grown, the evidence increasingly favors the latter.
This week, in an article written by 44 astronomers led by Steve Bryson of NASA Ames and accepted for publication in Astronomical Journal, scientists have come up with a new estimate of how common Earth-like planets are in the Milky Way.
The calculation, called eta-Earth, is an attempt to estimate the number of Earth-sized rocky planets in the habitable zone of sun-like stars. Also known as the Goldilocks zone, the habitable zone is the area where temperatures are neither too hot nor too cold for liquid water to form on planetary surfaces. It is also where we think life is most likely to arise.
This isn’t the first estimate of eta-Earth, but it may still be the best.
The team drew their number from the complete catalog of exoplanet discoveries made by the Kepler Space Telescope from 2009 to 2018. Kepler examined over 150,000 stars, found more than 4,000 candidate exoplanets, and confirmed over 2,800. Scientists have also spent years examining Kepler’s data with a fine-toothed comb, correcting various blips and artifacts to boost confidence that only real and confirmed exoplanets remain.
The team also used data from the European Space Agency’s Gaia project, which mapped the position and brightness of one billion stars, to improve habitable zone estimates.
Previous studies defined the habitable zone with a fixed band of orbital distances. This can be problematic because it doesn’t take into account the varying temperatures of the stars. The habitable zone for the hottest stars will, of course, be different from that around colder stars. This means that you may lose some habitable zone planets and include others that shouldn’t be there.
Combining data from Gaia and Kepler, the team instead defined the habitable zone with the “stream of instellations,” which is a whimsical way of saying how much sunlight actually hits a planet’s surface, rather than orbital distance. This, they say, makes a more accurate estimate.
“We are measuring the true temperature of the planet, regardless of whether it is in the habitable zone or not, for all the planets around all the stars in our sample,” Bryson said. MIT Technology Review.
The document offers a complex range of values with attached confidence levels, error bars, and conservative and optimistic estimates. Conservatively, they predict on average 37 to 60 percent of sun-like stars that have an Earth-like planet in the habitable zone. Optimistically, the balloon average is up to 88% of sun-like stars with a planet in a habitable zone.
Taking the document’s more conservative lower bound, 7 percent of the galaxy’s 4 billion sun-like stars could have an Earth-like planet in the habitable zone. This results in a population of at least 300 million such planets in the Milky Way. With the warp engine and a map, this is a potentially habitable planet for every 26 people in the world. (In fact, 4 of these planets could be within 30 light years of the sun, the closest within 20 light years.)
The key word here is potentially.
This may be our best guess on eta-Earth, but as all addictions should make clear, it will likely be a moving target for years to come. Although the calculation narrows down the uncertainty bands, those bands are still quite wide. Perhaps the biggest contributor to this is the fact that researchers are extrapolating from a very small population of planets.
Further observations should further reduce uncertainty. But even then, the actual conditions on these planets are anyone’s guess. They are too far away to be observed in detail (although the eventual launch of the James Webb Space Telescope could change that in some way).
It’s also worth noting that while sun-like stars make sense as a starting point in our quest for life – the only life we know of is right here in our solar system – other stars are also interesting targets. Smaller, colder, older red dwarfs are much more abundant than sun-like stars. An estimate by the University of California, Courtney Dresser of Berkeley, suggests that one quarter to half of these stars may host planets in the habitable zone. Kepler’s successor, the Transiting Exoplanet Survey Satellite (TESS), will scan the Earth’s neighborhood, where about 75% of the stars are red dwarfs.
Other discoveries could also broaden our reach. Finding life in the clouds of Venus, the icy methane lakes of Titan or the subterranean ocean of Europa could expand what we call habitable. Perhaps liquid water on the surface is not always needed. Perhaps we will add the moons of the outer solar system to the planets.
Whatever happens, the search for life in the galaxy is about to get interesting. This study suggests that the most basic conditions for life may have occurred hundreds of millions of times in our galaxy alone. By a conservative estimate, that’s still a huge number. And while it may shrink as we learn more, it seems more likely it will grow.
As Carl Sagan said, “If it’s just us, it seems like a terrible waste of space.”
Image credit: Kepler 186f illustration courtesy of NASA Ames / JPL-Caltech / T. Pyle
Source link
