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Rate of Earth-sized habitable zone planets
I was just looking at Anders Sandberg's 1999 RPG/worldbuilding project Big Ideas Grand Vision through an old link here on the forums, and something really struck me on the Planets and Life page:

Quote:When the second generation space telescopes went online in the 2020's, astronomers were surprised by the number of earth-like planets they found. Instead of being unique, the Earth was of a fairly common type. Estimates suggested that as many as 1% of all stars had a terrestrial planet orbiting them.

It's pretty cool that what he here describes as a surprisingly high number and "fairly common" has been wildly surpassed in real life. In this New York Times article about a 2020 analysis of the Kepler data (which as far as I can tell has not been posted here before), we get:

Quote:The team calculated that at least one-third, and perhaps as many as 90 percent, of stars similar in mass and brightness to our sun have rocks like Earth in their habitable zones, with the range reflecting the researchers’ confidence in their various methods and assumptions....According to NASA estimates there are at least 100 billion stars in the Milky Way, of which about 4 billion are sunlike. If only 7 percent of those stars have habitable planets — a seriously conservative estimate — there could be as many as 300 million potentially habitable Earths out there in the whole Milky Way alone.

Not to mention red dwarf stars:

Quote:The Kepler measurement of eta-Earth only pertains to stars like the sun, but in the galaxy those stars are vastly outnumbered by smaller, dimmer stars known as red dwarfs. One-quarter to one-half of red dwarfs also harbor habitable-zone planets, according to work by Courtney Dressing, now at the University of California, Berkeley, although some astronomers worry that radiation flares from such stars would doom any life trying to get started there. Red dwarf planets were not included in the new analysis of eta-Earth.

And on that note, a 2021 discovery from the TESS telescope had good news:

Quote:Ekaterina Ilin, Ph.D. student at AIP, and the team developed a method to locate where on the stars' surface flares are launched from. "We discovered that extremely large flares are launched from near the poles of red dwarf stars, rather than from their equator, as is typically the case on the Sun," said Ilin. "Exoplanets that orbit in the same plane as the equator of the star, like the planets in our own solar system, could therefore be largely protected from such superflares, as these are directed upwards or downwards out of the exoplanet system. This could improve the prospects for the habitability of exoplanets around small host stars, which would otherwise be much more endangered by the energetic radiation and particles associated with flares compared to planets in the solar system."

Of course, there are other difficulties for red dwarf planets, but this was a big one.

So, the galaxy and the universe truly have a previously unexpected abundance of Earth-like planets (in terms of size and insolation, at least). I sure hope that life also turns out to be really common - though past performance is no guarantee of future results.  Smile
Neat stuff! Personally, I suspect we will eventually that life is as common as doorknobs - a huge number of lifebearing worlds, often of a sort we haven't predicted - but that the number of non-lifebearing worlds will be vastly larger - and the overall numbers are so huge that it will initially seem to take a while to find each one individually. A million needles in a haystack of a trillion pieces of hay so to speak.

The definition of Earth-like needs some examination, of course. Currently Venus would count as an Earth-like planet, and so would Mars; neither are hospitable to humans.

Despite the discovery of thousands of exoplanets, few or none appear to be as Earthlike as the planets described in Big Ideas, Grand Visions, although that is probably a selection effect. We really haven't found a Nova Terra, or Pacifica, or Penglai yet. Many of the worlds we've found are too big - the planets described as 'habitable' are generally twice as massive as Earth, and if they have water oceans these are probably too deep to form Earth-like oceans and continents. Or they are orbiting red dwarf stars and are tidally-locked. I'm still looking for a good Earth-clone.
Here's 10 'Earth-like planets' from Jan 2022:
All of these are either too big or orbit red dwarfs, so would not be true EuGaian-type planets. Maybe some, or all, of them hold biospheres, but these would need to have significant differences from Earth.

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