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In the old days, especially in Sci-Fi, it was felt once we could
explore, we would find plenty of places to live, just like Earth.

from https://scitechdaily.com/is-earth-an-oddball-one-of-the-strangest-things-in-the-cosmos-might-be-us/

Is Earth an Oddball? One of the Strangest Things in the Cosmos Might Be rCo Us By Pat Brennan, NASA's Exoplanet Exploration ProgramJanuary 1, 202229 Comments4 Mins Read
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Earth Rotating Sun Space
How rare in the galaxy are rocky planets like Earth in similar orbits
around Sun-like stars? The question turns out to be surprisingly
difficult to answer.
Despite thousands of exoplanets discovered, Earth remains unique in its
size, orbit, and type of star. Technical challenges in current detection methods make finding Earth-like planets difficult, especially around
Sun-like stars, leading to uncertainties about whether our solar
systemrCOs arrangement is rare or just hard to detect.

One of the strangest things in the cosmos might be rCo us.

Among the thousands of planets confirmed to be in orbit around other
stars, werCOve found nothing quite like our home planet. Other planets in EarthrCOs size range? Sure, by the bushel. But also orbiting a star like
our Sun, at a comparable distance? So far itrCOs just one, lonely example.
The one beneath our feet.

A big part of this is likely to be the technical difficulty of finding a sister planet. Our telescopes, in space and on the ground, find planets
around other stars by two main methods: wobbles and shadows.

The rCLwobblerCY method, or radial velocity, traces the subtle
back-and-forth motion as orbiting planets tug their star this way, then
that, because of gravity. The larger the tug, the rCLheavierrCY the planet rCo that is, the greater its mass.

In the search for shadows, planet-hunting telescopes wait for a tiny dip
in starlight as a planet crosses the face of its star rCo a crossing known
as a rCLtransit.rCY The bigger the dip, the wider the planet.

In both cases, large planets are much easier to detect than small ones.
And in the case of transits, small, rocky planets about the size of
Earth show up much better against very small stars known as red dwarfs.
In a sense, they cast a bigger shadow that blots out proportionally more
of a small starrCOs light, so instruments like NASArCOs TESS space telescope can more readily find them. A Sun-sized star wonrCOt dim as much when an Earth-size planet passes by, making their transits harder to detect.

Apollo 11 Earth Image
Apollo 11 Earth image. Credit: NASA Johnson Space Center
And thererCOs another troubling issue: time. A planet orbiting a star at EarthrCOs distance from the Sun would take about 365 days to make one revolution rCo just like our planetrCOs rCLyear.rCY But to confirm such an orbit, your telescope would have to stare at that star for, say, 365
days to catch even one transit rCo and to be sure itrCOs truly a planet, yourCOll want to see at least two or three of these transit signals.

All of these difficulties have placed such planets largely out of reach
for todayrCOs instruments. WerCOve found plenty of small, rocky planets, but theyrCOre nearly all orbiting red dwarf stars.

In our galaxy, red dwarfs are far more common than larger yellow stars
like our Sun. That still leaves room for billions of Sun-like stars and, maybe, a significant number of habitable, Earth-sized worlds circling them.

Or maybe not.

Rare or Just Difficult?
The apparent oddness of our home system doesnrCOt end with Earth. Our particular arrangement rCo small, rocky worlds in the nearest orbits, big
gas giants farther out rCo also is something we havenrCOt yet detected in close parallel anywhere else. Whether this is because they are truly
scarce or because they are hard to detect is unclear.

Jupiter takes one trip around the Sun every 12 years. But Jupiter-type
planets in long orbits are comparatively rare around other stars, and
that could be important. Theorists say Jupiter might well have cleared
the way for Earth to become a habitable world, quite literally. The
giant planetrCOs intense gravity could have hoovered up small rocky bits
that might otherwise have smashed into Earth, sterilizing it just as
life was getting its start.

rCLThe planetary systems we are finding do not look like our solar
system,rCY said Jessie Christiansen, a research scientist at NASArCOs Exoplanet Science Institute. rCLIs it important that our solar system is different? We donrCOt know yet.rCY

Christiansen, who studies exoplanet demographics, does not think
rCLEarthsrCY will turn out to be rare, but says scientific literature on the question rCLis all over the place.rCY

Far more data are needed, scientists tell us, to determine the frequency
of planets similar to Earth in both size and circumstance.

Future space telescopes could examine the atmospheres of distant, rocky
worlds for signs of oxygen, methane, or carbon dioxide rCo in other words,
an atmosphere that reminds us of home.

For now, we remain in the dark. Earth-like planets around Sun-like stars
might be plentiful. Or, they could be the true oddballs of the galaxy.

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