Starship Asterisk*

Ten Billion Earths

Explanation: How common are Earth-sized planets? Quite common, according to extrapolations from new data taken by NASA's orbiting Kepler spacecraft. Current computer models are indicating that at least one in ten stars are orbited by an Earth-sized planet, making our Milky Way Galaxy the home to over ten billion Earths. Unfortunately, this estimate applies only to planets effectively inside the orbit of Mercury, making these hot-Earths poor vacation opportunities for humans. This histogram depicts the estimated fraction of stars that have close orbiting planets of various sizes. The number of Sun-like stars with Earth-like planets in Earth-like orbits is surely much less, but even so, Kepler has also just announced the discovery of four more of those.

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Soooooo, I am thinking Sunscreen just isn't going to cut it....

And considering how long it would take to get there...

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APOD Robot wrote:Kepler has also just announced the discovery of four more of those.

To be very pedantic, it isn't Kepler as such that has announced the discovery but the Kepler team.

With the talk about other planets and their relative size to earth, my question is - (speaking of size only) on how large of a planet can a human live comfortably? Can we live comfortably, or maybe with minor adjustment needed, live on a planet twice as big as earth? On a planet twice the size of earth, would I weight twice as much, or might I weight 1.5 times as much? Or what would I weigh?

I think that in the Universe, many stars must be orbited by planets of different sizes.
There are more planets in Universe than we could imagine, but do not see them.

Stefan48 wrote:With the talk about other planets and their relative size to earth, my question is - (speaking of size only) on how large of a planet can a human live comfortably? Can we live comfortably, or maybe with minor adjustment needed, live on a planet twice as big as earth? On a planet twice the size of earth, would I weight twice as much, or might I weight 1.5 times as much? Or what would I weigh?

Assuming the same average mass density, your weight would scale directly with planet size. So for your example, you would weigh twice as much.

Stefan48 wrote:With the talk about other planets and their relative size to earth, my question is - (speaking of size only) on how large of a planet can a human live comfortably? Can we live comfortably, or maybe with minor adjustment needed, live on a planet twice as big as earth? On a planet twice the size of earth, would I weight twice as much, or might I weight 1.5 times as much? Or what would I weigh?

There is so much more than the relative size of the Earth that makes it habitable to us humans. We are a product of the entire biosphere of the Earth. We don't need all the species on the Earth to survive as species ourselves, but the underlying balances and feedbacks are certainly necessary for our long-time survival. Consider, for example, the fact that all the Earthly animals and plants are extremely closely related to us. That makes it easy for our bodies to process their bodies and use them as food.

There's more. Is the water on the Earth exactly like the water on other planets? Astronomers have recently discarded the idea that the water on the Earth has been carried here by comets, since the water ice of comets is typically chemically and isotopically different than the water on the Earth.

If we went to another planet, any other planet, could we create a suitable and stable biosphere there from scratch? If there already was a biosphere there, could we change it so that it was suitable for us? Could we drink the water? Could we breathe the air?
Tom Lehrer warned about the water and air in American cities. What about the water, air and biosphere on other planets?

Does that mean that planets must be exactly be like the Earth in order to be habitable? Of course not. Quite different planets could very well be habitable. There may be billions of not really Earth-like planets in the Milky Way that are habitable.

But not to us.

Ann

"depicts the estimated fraction of stars"
Fraction? Is 20 a fraction? Did you mean percentage?

amserhwylio wrote:"depicts the estimated fraction of stars"
Fraction? Is 20 a fraction? Did you mean percentage?

I was wondering that. The bar for Earth comes to about 16. Is that 16%? One in 16?

I followed the link to the Harvard-Smithsonian site, http://www.cfa.harvard.edu/news/2013/pr ... mages.html, where it seems to imply a percentage.

The results show that one in six stars has an Earth-sized planet in a tight orbit. About a fourth of all stars in the Milky Way have a super-Earth, and the same fraction have a mini-Neptune. Only about 3 percent of stars have a large Neptune, and only 5 percent a gas giant at the orbital distances studied.

So terrestrial planets orbit 16% of stars. Fine. Super-Earths and mini-Neptunes, 25% each. The bars show 20, which was called a fifth when I was at school Large Neptunes and gas giants, 3% and 5% respectively - so why are the two bars the same length, at no more than 2%, however you allow for artificial perspective within the chart?

In all, I'm still puzzled.

I'm wondering how life would have developed if the Earth had more mass. I guess that fish as we know them would all sink to the bottom. Would plants grow as high as ours? Would land creatures be built more massively, or lighter than we are? I guess swimming would be out of the question for us. Would huge dinosaurs have been possible? Or birds?

If the average planet mass is higher than ours, I guess that beings from other planets would on average be stronger than us.

Interesting to think about, but hard to imagine what it would be like.

Stefan48 wrote:With the talk about other planets and their relative size to earth, my question is - (speaking of size only) on how large of a planet can a human live comfortably? Can we live comfortably, or maybe with minor adjustment needed, live on a planet twice as big as earth? On a planet twice the size of earth, would I weight twice as much, or might I weight 1.5 times as much? Or what would I weigh?

That’s a good question, for while these planets are far too hot, many others have been and will continue to be discovered that have orbits in the “habitable zone” of the stars they orbit. Ignoring cases of rapid rotation, the force that something would feel on the surface of a planet depends on only two factors; the planet’s mass and its radius. The more massive a planet is, the greater the G force would be, but this is somewhat counter-acted by the size of a planet, because this force drops with increasing distance. Therefore you cannot accurately say that we would be twice as heavy on a world that was twice the radius of Earth, nor can this be said if the world is twice the mass of Earth. Both planetary mass and size must be known for surface gravity to be calculated. I don’t remember the exact formula, I’m sure someone will post it soon if they haven’t already done so.

You say, "The number of Sun-like stars with Earth-like planets in Earth-like orbits is surely much less..." but I don't see the logic in that statement. What is counted in this APOD are close-in orbits, which is a restricted set; "Earth-like orbits" (presumably in the "habitable zone") is a different restricted set. How do we know that the number in the first set is fewer than the number in the second? In the solar system at least, there is a lot less space in the first area than the second.

Sorry to be pedantic, I really, really love the site, but..

Those figures are percentages not fractions and it's a Bar Chart not a Histogram.

Unless it means something different in the States than it does in the UK...?

... And the bars are labelled incorrectly, if they're going to be separated...

10 billion Earths; 7 billion people! Get your planet now! I remember hearing that there are more stars in the universe than grains of sand on the beach! I don't know how accurate that is but I believe that there are more planets than stars! Only time will tell!

inertnet wrote:
I'm wondering how life would have developed if the Earth had more mass.

I guess that fish as we know them would all sink to the bottom.
... I guess swimming would be out of the question for us.

Swimmers don't care much about gravity (just their density vs-a-vis water).
...which is why blue whales can be as large as they are.

inertnet wrote:
Would plants grow as high as ours?

Not land plants that depend upon ground water:

http://en.wikipedia.org/wiki/Sequoia_sempervirens wrote:
<<Sequoia sempervirens includes the tallest trees on Earth. The current tallest tree is Hyperion, measuring at 379.3 feet (115.6 m). A tree claimed to be 424.08 ft (129.26 m) was felled in November 1886 by the Elk River Mill and Lumber Co. at the south fork of Elk River in Humboldt County. The theoretical maximum potential height of coast redwoods is limited to between 122 and 130 m (400 and 427 ft), due to gravity and the friction between water and the conduits through which it flows.>>

inertnet wrote:
Would land creatures be built more massively, or lighter than we are?
Would huge dinosaurs have been possible? Or birds?

Land creatures including dinosaurs would be smaller and (for the same size) built more massively.

Flying creatures including dinosaurs (i.e., birds) would be smaller and (for the same size) built less massively.

Spiders & insects don't care much about gravity (just the surface tension of water).

inertnet wrote:
If the average planet mass is higher than ours,
I guess that beings from other planets would on average be stronger than us.

http://en.wikipedia.org/wiki/Krypton_%28planet%29 wrote:
<<Krypton was shown to have been a planet similar to Earth, older by eons and possessed of all the beneficial progress that implied. All Kryptonians possessed a level of heightened physical abilities, including super-strength and super-speed.>>

http://en.wikipedia.org/wiki/Lake_Wobegon wrote:
<<Lake Wobegon, Minnesota: "the little town that time forgot, and the decades cannot improve," and "where all the women are strong, all the men are good looking, and all the children are above average." The Lake Wobegon effect, a natural human tendency to overestimate one's capabilities, is named after the town.>>

Many years ago, Isaac Asimov wrote a book entitled "The Double Planet" in which he contended that Earth is truly a special case. Although there may be many Earth-like planets, what makes our Earth unique, it's the disproportionate Moon -- a satellite nearly 1/4 the size of the primary planet -- which has never been observed in any other system, may be a critical factor. Having the Moon as large and as close as it is, he contends, is only one of the very many factors which enabled life to evolve. Firstly, the Moon acted as a shield to absorb or deflect much of the cosmic detritus floating around during creation, stuff that might have otherwise made it to Earth and destroyed whatever life happened to be evolving. Secondly, the Moon induced huge tidal forces upon Earth. When things finally settled down and the oceans began to support life, the tides enabled life to slowly adapt to the land. Finally, he contends, the Moon serves as a springboard for humanity to reach the rest of space. How difficult would it be to go from Earth to Mars in one shot?

In short, looking for a DOUBLE planetary Earth-Moon system, which has made the Earth be what the Earth is, may, unfortunately, be incredibly rare, but also might be one of the critical requirements for life to evolve and flourish.

GoddardOffTheGround wrote:
In short, looking for a DOUBLE planetary Earth-Moon system, which has made the Earth be what the Earth is, may, unfortunately, be incredibly rare, but also might be one of the critical requirements for life to evolve and flourish.

Some people believe that there is another such "planetary" system:

i'm interested to see how the new population distributions affect the drake equation (or its equivalent). i haven't yet seen data published in a useable format; the probability of life evolving on earth mass planets depends on many factors, including the parent star spectral type, the mass limits for holding an atmosphere and forming a solid surface, the expected quantity of water in protoplanetary disks that produce earth sized (rather than jupiter sized or binary star) systems, and so on. i used to believe that there were perhaps half a dozen technically advanced civilizations at any epoch in our galaxy of ~100 billion stars. now i might think it's perhaps a full dozen. the uncertainties are certainly shifting from astronomical factors to mechanisms of biological evolution, and the question of how useful a technically advanced civilization really is to the career of a predator species. for all we can tell, technical culture may accelerate a predator species through great achievements and into prompt extinction: at present, humans have been around for a tiny fraction of the typical species endurance of millions of years. but the astronomical data help to limit the range of probabilities.

neufer wrote:

GoddardOffTheGround wrote:
In short, looking for a DOUBLE planetary Earth-Moon system, which has made the Earth be what the Earth is, may, unfortunately, be incredibly rare, but also might be one of the critical requirements for life to evolve and flourish.

Some people believe that there is another such "planetary" system:

[b][color=#0000FF][size=135]Artist's concept of Charon seen from the surface of Pluto.[/size][/color][/b]

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Perhaps a "planetary" system where an Earth-like moon orbits a gas giant in a star's habitable zone would meet the criteria for the evolution of life (or something like terrestrial life) as in James Cameron's "Avatar".

drollere wrote:i'm interested to see how the new population distributions affect the drake equation (or its equivalent).

Not much, I would think.

The problem with the Drake equation is that the term for habitable planets has never been dominant. It is the terms associated with the probability of life forming in the first place that are so uncertain, and this new data does nothing to change that.

GoddardOffTheGround wrote:... it's the disproportionate Moon -- a satellite nearly 1/4 the size of the primary planet -- which has never been observed in any other system, ...

True, but we simply have no way to detect such moons around smaller rocky planets in the "water zone" of any extra-solar system -- yet. We will have soon, and since there's no particular reason to assume such moons are rare (and no reason to assume they're not rare either) we'll just have to wait and see.

If we eventually find that such moons are extremely rare, and that life only exists in their presence, your suggestion will become a conclusion. But not yet.

Rob

http://asterisk.apod.com/viewtopic.php?p=190587#p190587

"Planets closer to their stars are easier to find because they transit more frequently. As more data are gathered, planets in larger orbits will come to light. In particular, Kepler's extended mission should allow it to spot Earth-sized planets at greater distances, including Earth-like orbits in the habitable zone. "

So more earth size planet might be find. The number of earth size planet will increase as the capacity to detect them grow and the time to see the transit come. Billion of earth size planets will add up. Cool.

I'm reminded of Plato's parable of the cave. We're sitting here in the dark, watching nearly infinitesimal dips in the brightnesses of stars in a tiny patch of our galaxy. The patterns we're able to detect and the inferences we can draw from them are truly exciting! But it seems much too early to speculate too grandly about life on other planets and our prospects for interstellar colonization. To date, there is one body in the universe known to harbor life. It's hard to do inferential statistics with a sample size of one.