APOD: Ten Billion Earths (2013 Jan 12)

[George K]

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, I meat to say, How do we know the number in the first set is greater than the number in the second? The second set, planets with Earth-like orbits, are harder to detect with current techniques, so we count fewer of them, but that doesn't mean there are intrinsically fewer of these planets. It's the usual selection-effect issue.

[GoddardOffTheGround]

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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Well, although Pluto-Charon might be considered a double planet, they both lack all of the criteria for an Earthlike environment. What I was hinting was that the focus is much on determining if planets are like Earth. Perhaps the focus should be determining if some planets like Earth have a very large moon nearby. As Asimov suggests, LAWKI (Life As We Know It) would probably not be possible without our Moon. How much probable? Who knows yet?

[ddale51]

Earth is a "Goldilocks" planet. Everything is just right. Just right air. Just right sun. Just right gas giant neighbors. Just right size moon. Just right rotation rate, axial tilt, composition, weather patterns. Just right, well, everything needed to support advanced life. There are nearly countless factors that had to be just right for creatures like us to exist on this planet. Earth is truly a miracle. Who knows? it's conceivable it may be unique in all the cosmos. I believe it was created to be humanity's home. You may disagree of course; but you also have to concede that this beautiful blue marble is a wonder indeed.

[geckzilla]

And yet we also know it was not always just right for human habitation as we currently know it. Humans, along with all modern animals, evolved into and with their own niches, which have also changed dramatically over time.

[BDanielMayfield]

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, I meat to say, How do we know the number in the first set is greater than the number in the second? The second set, planets with Earth-like orbits, are harder to detect with current techniques, so we count fewer of them, but that doesn't mean there are intrinsically fewer of these planets. It's the usual selection-effect issue.

That's very true George. Detecting solar systems like ours is still beyond our abilities, so it's too soon to say that systems like ours are rare, IMO.

Earth is a "Goldilocks" planet. Everything is just right. Just right air. Just right sun. Just right gas giant neighbors. Just right size moon. Just right rotation rate, axial tilt, composition, weather patterns. Just right, well, everything needed to support advanced life. There are nearly countless factors that had to be just right for creatures like us to exist on this planet. Earth is truly a miracle. Who knows? it's conceivable it may be unique in all the cosmos. I believe it was created to be humanity's home. You may disagree of course; but you also have to concede that this beautiful blue marble is a wonder indeed.

I couldn't agree more. Well stated.

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.

Since no one else supplied the answer I looked it up. The formula for the gravitational force at the surface of a planet is F=GM/r^2 where G is the gravitation constant, M is the mass of the planet and r is the distance to the planet's center (radius). So the weight of an object on a planet is directly proportional to the planet's mass, but it's inversely proportional to the square of the radius. I hope that helps answer your question Stefan.

[rstevenson]

Earth is a "Goldilocks" planet. ...

That's a restating of the Rare Earth hypothesis, about which you can read at this Wikipedia page and its many linked-to pages and articles. Here's the opening definition:

In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the emergence of complex multicellular life (metazoa) on Earth (and, as follows, intelligence) required an improbable combination of astrophysical and geological events and circumstances. The hypothesis argues that complex extraterrestrial life requires an Earth-like planet with similar circumstance and that few if any such planets exist.

Which contrasts with...

The principle of mediocrity concludes that the Earth is a typical rocky planet in a typical planetary system, located in a non-exceptional region of a common barred-spiral galaxy. Hence it is probable that the universe teems with complex life.

At this point in time, choosing one of those as "true" is a philosophical decision, not a scientific one. Perhaps in a few thousand years we'll have enough data to state a conclusion.

Rob

[neufer]

And yet we also know it was not always just right for human habitation as we currently know it. Humans, along with all modern animals, evolved into and with their own niches, which have also changed dramatically over time.

The Earth was made for bacteria.

Heck...everything from humans to termites & cockroaches was made for bacteria:

[b][color=#0000FF]Stromatolites provide some of the most ancient records of life on Earth by fossil remains which date back more than 3.5 billion years ago.[/color][/b]

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<<The ancestors of modern bacteria were single-celled microorganisms that were the first forms of life to appear on Earth, about 4 billion years ago. For about 3 billion years, all organisms were microscopic, and bacteria and archaea were the dominant forms of life. Although bacterial fossils exist, such as stromatolites, their lack of distinctive morphology prevents them from being used to examine the history of bacterial evolution. However, gene sequences can be used to reconstruct the bacterial phylogeny, and these studies indicate that bacteria diverged first from the archaeal/eukaryotic lineage.

Bacteria constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals. Bacteria are present in most habitats on the planet, growing in soil, acidic hot springs, radioactive waste, water, and deep in the Earth's crust, as well as in organic matter and the live bodies of plants and animals, providing outstanding examples of mutualism in the digestive tracts of humans, termites and cockroaches. There are approximately ten times as many bacterial cells in the human flora as there are human cells in the body, with large numbers of bacteria on the skin and as gut flora.

There are typically 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water; in all, there are approximately five nonillion (5×10³⁰)* bacteria on Earth, forming a biomass that exceeds that of all plants and animals. Bacteria are vital in recycling nutrients, with many steps in nutrient cycles depending on these organisms, such as the fixation of nitrogen from the atmosphere and putrefaction. In the biological communities surrounding hydrothermal vents and cold seeps, bacteria provide the nutrients needed to sustain life by converting dissolved compounds such as hydrogen sulphide and methane.>>

* Note: Sagan's number is the number of stars in the observable universe presently estimated to be ~70 sextillion (7×10²²).

[geckzilla]

Made for bacteria or made OF bacteria?

[ro_star]

should say percentage not fraction!

on a planet twice as massive of course you would not weigh twice as much! look up Surface_gravity on wikipedia.

look up planetary factsheets - if earth gravity is 1 and mars is 0.38, is mars 0.38 the mass of earth? of course not!

ALL stars have planets - at least 8 each (our star being an average example)

if you read the article carefully, the one linked at kepler.nasa.gov, you see that these PERCENTAGES refer to planets with orbits up to 400 days; once telescopes are available to see all planets, we'll know ALL stars have planets, would be highly improbable to find one without planets

[xarkonnen]

hohoho, "Please give another globe" is not a joke now!

[Cal]

I'm a huge fan of APOD, but I must say this is one of the most disappointing "pictures". This graph is poorly designed for actual scientific study. As others have pointed out, the Y axis is mislabeled, and the values on each bar don't match the text. I also noticed that the graph is physically distorted: the horizontal lines aren't actually horizontal, but converge toward the right, so the scale on the right side of the graph is not the same as the left.

I recommend that anyone who wants to draw a conclusion from this graph should not; instead, look up the original data and study it carefully.

[neufer]

I'm a huge fan of APOD, but I must say this is one of the most disappointing "pictures". This graph is poorly designed for actual scientific study. As others have pointed out, the Y axis is mislabeled, and the values on each bar don't match the text. I also noticed that the graph is physically distorted: the horizontal lines aren't actually horizontal, but converge toward the right, so the scale on the right side of the graph is not the same as the left.

I recommend that anyone who wants to draw a conclusion from this graph should not; instead, look up the original data and study it carefully.

Unfortunately, the folks in charge of press (media) releases for large scientific organizations
are primarily publicity agents who know very little about either science or math.

The [Harvard/Smithsonian Center for Astrophysics] Public Affairs Office (PAO)
issues press (media) releases and other materials pertaining to CfA's research and projects.

[Macropus]

I love this site but not the occasional bit of easily avoidable sloppiness, such as the "fractions" on the y axis appearing as whole numbers. Likewise what does "size" mean on the x axis? Is it mass, volume, diameter...? It was always drilled in to me that graphs mean nothing without units of measurement and they certainly would have helped here.

[inertnet]

Earth is a "Goldilocks" planet. Everything is just right. Just right air. Just right sun. Just right gas giant neighbors. Just right size moon. Just right rotation rate, axial tilt, composition, weather patterns. Just right, well, everything needed to support advanced life. There are nearly countless factors that had to be just right for creatures like us to exist on this planet. Earth is truly a miracle. Who knows? it's conceivable it may be unique in all the cosmos. I believe it was created to be humanity's home. You may disagree of course; but you also have to concede that this beautiful blue marble is a wonder indeed.

Yes, but if a planet exists somewhere, with slightly different air, different sun, different moon etc. If life evolved there too, they could say exactly the same thing. They might think that they're unique too. We exist because circumstances have been exactly right for us to evolve, but that doesn't mean that ours are the only circumstances where the emergence of intelligent life would have been been possible.

[Raven]

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,....

Closer orbits around colder stars, or more distant orbits around hotter stars, should provide equally "habitable zones", to adjust the odds of finding livable Earth-like planets back up again.

[Raven]

They might think that they're unique too.

Goldilocks thought that only Little Bear's stuff was "juuuust right!"

But Papa Bear's stuff was just right for Papa Bear, and Mama Bear's stuff was just right for Mama Bear.

That aspect of the "Goldilocks" story is worth remembering.