Exoplanet Census Suggests Earth Is Special after All

Feb 19, 2016

Photo credit: NASA/ESA/ESO

By Shannon Hall

More than 400 years ago Renaissance scientist Nicolaus Copernicus reduced us to near nothingness by showing that our planet is not the center of the solar system. With every subsequent scientific revolution, most other privileged positions in the universe humans might have held dear have been further degraded, revealing the cold truth that our species is the smallest of specks on a speck of a planet, cosmologically speaking. A new calculation of exoplanets suggests that Earth is just one out of a likely 700 million trillion terrestrial planets in the entire observable universe. But the average age of these planets—well above Earth’s age—and their typical locations—in galaxies vastly unlike the Milky Way—just might turn the Copernican principle on its head.

Astronomer Erik Zackrisson from Uppsala University and his colleagues created a cosmic compendium of all the terrestrial exoplanets likely to exist throughout the observable universe, based on the rocky worlds astronomers have found so far. In a powerful computer simulation, they first created their own mini universe containing models of the earliest galaxies. Then they unleashed the laws of physics—as close as scientists understand them—that describe how galaxies grow, how stars evolve and how planets come to be. Finally, they fast-forwarded through 13.8 billion years of cosmic history. Their results, published to the preprint server arXiv and submitted to The Astrophysical Journal, provide a tantalizing trove of probable exoplanet statistics that helps astronomers understand our place in the universe. “It’s kind of mind-boggling that we’re actually at a point where we can begin to do this,” says co-author Andrew Benson from the Carnegie Observatories in California. Until recently, he says, so few exoplanets were known that reasonable extrapolations to the rest of the universe were impossible. Still, his team’s findings are a preliminary guess at what the cosmos might hold. “It’s certainly the case that there are a lot of uncertainties in a calculation like this. Our knowledge of all of these pieces is imperfect,” he adds.

Take exoplanets as an example. NASA’s Kepler space telescope is arguably one of the world’s best planet hunters, but it uses a method so challenging that it is often compared with looking across thousands of kilometers to see a firefly buzzing around a brilliant searchlight. Because the telescope looks for subtle dimming in a star’s light from planets crossing in front of it, Kepler has an easier time spotting massive planets orbiting close to their stars. Thus, the catalogue of planets Kepler has found lean heavily toward these types, and smaller, farther-out planets are underrepresented, leaving our knowledge of planetary systems incomplete. Astronomers do use other techniques to search for smaller planets orbiting at farther distances, but these methods are still relatively new and have not yet found nearly as many worlds as Kepler. In addition, “everything we know about exoplanets is from a very small patch in our galaxy,” Zackrisson says, within which most stars are pretty similar to one another in terms of how many heavy elements they contain and other characteristics. The team had to extrapolate in order to guess how planets might form around stars with fewer heavy elements, such as those found in small galaxies or the early universe.


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10 comments on “Exoplanet Census Suggests Earth Is Special after All

  • @OP – In addition, “everything we know about exoplanets is from a very small patch in our galaxy,” Zackrisson says, within which most stars are pretty similar to one another in terms of how many heavy elements they contain and other characteristics. The team had to extrapolate in order to guess how planets might form around stars with fewer heavy elements, such as those found in small galaxies or the early universe.

    Despite the enthusiasm of some SETI enthusiasts, and wishful misusers of the Drake Equation, Earth-like planets are likely to be extremely rare. This has been known for some decades now, and as more information has become available, the understanding of the extent of this rarity has increased.

    The Rare Earth Hypothesis (Which describes the rarity of the Earth-Moon system) has been around for a long time, and since it was postulated, the recognition of “Goldilocks Zones” in solar-systems, the rarity of our Solar System, the rarity of the our type of galaxy, and the recognition of galactic habitable zones, have all reduced the likely incidence of Earth-like planets existing in the universe or the galaxy.

    http://www.astrobio.net/news-exclusive/galactic-habitable-zones/
    Keeping out of the way of the Galaxy’s spiral arms is another requirement of the Galactic Habitable Zone.

    The density of gases and interstellar matter in the spiral arms leads to the formation of new stars. Although these spiral arms are the birthplaces of stars, it would be dangerous for our solar system to cross through one of them. The intense radiation and gravitation of a spiral arm would cause disruptions in our Solar System just as surely as if we were closer to the center of the Galaxy.

    Luckily, our Sun revolves at the same rate as the Galaxy’s spiral-arm rotation. This synchronization prevents our Solar System from crossing a spiral arm too often.

    “At our location, our orbital period is very similar to that of the pattern speed of the spiral arms,” says Gonzalez. “This means that the time interval between spiral arm crossings will be a maximum, which is a good thing, since spiral arms are dangerous places. Massive star supernovae are concentrated there, and giant molecular clouds can perturb the Oort cloud comets leading to more comets showers in the inner solar system.”

    The unusually circular orbit of our Sun around the galactic center also tends to keep it clear of the spiral arms. Most stars the same age as our Sun have more elliptical orbits.

    “If the Sun’s orbit about the galactic center were less circular,” says Gonzalez, “the Sun would be more likely to cross spiral arms.”

    Thus, thanks to a lot of unusual characteristics of our Sun, our Solar System is lucky enough to lie in a Galactic Habitable Zone. Gonzalez argues that these characteristics made it possible for complex life to emerge on Earth. More than 95 percent of stars in the Galaxy, says Gonzalez, wouldn’t be able to support habitable planets simply because their rotation is not synchronized with the rotation of the galaxy’s spiral arms. Add all the other factors involved in keeping a solar system habitable, and it seems that the odds of finding another solar system in a Galactic Habitable Zone are close to impossible.

    “everything we know about exoplanets is from a very small patch in our galaxy,” Zackrisson says, within which most stars are pretty similar to one another in terms of how many heavy elements they contain and other characteristics.

    This means that star systems near to the Solar-System are likely to contain far more that rocky planets than average.



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  • A key factor in planet formation is metalicity – the percentage of heavy elements in relation to Hydrogen and Helium in the accretion disk, the star and any planets.

    http://www.space.com/15341-planet-formation-stars-heavy-elements.html
    Heavy Elements Key for Planet Formation, Study Suggests –
    Planets form more commonly in star systems with relatively high concentrations of elements heavier than hydrogen and helium, a new study suggests.

    Such heavier elements are necessary to form the dust grains and planetesimals that build planetary cores, according to the study, which was carried out by researchers Jarrett Johnson and Hui Li of Los Alamos National Laboratory in New Mexico.

    Additionally, evidence suggests that the disks of dust that surround young stars don’t survive as long when the stars have lower concentrations of heavy elements, or lower “metallicities” in astronomers’ jargon. The most likely reason for this shorter lifespan is that light from the star causes clouds of dust to evaporate.

    Our cosmic history has several defining epochs, one of which is the point at which star systems began to form planets. Heavy elements such as carbon, silicon and oxygen first needed to be created from huge star explosions called supernovas and the stellar cores of the first generations of stars before the first planets could form.

    According to Johnson and Li, a successful theory of planet formation should make predictions about the properties of the earliest planets and their host stars. Such a theory could be tested by studying very old planetary systems in our galaxy. The enrichment of gas with metals from supernovas is thought to affect not only planetary formation, but the formation of low-mass stars like our sun as well.

    “A planet as massive and dense as the Earth could only form once stars and supernovae had enriched the gas with an abundance of heavy elements that is at least 10 percent that in the sun,” Johnson said. “This suggests that many generations of stars had to form and evolve before habitable planets could form.”



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  • AG #5
    Feb 22, 2016 at 5:43 pm

    The link is just a small example of possibilities.

    These are indicative of possibilities, but on the down side, I did not see any reference to large moons, and at that distance, they are well beyond the reach of even potential future Earth created space-craft.



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  • Much interesting stuff in the article and the paper it references. Of course, like with the Drake equation, there are still too many unknowns for us to get any meaningful estimate of the chances of other life in the universe. This will depend greatly on the likelihood of life arising where conditions can support it and we have little data.

    How sad to read all this and then just a few minutes later come across such an appallingly titled article as below on CNN.

    http://www.greatbigstory.com/stories/hpe-exploring-the-other-side-of-the-universe

    The headline is “TELESCOPE SO POWERFUL IT CAN SEE INTO THE PAST”

    I want to vomit. The naked eye can see hundreds or thousands of years into the past every time you look at the night sky. I see eight minutes and 20 seconds into the past every time the sun comes out. Still, when a considerable proportion of Americans don’t even know the earth orbits the sun or how long that takes I suppose it’s too much to expect them to know what a telescope does.



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  • I very much enjoyed the comments section.

    My first comment here, so I’ll state the obvious:

    “…out of a likely 700 million trillion terrestrial planets in the entire observable universe.”
    Let’s say earth is one in a million trillion, that leaves us with 700 earth-like planets?

    That’s really nothing. Negligible to one stunningly big to another.



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  • Bolesław Fazurczak Nullo #8
    Feb 23, 2016 at 4:44 am

    I very much enjoyed the comments section.

    My first comment here, so I’ll state the obvious:

    Welcome to the discussion.

    “…out of a likely 700 million trillion terrestrial planets in the entire observable universe.” Let’s say earth is one in a million trillion, that leaves us with 700 earth-like planets?

    We still don’t know numbers, but on the one hand there appear to be many planets which are not suitable to support life (as we know it), but on the other hand there are so many stars that the odds must favour the existence of some!

    The key question for humans is the proximity of possible life sustaining planets in our galaxy, so other galaxies with low metalicity stars, or distant areas within our galaxy which are hostile to life, are not immediately relevant.

    It may be that there are some high metalicity star systems within tens of light years of our Solar System with habitable planets, so it may be possible for humans to visit these by star and planet hopping.
    There are enough moons, dwarf planets, Kuiper Belt Objects, and Oort Cloud bodies, to resource star-ships or star probes, if (robotic?) mining bases can be set up out there.
    https://en.wikipedia.org/wiki/Asteroid_mining



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  • “If you have these civilizations that had a 3.5-billion-year head start on us, why haven’t they colonized our
    galaxy?” asks Tegmark. “To me, the most likely explanation is that if the planets are a dime a dozen, then highly
    intelligent life evolves only rarely.”

    Or, intelligent life is plentiful but rarely survives long enough to colonize a galaxy. Or, intelligent life is plentiful and has colonized sections of our galaxy, but we aren’t able to observe the evidence of that colonization. Or, intelligent life is plentiful and capable of colonizing a galaxy, but nobody has colonized our galaxy yet (and other galaxies are too far away to observe the evidence of colonization). Or, intelligent life is plentiful, but it takes billions of years to get to the point where it can begin to colonize a galaxy and nobody has gotten to that stage yet.

    Or (and this is my personal favorite), it’s simply NOT POSSIBLE TO COLONIZE A GALAXY! Given the vast distances involved, the limits imposed by the speed of light, the amount of resources required to colonize even one other stellar system, why does everybody just assume it’s possible, let alone probable, that other civilizations would have done this by now if they existed?

    But, yeah, I tend to agree that there’s a very good chance that intelligent life as we know it may be vanishingly rare in the cosmos, if only because of all the conditions that needed to be “just right” in order for it to happen here on Earth. Creationists are that this is proof that God must have done it, since the odds of it happening by itself are so small. I argue that it’s just proof that we are lucky and it’s probably a very lonely place out there…



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