Stunning Revelations from Kepler Space Telescope Exoplanet Survey: MANY small planets
On June 19, 2017, NASA announced the release of the eighth Kepler catalog of exoplanet candidates—now totaling 4,034, of which 2,335 have already been verified as planets. Here’s a link to the press briefing materials on the NASA website, that includes some absolutely cool visuals.
Kepler survey announcement June 2017
The news in June was that another 219 candidates had been added since the last announcement, 10 of which had characteristics of size, distance from host star, and sun-like nature of the host star, that suggest conditions hospitable to life as we know it. Rocky planets with diameters between 0.7 and 2 Earth diameters are the most promising if they fall within the “Goldilocks Zone” where liquid water could exist on their surfaces.
Turns out, contrary to what I had been expecting from the news about huge exoplanets dribbling out over the last decade, now we hear that “small planets are common” according to NASA. Note that “small” is in the eye of the beholder. NASA qualifies a planet as “small” if it is less than four Earth diameters. The trend toward the discovery of “gas giants” exoplanets early on was due to the difficulties, with less sensitive instruments and less sophisticated methodology, of detecting the smaller objects.
(“Large” planets, on the other hand, can be really large. Our solar system’s Jupiter, possessing more than twice the mass of all the other planets put together, is eleven times the diameter of Earth. Big, but there’s a “super Jupiter” 170 light years from us with 13 times the mass of Jupiter, and thought by some even to have reached quasi-stellar status as a “brown dwarf” heating itself by fusion.)
The candidates found by Kepler are then studied by observations from the Keck observatory to confirm their status.
As surveyed so far, planets up to four times the size of Earth are more common than the “gas giants” that were the first, and easiest, to be found.
In an earlier post, to be found at TRAPPIST-1 and other spacey stuff, I discussed the discovery of the TRAPPIST-1, a solar system just 40 light years from us with a clan of promising small planets. At the time the news came in, in February of 2017, I had little idea of the wealth of small planets Kepler was finding.
How small or large is life-friendly?
For starters, too small to hold a sufficiently dense atmosphere would be a nonstarter for organisms with a high metabolic rate, even if the atmosphere was mostly oxygen. A hummingbird released on Mars would expire within a minute or so, even if Mars’s atmosphere was 1/5th oxygen, like Earth’s. Since Mar’s atmosphere is mostly carbon dioxide, with an oxygen content 0.13%, the hummingbird would survive for perhaps 20 seconds.
Mars, with a mass a tenth that of Earth, has a surface pressure only one-hundredth that of Earth. Low mass, and low escape velocity means that, over long time periods, what atmosphere there is will leak away from a small planet over time—less time than it would take for live to evolve into complex forms. That’s because gas molecules are in constant motion, and by matter of chance, some will achieve escape velocity and fly away to parts unknown. It adds up. Another atom-robbing effect with small planets is the lack of a magnetosphere (see three paragraphs on), allowing solar winds to tear off ions from the top of the atmosphere down. (As that mythical trickster’s namesake, tiny Mercury, has shown us, even a planet 1/20th the size of Earth can have a magnetosphere.)
And what about Venus? With a mass and gravity close to that of Earth’s, the second rock from the sun is swathed with an atmosphere yielding a surface pressure of 92 times that of Earth, the highest in the Solar System. It has no detectable magnetosphere, so what gives? A mystery wrapped in an enigma. You can research it for yourself, since I don’t understand the explanations
You might think that a planet three times the radius of Earth might have a prohibitively high gravity to accommodate terrestrial life, but that depends on the density. In our solar system, the density of the rocky planets is very similar—if Earth is 1.0, Mercury is 0.984, Venus 0.951, and Mars 0.713. A planet twice the radius of Earth with the same density would have twice the surface gravity;* at half Earth’s density it would have the same surface gravity. A planet three times the diameter of Earth but with half the density would have a gravity only 50% stronger than Earth’s at the surface.
For a matrix of properties of our solar system’s planets compared with Earth, see Earth vs the rest.
It’s suspected that a planet with half Earth’s density, no matter how voluminous, would lack a global magnetic field (magnetosphere). Earth has a rotating molten iron-nickel core that generates our magnetosphere, and such a structure is unlikely in a less dense planet.
Lacking the protection of such an Earth-like electromagnetic shell, a planet would be subject to far more intense radiation from their star. As we mentioned above, such is the case with Mars, a tenth the mass of Earth, which has only a minimal magnetic field.**
Note also that cosmic rays—fired from distant supernovae [the most probable source] and shooting unhindered through interstellar space—also bombard the Earth from every direction. It’s not just the Sun that sprays us with radiation. )
Such electromagnetic radiation would put a dire stress on the evolution of life.
Farfetched for life to withstand drenching by radiation?
Would the lack of a strong magnetosphere prohibit the evolution of complex life forms? Well, consider that despite our own magnetosphere, we tolerate blasts of solar radiation that get to the surface anyway. Methinks, given that terrestrial beings have armored themselves against a not-negligible amount of solar radiation, that alien life might evolve to tolerate harsher radiation regimes. However, it might take a lot longer to get to the level of multicellular organisms.
BUT PERHAPS THE OPPOSITE IS TRUE!?! There is a wild (outlandish?) hypothesis that life evolved in the distant past on Venus and was conveyed to Earth (accidentally) on space rocks, before Venus enshrouded itself with its now thick, toxic atmosphere. The startling premise of this hypothesis is that high rates of solar irradiation of Venus (it being much closer than Earth to the sun, and without a magnetosphere) would have caused high mutation rates, and therefore have promoted early evolution more than held it back. This is the brainchild of Annabel Cartwright at Cardiff University in Wales. I say no more except to note that (1) Cartwright is not a crank but rather a respected scientist; (2) she is an astronomical physicist, not a biological scientist; (3) the laws of physics do not rule it out; and (4) you can ponder “The Venus Hypothesis” published in 2016 for yourself at the following locations: https://arxiv.org/ftp/arxiv/papers/1608/1608.03074.pdf
and https://arxiv.org/abs/1608.03074v1
Distinct classes of planets by size: a new finding
One of the most fascinating findings from the Kepler survey so far is an apparent bifurcation in frequencies of sizes of the “small” planets. As discussed and graphically represented in the NASA report linked to above, it appears that smallish planets fall into two main groups: (1) those smaller than 1.7 Earth-size, with subdivisions of “rocky Earth-sized” and “super-Earth sized;” (2) those 2.0 Earth-sized and larger, with the larger resembling Neptune (3.88 the diameter of Earth).
There are more of the larger-mass group in number. For a graphic illustration, with a simple histogram See the “Planets come in two sizes” chart in the NASA report.
In between the two groups there is a valley at around 1.75 Earth-sized, much less common than either the smaller or the larger. One speculation for the valley’s existence is that, at about 1.75 Earth size, there’s a jump of the gap to Neptune-sized as those planets take on small amounts of hydrogen and helium gas . Once the James Webb telescope gets cracking, we’ll find out more about exoplanet atmospheres and proto-atmospheres.
Life on other planets probable?
Short answer: yes.
The foundation is the estimate that the number of stars in the Universe exceeds that of the number of grains of sand on all the beaches of Earth.
You may have asked yourself—since this idea is being bandied about in the Earthbound news quite a bit recently—”how can that be?” If so, you have a lot of company. Doubters, you can find the calculations by “The Math Dude” at this location: https://www.universetoday.com/106725/are-there-more-grains-of-sand-than-stars/
Given so many stars (who am I to question “The Math Dude?”), and the abundance of Earthlike planets already discovered by Kepler, it is pretty much impossible to think of life not existing elsewhere in the Universe. If creatures at least as intelligent as ourselves might exist is more of a stretch, but I would give it better than even odds. If they exist, sadly it is next to impossible that we will ever communicate with them. Odds are, given the age and scale of the cosmos, that they have already gone extinct, or are at a primitive stage on their birth-world—so primitive that we will be extinct even before they evolve enough to send their message. Or become extinct while the message crosses hundreds of thousands of light-years. Or, arise after humans have become extinct. Mind, we’re talking multiples of billions of years.
(Yes, I’m sure humans will either go entirely extinct within at most the next 100,000 years, or we will have “evolved” (more likely, engineered ourselves) into creatures whom we would not recognize as members of our species. In other words, extinct. That’s if machine intelligence has not already wiped us out. (More on that pleasant topic in posts to come.))
So the question is not if life, or intelligent life, exists elsewhere, but when. If our stage of evolution happens to overlap with a similar stage in theirs, and they are close enough we can make contact, in my opinion that would be little short of a miracle.
Whether that would be a good or bad miracle is a subject of longstanding debate, but if they could make it here anytime soon and their level of acquisitiveness is anything like ours, I would say “Watch Out!”
================ footnotes below ==============
* At a similar density, a planet twice the radius of Earth would have a surface gravity twice as strong (it would have eight times the mass, but, since the surface is twice as far from the center of mass, the inverse square law reduces the force by a factor of four.). However, at a density half that of Earth’s, its mass would be only four times greater, so at the same radius the surface gravity would be the same. The same sort of computations hold for a body three times the radius of Earth—three times Earth gravity with the same density, but only 150% Earth gravity at half Earth density. .
** Mars may once have had a magnetosphere, but bombardment by comets and asteroids might have undone it.