Monday, November 30, 2020

How Really "Earth -like" Is The Newly Discovered Planet Kepler 186f?


    Artist concept of the planet Kepler 186f in the constellation Cygnus, 500 light years away.

Since the Kepler mission was launched in 2009 to monitor over 150,000 stars for signs of exoplanets, the discoveries have come fast and furious, e.g.

And now it appears that after crunching Kepler's data for two years  a team of 44 astronomers led by Steve Bryson of the Ames NASA research  facility - may have arrived at the definitive answer to the question: How many of the billions of exoplanets in the Milky Way are actually habitable?    What that means is basically that the planet or planets occur in a "Goldilocks" zone around the target star - such that the environment is warm enough to retain liquid water. 

Thus far, the Ames team has calculated that at least one-third - and possibly as many as 90 percent of stars- similar in mass and brightness to our Sun -  have planets like Earth in their habitable zones. (The range reflects the variation in  confidence in the researchers' methods of detection and their attendant assumptions).

If then there are 100 billion stars in the Milky Way galaxy, of which 4 billion are defined as "Sun-like" - then we are talking of a lot of Earth- like planets. Any of which could support life of some kind.  If half of these planets met the habitable zone threshold we are looking at two billion worlds like our own.  But even if only 7 percent of those Sun-like stars have habitable planets we are looking at as many as 300 million "Earths" - most with potential life forms. 

This brings up the question of what the average distance might be between such planets.  The Ames team calculated the nearest such planet is about 20 light years distant.  Also there should be four such planets within 30 light years or so from the Sun. 

  Of particular attention now is the first Earth-sized  planet discovered in the habitable zone by Kepler, designated Kepler 186f, about 500 light years from Earth in the constellation Cygnus.   The "Earth -sized" aspect is important, because if a planet is a giant like Jupiter and in the habitable zone, it still wouldn't likely support life because the surface gravity is too high.

In the case of Kepler 186f, we are looking at a world with  about 1.11 times the radius of Earth.  So even if the interior was rocky and roughly the density of our Earth, the surface g -value would not be too terribly great.   We also know the planet orbits its central  M-type star  in about 130 days.  Using Kepler's harmonic law:   

Given the cited distance (»  0.40 AU) and the fact that the central star in the 186 system has a temperature of 3755 K, we can determine the radiant energy  received on the planet (or "solar constant") would be roughly one-third of that for Earth. So if Earth receives 1360 W/ m2  then Kepler 186f would receive about 450 W/ m2   .  In other words, we are talking about a fairly chilly planet, even at its equator.   

One of the more down notes of the project is that:  "The Kepler mission didn't detect many (arguably any) true Earth analogues, i.e. planets with the same radius of Earth and orbiting at the same period - and hence receiving the same amount of light."   

This,  according to an email  to the media from David Charbonneau of the Harvard-Smithsonian Center for Astrophysics.  Indeed, even the distance from the star cited of 0.40 AU is open to skepticism, being more likely the distance at perihelion. Because when one applies the Kepler 3rd or harmonic law, e.g.

(P1/ P2)2 = k(a1/ a2)3

One obtains for a1,  the semi-major axis (mean distance ) of the planet from its star:

P1 = (129.9/365.25) yr. = 0.353 yr.

a1 = {[P1]2}1/3 = [(0.353)2]1/3

a1 = 0.5 AU

This  actually turns out to be just beyond the computed habitable zone terminating at 0.46 AU (according to the Wikipedia entry, though noting the range is a "conservative one")   It would certainly be good if one of the Ames team could explain the discrepancy!   In the meantime, we need to apply a solid dose of skepticism to any of the Kepler results we see and any extrapolations from them.  

As Natalie Batalha, quoted in a recent press account in The Denver Post  (Nov. 29, p. 18A) put it :

"We don't yet have any planet candidates that are exact analogues of the Earth in terms of size, orbit or star type.

As a result, and left unsaid, is that the research team has had to extrapolate data from the planets they did observe.  Is this good enough?  Time will tell, but at this stage I'd say we have to understand the limits of the Kepler project and not let expectations run ahead of the actual data.

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