Thursday, February 27, 2014

Kepler Finds 715 New Planets - Does It Mean A Greater Probability Of Life On Them? No

Planetary graphs data-near Earthlike planets
Two representations of 603 exoplanets around Sun-like stars found up to the end of 2013. The data are plotted with planet radius (R) vs. orbital period in days - P (a), and against stellar irradiation flux (b) (From Physics Today, January)

The news that the Kepler telescope has found 715 new planets is now making the general media rounds, but many outlets seemed to have jumped the gun and extrapolated from these a "higher possibility of life being found". This isn't true at all. As I noted in a previous blog post, one of the most critical first objectives  - to assess for an "Earth-like planet",  is to obtain the periodicity or the time it takes the planet to circle its Sun. This is based on the fact that with the period, one can determine a planet's distance, i.e. from Kepler's 3rd law, see e.g.

If then the period is too short,  a planet will be too close to its host star, never mind it may have the mass or radius of Earth. Then it can't really be called "Earth like" in the strict technical sense of actually being a place one could survive.  With those limitations in mind, Geoffrey Marcy of the University of California, Berkeley and Andrew Howard of University of Hawaii, last year reported an analysis of 603 planets (found via Kepler)  orbiting Sun-like stars, including with a few having periods longer than 300 days.

Bear in mind the Earth's orbital period is 365 days so any exoplanets with P > 300 days are of major interest since they could mark a genuine Earth-like planet, i.e. which could actually be colonized one day. This makes the Marcy-Howard research especially significant given they also corrected for observational biases and limitations, and restricted their searches to stars less than 3,000 light years away - with low photometric noise and Sun-like surface temperatures, or around 11,000 F.

But at the end of the analysis, a core of only one single planet of the 603 was adjudged to be in any way "livable". (Which still doesn't mean anything actually inhabits it)  This was identified within the small green inset box (from 10 plausible initial candidates) seen in the graph (b) at the right. (Plotting the radius vs. stellar irradiation flux.)  The candidate planet met the key criteria for in occurring in the vicinity of stellar flux  Fo = 1 and planetary radius R =1 (determining the acceleration of gravity g) .

 NASA on Wednesday announced the discovery of 715 new planets, but of these only four were designated as existing in any habitable zone.  The 715 planets were found to orbit 305 different stars, and were discovered by the Kepler space telescope using a new technique called "verification by multiplicity".  It relies in part on the logic of probability. Instead of searching blindly, the team focused on stars that the technique suggests are likely to have more than one planet in their orbit.

Let's examine briefly an assay of the planets thus far discovered. According to NASA, 95% of the planets discovered by Kepler are smaller than Neptune, which is four times as big as Earth. With such large radii, and likely accompanying density, none of them are plausible habitable worlds.

Meanwhile, one of the new planets is about twice the size of Earth and orbits a star half the size of Earth's sun in a 30-day cycle. This again eliminates it as being habitable based on the Kepler harmonic law. It would be too close to its parent star, hence too hot.  The other three planets in habitable zones also are all roughly twice the size of Earth, again likely making their gravity too great for any life to evolve.
Of course, more observations are needed, and planetary astronomers expect to find a higher percentage of new planets that could potentially have a life-supporting climate like Earth's.
But still, the best approach is not to get ahead of ourselves. While the more than 1,700 new planets discovered is exciting to be sure it doesn't mean that any of them harbor any life forms.  We have a plethora of planets, thanks to Kepler, but so far zero evidence for life on any of them.

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