Friday, January 24, 2014

Finding a "Second" Earth? We Better Make the Best of THIS One!

Two representations of 603 exoplanets around Sun-like stars. 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)

In  recent TIME article ('Finding A Second Earth') we learn that Harvard lecturer Lisa Kaltenegger is busy modeling previously discovered exoplanets. Of coure, many astronomers are also doing this. What's different with Kaltenegger is her incorporation of data about our own Earth: its meteorology, geology and its history. The point is, our own planet - seen by any alien civilizations, would look very differently depending on the time point it was being observed. For example, the Earth observed 3.9 billion years ago, would have appeared very differently - as a brownish globe with an atmosphere mostly of hydrogen sulfide (the rotten egg smelling gas), CO2 and nitrogen.

Data collected by hypothetical advanced aliens-  using many of the same methods we're using today - i.e. parsing exoplanets' atmospheres during transits of their parent stars,  likely would have concluded an inhabitable world - never mind its placement in a "habitable" zone.  (Assuming the observing aliens have roughly the same biology)

Now, in terms of recent advances, we're actually able to discern the approximate number of livable, Earth-like worlds - from a photometric record of some 43,000 stars observed using the Kepler telescope. Launched in March, 2009, Kepler's primary mission was to continuously monitor  large numbers of stars over long periods to in search of the very slight dimming that indicates the repeated transit of an exoplanet across the face of its host star.

One of the most critical parameters to obtain - to assess for an Earth-like planet, is 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 a planet is 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, recently reported an analysis of 603 planets 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.

To perform their automated searches they used a software package called TERRA - developed by Marcy's grad student Erik Petigura. The program cleansed the photometric records of suspected outliers, intrinsic stellar variations, and a variety of other systematic errors.  Incredibly, Petigura next vetted by eye thousands of records for which the program found transit evidence - to weed out any anomalies due to instrumental or cosmic ray effects.

The team also eliminated any candidates with binary companions with radii (measured by the depth of the dimming during transit  trough) that exceeded 20 times the Earth's radius. (The fractional dimming of a star of radius R* during transit of a planet with radius R(p) is (R(p)/R*)2

The graphic shown gives the Marcy-Howard final sample of 603 planets, plotted in two distinct graphs, with (a) showing planet radius vs. orbital period in days (P) and graph (b) plotting the radius vs. stellar irradiation flux. (Note: uncertainties in stellar size, diameter contribute to uncertainties in both R* and the stellar flux Fp.)

Figure (b) is perhaps the more crucial graphic, with the stellar flux Fp  calculated from the orbital separation and the star's intrinsic luminosity (which is determined from its surface temperature and area (deduced from radius), see e.g.

The solar flux ( Fo ) at Earth's mean distance from the Sun (1 A.U. = 1.5 x 10 11 m )  is 1.36 kW/ m2  for reference.  The green patch in (b) contains those few candidates occurring in Earth-like habitable zones, with fluxes limited to 0.25 - 4Fo   and with planet sizes 1-2 R.  

So  how many of those 603 planets might actually be livable habitats for Earthlings? As it turns out not very many! Inside the green box one counts 10 in all. the vicinity of stellar flux  Fo = 1 and planetary radius R =1 (determining the acceleration of gravity g) we find exactly one candidate planet. Given the photometric uncertainty (vertical-horizontal crosses on right side of (b) even this candidate could conceivably be knocked out.

The takeaway? We had better be good stewards of this planet and not look afar for colonizing opportunities (assuming we reach the level of starship builders and don't blow ourselves up, squander all our resources on stupid wars, or perish in a greenhouse holocaust). At the rate we are going, consuming the equivalent of 1.5 Earths every year, the future isn't promising.

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