The Galactic Habitable Zone or GHZ

Re-emergence of life after catastrophic extinction event
(Chen & Benton 2012)

Many times we hear about habitability or the habitability zones within stellar exoplanetary systems which is closely related to the presence of liquid water as a precursor to life. However, to have the heavier elemental components making up planets much like our Solar System required generations of stellar evolution to create those elements in the abundances we measure today. If we make the assumption that life evolves in a similar environment, then there is a dependence on the age, evolution and position of an exoplanetary system in a galaxy--or, put another way, a galactic habitable zone (GHZ) (e.g., Lineweaver et al. 2004, Dosovic et al. 2019).

Another way to think of GHZs is to assess the components within a galaxy that could hinder or damage the formation of life on an exoplanet. For example, an exoplanetary system being too close to a number of supernovae or where the star formation rate is high creating gamma rays, X-rays and cosmic rays could strip atmospheres of UV blocking. Also, complex life seems to require complex heavy elements which implies that the location of the exoplanetary system in a galaxy would need to lie somewhere in the very narrow 'young thin disc' (how narrow? see Yoachim & Dalcanton 2006). This is the region where star formation and evolution is most active and produces the heavier elements through supernova explosions.

One hopeful component for habitability in general is that life seems robust. On Earth, for example, life rebounded quickly after catastrophic events or within millions or tens of millions of years (e.g. see Chen & Benton 2012) which is only approximately 1 to 5% of the age of the planet. Given that, any life on exoplanets would have the ability to rebound and evolve to some form of intelligence over 10x within a habitable galactic zone once critical local habitability conditions for life were established.

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Unusual Naivete by Nobel Laureate Michel Mayor

Well, perhaps I'm being too strong in my title, but, to have the 2019 Nobel laureate Michel Mayor say that we will never live on another planet is astonishingly silly and naive. In a recent article with the Agence France-Presse, Mayor notes that we need to kill the idea that humans migrating to exoplanets will ever happen. "If we are talking about exoplanets, things should be clear: We will not migrate there" Mayor noted. Perhaps he was attempting to focus on fixing things on Earth without dreaming of migrations to other exoplanets, but he seems to be strong on "never" in his statements.

Note, however, he's referring to a full migration if things get bad enough on Earth, but his rationale for that statement is focused on the time it would take to get to any exoplanet. While I might agree that a full migration might be more problematic and a more reasonable statement, the reason why is flawed in my view.

Probably what distresses me the most about his statement on traveling to exoplanets is just how many revolutions in science have occurred  and what theoretical physics is saying about the possibilities of manipulating space-time to travel to other stellar systems in the galaxy. Also, history has proven naysayers wrong more often than not (just look at this list just for a more popular view).

Don't get me wrong - I think Mayor and Queloz's Nobel was well deserved for the first exoplanet discovery. However, Mayor clearly jumped in the history of science naysayer list of something 'that will never happen' which demonstrates a significant lack of scientific vision.

As Lord Kelvin noted in 1895, "Heavier-than-air flying machines are impossible" QED.

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Mars 2020 ExoScience

With less than a year away for the next Mars rover mission of Mars 2020, it seems that a quick overview of one of the science payloads is worth mentioning.  Mars 2020 is expected to launch next July with a landing on Mars in Feb of 2021. Some of the most exciting scientific experiments relate directly to exoastronomy. For the mission, the search for biosignatures is Science Goal #1.

Attempting to detect both past and present biosignatures is no easy feat considering how difficult it is to measure and not contaminate the samples. For the 2020 science team, "a biosignature (a 'definitive biosignature' or DBS) is an object, substance and/or pattern whose origin specifically requires a biological agent. Examples of DBS are complex organic molecules and/or structures whose formation  and abundances relative to other compounds are virtually unachievable in the absence of life." (Mustard et al. 2013). But, how to achieve a DBS is the problem.

We've seen some false positives/tentative results decades before through the Viking Mars missions--such as the metabolic reaction in the Labeled-Release experiment (still controversial) and failure of the gas chromatograph mass spectrometer (GCMS) to detect organics. But, both the Mars Global Surveyor and Mars Exploration Rovers demonstrated Mars was wet in the past hinting at the strong possibility of life during a warmer, wetter period.

For Mars 2020, the primary biosignature instrument will be SHERLOC or the Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals. Raman spectroscopy utilizes a laser to interact with the vibrational and rotational states of molecules which shift the scattered photon frequency. For SHERLOC this is a 248.6 nm UV laser which provides both fluorescence and resonant Raman scattering at a wavelength suited to condensed carbon and aromatic organics. The good news is that the power level of the UV laser can be low enough to reduce concern on heating and/or destroying the organic samples.

It's important to note that a detection of a DBS does not complete the 'life on Mars' journey. There is still a second stage to secure a sample, seal it in containers and make it available for return to Earth in future missions thus allowing for more definitive analysis. Small steps but important ones.

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