So Many Planets, So Little Budget
As is virtually always the case where scientific research is concerned, one has to decide on what to spend the always-to-small research budget. So how, I asked myself, do they decide to spend cash on relatively large projects for searching for searching for life in the universe (only because I’m interested on the subject). My own research lead me to the work of an astrobiologist named Margaret (Maggie) Turnbull; the namesake of asteroid 7863 Turnbull. In November 2003, she produced a paper which included a list of preferred targets for the Terrestrial Planet Finder project.
Of course, it all seems real simple when a professional of the caliber of Turnbull explains it. The criteria for selecting candidate stars for Earth-like planets are:
- have fractional parallax uncertainties less than 5%,
(Despite some background in astronomy, I’m afraid I had to look this first item up to be sure. From my dabbling in the area of 3D programming using Microsoft’s DirectX API, I’ve learned that parallax is the measure of the relative, perceived change of two points from a fixed point of observation. But this is also used in the field of astronomy as a component in measuring the distance between Earth and other astronomical bodies in the sky. Of course, there is a degree of uncertainty in calculating such distances on an astronomical scale; caused by a variety of factors, including the gravitational pull of undiscovered objects between here and the target point. Presumably, surveying stars with lower uncertainties will eliminate the likelihood of mistakenly surveying star systems where a planet "false positive" has occurred, since it would ultimately prove a waste of time.)
- are on the main sequence,
("Main sequence" stars are those which are stable; with limited flare activity, not undergoing sudden, explosive changes in output radiation or size – stars which are in their prime, burning hydrogen instead of being mature, toward the end of their lives, burning heavier metals.)
Radius Mass Luminosity Temperature R/R☉ M/M☉ L/L☉ K O2 16 158 2,000,000 54,000 O5 14 58 800,000 46,000 B0 5.7 16 16,000 29,000 B5 3.7 5.4 750 15,200 A0 2.3 2.6 63 9,600 A5 1.8 1.9 24 8,700 F0 1.5 1.6 9.0 7,200 F5 1.2 1.35 4.0 6,400 G0 1.05 1.08 1.45 6,000 G2 1.0 1.0 1.0 5,700 G5 0.98 0.95 0.70 5,500 K0 0.89 0.83 0.36 5,150 K5 0.75 0.62 0.18 4,450 M0 0.64 0.47 0.075 3,850 M5 0.36 0.25 0.013 3,200 M8 0.15 0.10 0.0008 2,500 M9.5 0.10 0.08 0.0001 1,900
- have B-V color consistent with F, G and K stars,
(Another element in the search for life is thought to be stars capable of supporting a ring or "habitable" zone. Stars which are too large or too small & burn too cold or which are too large or small & burn too hot are not good candidates for finding life. Stars, like or close enough to our sun, which is known to have such a habitable zone & burn at the right temperature & can have planets at just the right distance are good candidates for finding life. Stars are categorized into classes, labeled alphabetically. The table, right, expresses these classes and a few of their respective characteristics.)
- are older than ~2 billion years,
(Life, like any great souffle, takes time to "cook up" properly. If the main parent star has only been stable, burning hydrogen for a few million years, the likelihood mother nature has had enough time to ignite life in a puddle of amino-acid-rich primordial goo isn’t great.)
- are not variable,
(Variable stars can be technically "main sequence", but yet have other circumstances making them too unstable to have any likelihood of planets with life. Such circumstances can include changing – sometimes explosively changing – surface area or volume, exhibit flares or other mass ejections, or have unusual sunspot activity.)
- have metallicities of at least half solar ([Fe/H] > -0.3),
(Here I had to make a bit of a semi-educated guess. Metallicity is, as the name suggests, an expression of the star’s metallic composition – essentially what elements are present in the star’s structure. As a star’s gravity causes nuclear fusion of hydrogen during its earlier stages, the product of the process is, invariably, heavier elements – like Helium, initially – then heavier and heavier elements the older the star gets. It stands to reason that the earlier point of how old a star is comes into effect here – but that some stars burn longer or shorter than others. As an indicator of whether the star is at a stage in its life span to support life-harbouring planets, one measures it metallicity to be at least half that of the Sun.)
- are thin disk members, and
("Thin disk" appears to refer to star placement in a galaxy. Our Milky Way galaxy is a barred-spiral-class galaxy¹ which means there’s a difference in the structure of the galaxy across its "length". In other words, viewed "edge-on", the galaxy has a depth which is variable at the centre versus the extremities where matter depth is "thinner". Stars in this latter part of the galaxy are likely thought better candidates for finding life because radiation and other extremes of environment can be experience in stellar neighborhoods of increasing density – such as that toward the centre or core of the galaxy. For one thing, more stars would equal more stellar explosions like supernovae which, over time, would eradicate any life that had been sparked.)
A composite image of our Milky Way Galaxy, taken from Earth at two locations;
one in the United States, the other in Australia, to create a 360°, panoramic view.
Because of our solar system’s position in the galaxy, the photo appears to be taken "edge-on",
in the midst of the galactic plane – maybe just slightly lower than absolute centre.
- and have no known stellar companions closer than 10 arcseconds.
(Finally, the ideal is to have star systems like ours with a single star so as not to complicate or narrow the "habitable" ring in which life can occur. Adding stellar companions like brown dwarves or super-gas-giants that exhibit extreme radiation or other stars that compromise planets which would otherwise evolve life erodes the chance of that life occurring. An arcsecond is the measure of an angle, expressed in a subset of degrees. A degree is comprised of 60 arcminutes, which – in turn – is comprised of 60 arcseconds. A degree, thus, is comprised of 3600 arcseconds.)
The above list is in no way an expression of where life can occur. Life has been found in environments on Earth – for example, living in clouds of sulfur at the bottom of our oceans next to volcanoes – thought at one point to be completely uninhabitable. So the "rules" for where life can occur are at this point still a big unknown. But one must prioritize research for reasons outlined at the beginning of this article – and the best reasons are arrived at using the available data. At this point, it’s safe to say we’re looking for life derived from compounds that are carbon-based, for example. We simply have no other examples of life upon which to draw any other likelihood for its existence, after all. We also have established some parameters about how much heat and radiation any carbon-based life can exist in to get these metrics established.
One wonders if we’ll find something or anything at all like us out there. Given the criteria, I wouldn’t be surprised if/when life is found that there are some parallels we can draw with ourselves as a species.
¹ Until recently, scientists thought our galaxy’s structure was simply a spiral. In many popular television shows (mostly sci-fi), the Milky Way galaxy is still referred to or depicted as a standard spiral galaxy.