Variable Stars and the Stories They Tell: Exoplanets and the Search for Extraterrestrial Life
©February 2018 by Dale Alan Bryant
(In memory of M.I.T. astrophysicist Dr. Philip Morrison - who started all this...)
Though the figures are tough to keep up with - they are changing, almost daily - the Kepler Space Telescope (KST) has discovered, to date, over 4,500 exoplanet candidates, ~3,400+ of which have been confirmed.
An exoplanet is an extra-solar planet that orbits a star other than the Sun, and is, therefore, not a member of our Solar System.(1) Almost all stars host at least one orbiting planet - and many of these 'alien suns' bestow a large number of planets; we have found as many as eight, in one system. These systems - the planet(s) and their Sun(s) - are known as planetary systems, as is our own Solar System.
The number of known planetary systems are just a few of the projected billions of systems in existence in the Milky Way galaxy, and are only those systems whose orbital planes lie at a favorable, visually discernable, incline from the Kepler telescope's perspective. Of those 3,400+, at least 70 are Earth-sized, and at least 48 lie within the so-called 'Goldilocks Zone', or Habitable Zone.(2)
Kepler, which was launched on March 7, 2009, achieves its miracle of detection of extra-solar planets, or exoplanets, by using a photometer to perform a time-series measurement of the varying light curves of their host, or parent star(s). By using this technique, known as the photometric method of detection, stars were found to vary in their light output over periods of from several hours to several days. These are known as extrinsic variable stars. (Intrinsic variable stars are those stars whose actual luminosities vary, due to some internal factor.)
As a planet transits, or passes in front of its sun as seen from Kepler's perspective, there is a barely measurable, but significant, decrease in the star's light output, on average about 2%, with a 0.03% added decrease in the presence of a planetary atmosphere. This is roughly equivalent to the amount of light lost by an observer of a house fly passing in front of a car headlight as seen from several miles away! When astronomers can determine a regular pattern of dimming and brightening, they can then deduce that the star has at least one orbiting planet. Stars vary in their brightnesses for other reasons, as noted earlier, but - this is by far the most exciting reason!
If (when?) life is discovered elsewhere in our universe, does it necessarily mean that it will be found on an 'Earth-like' exoplanet (planets with sizes comparable to Earth and whose orbits carry them into the system's Habitable Zone)? We humans like to think so, and it may very well be, but life might also be found in much less likely environments. We simply do not yet know. The diversity of life in Earth's biosphere is staggering in that it can be found in the deepest ocean trenches, in waters well above 800 degrees Fahrenheit, and even pushing well north and south of the Arctic and Antarctic Circles, respectively, as well as on land, at temperatures far below freezing. But, it needs to be remembered that we are products of our own struggle to survive, contingent on our ability to adapt to an ever-changing environment. We must also yield to modification by genetic mutation. Nature has experimented with many kinds of organisms, through the process of Natural Selection, but most of those organisms were ill-suited to the task of survival: greater than 98% of species - terrestrial and marine - have become extinct over geological time.
Nevertheless, our planet is teeming with life.
The first step in the quest to find extraterrestrial life is, as previously mentioned, to find exoplanets whose orbital planes are suitably inclined to our line-of-sight. The first technique used to detect exoplanets was through the measurement of the shift in a star's radial velocity. A planet in orbit around a star will produce a shift in the star's spectral lines as the star is tugged on by the planet's mass, making the star appear to "wobble", back and forth, to and fro, as the two revolve around a common barycenter, or center of gravity. This is known as the radial velocity method of detection. And, in 1952, Russian-American astronomer Otto Struve suggested that extra-solar planets might be detected through dips in the host star's light during a planetary transit. Even then, techniques were available to detect such a drop in light, but the subject was forgotten about until decades later.
By 1999, two professional astronomers, using a 10-centimeter telescope, discovered the first tell-tale signs of such a transiting exoplanet. Amateur and professional astronomers have since detected countless candidates.
NASA's Ames Research Center lists a table of 70-plus confirmed exoplanets, the earliest of which were discovered by the Kepler telescope in May 2012, which were given the designation, 'Kepler', followed by a letter. (Since then - thousands more exoplanets have been confirmed and named, in this convention). Planetary characteristics in the table for each planet include: Jupiter Masses; Earth Masses; Jupiter Radii; Earth Radii; Density; Temperature; Transition Duration; Period; Semi-Major Axis (UA); Eccentricity; Inclination (in degrees); and Distance (in parsecs). The table also lists the characteristics of the host star. It should be noted here that Kepler-23b through Kepler-23d are planets that are within just a few Earth radii - though they are several hundred times more massive - and their orbital periods seem much too short - just a few days - to lie within their star's habitable zones. Nonetheless, it tells us that exoplanets roughly the size of Earth are detectable, and are, indeed, out there.
Within the first 45 days of operation, KST, combined with follow-up ground-based observations, confirmed the discovery of five new exoplanets, including Kepler-7b, the least-dense planet discovered at that time. Kepler has also been credited with the discovery of two 'super-hot' orbiting companions - companions that appear to be hotter than their respective host stars! That discovery, first announced at the 215th American Astronomical Society meeting in Washington, DC, on January 4, 2010, revealed that the data from Kepler, along with the ground-based data, had yet to confirm just what these objects are. One of the objects, KOI-74b, (KOI = Kepler Object of Interest ), had a surface temperature of 70,000 degrees Fahrenheit; its host star, in comparison, is a mere 17,000 degrees Fahrenheit! The object is roughly the size of Jupiter, and orbits its host star every 23 days. The hottest confirmed exoplanet, to date, has a surface temperature of ~3,700 degrees Fahrenheit.
As of June 15, 2010, Kepler had identified 706 stars hosting exoplanet candidates, with sizes ranging from as small as that of Earth to significantly larger than Jupiter. On August 26, 2010, two new exoplanets orbiting the same star were discovered via the transit method. Two planets orbiting the star Kepler-9, roughly 2,300 light years distant, have been designated Kepler-23b and Kepler-9c, and were discovered over a seven-month period. Astronomers at the W. M. Keck Observatory in Hawaii have estimated the masses of these two confirmed planets: Kepler-9b is the larger of the two, and the other, then, still unnamed, was found to be only 1.5 Earth radii, making it one of the smallest exoplanets known.
Life - as we know it - would require that it evolve on planets with exactly the same physical make-up as the Earth, and it can be assumed that life-forms on other worlds should evolve, specific to their planet's/star's chemistries, as well. Earth life is carbon-based - but we should not necessarily exclude, say, silicon-, or even ammonia-based life-forms on other worlds. Our body chemistry is that of Earth. With that in mind, astrobiologists expect to be able to detect the presence of life, by using a technique known as spectropolarimetry - looking for bio-signatures in polarized starlight reflected from one of a star's planets. This would measure atmospheric gaseous emissions - similar to those oxygen bio-signature emissions given off by vegetation here on our planet. So, we can determine which signs to look for. The presence of atmospheric gases on a planet can be found by measuring the planet's transmission spectrum during its transit across the face of its host star. If certain elements are present in a planet's atmosphere, the atmosphere will absorb some of the star's light as light fall-off. If no planetary atmosphere is present, the light fall-off will be the same at all wavelengths. In one case, the presence of sodium in a planetary atmosphere made one planet appear to be six percent larger than at other wavelengths! This technique is the spectropolarimetry method of detection. To our benefit, in the search for extraterrestrial life, the universe operates the same everywhere else as it does here on Earth.
Although Kepler was originally slated to last 3 1/2 years and was initially designed to find transiting Earth-like exoplanets by continuously monitoring a field of more than 100,000 stars in an area just east of the constellation Cygnus, steps were taken by its team of engineers to extend its mission another 3+ years. This extended mission is called, simply, 'K2'. Nine years later and into its 'Second Light' mission phase, K2 is also providing an extraordinary collection of time-series data for studying the variability of the stars in our galaxy. There are on-line tools available for studying the periodicities of these stars, including the NASA's Star and Exoplanet Database's Periodogram Tool. A periodogram finds the periodicities present in time-series data sets, and the probability that an individual period arises by chance. Other discoveries by Kepler, using time-series photometry, include solar-like oscillations in the light curves of red-giant stars, and solar-like astroseismic events in relatively nearby type-G stars, as well as in the open star cluster NGC 6819.
To date, we know there are at least 3,000 confirmed exoplanets, in 2,657 planetary systems in the Milky Way galaxy - but those numbers are constantly rising, as astronomers, as well as Citizen Scientists**, sift through the still-incoming mountains of downloaded data from Kepler. Based on current numbers, astronomers estimate that our galaxy contains over 50 billion (50,000,000,000) Earth-sized exoplanets - 500 million (500,000,000) of which lie in their sun's habitable zone. Ideally, we should not limit our search to Earth-like planets exclusively, but it's a good place to start. Life, as we know it - or, do not know it - will surely be a far more interesting story than we can even imagine - and one thing is becoming very clear - the Habitable Zone of a given star may be far less distinct than we think!
Nevertheless, life, almost surely - somehow - will find a way...
(1) Our Solar System consists of the star we call the 'Sun', the eight currently known planets and their numerous satellite 'moons', several dwarf planets, hundreds of thousands of comets, asteroids, and other planetesimals, various gases, and dust. Our star, the Sun, has a proper name, just as the planets and many other objects within our planetary system do; our star's name is "Sol". Hence, we refer to our planetary system as the "Solar" system, or, Solar System. I will refer to other planetary systems here as 'planetary systems', regardless of the names of the parent stars.
(2) The Habitable Zone, also affectionately known as the 'Goldilocks Zone', of a star, is the region where, measuring from the star's center outward, the temperatures are temperate enough to support a potentially life-bearing planet, where water in the liquid state exists.
** If you would like to join NASA, and the SETI (Search for Extraterrestrial Intelligence) Institute as a Citizen Scientist, in the search for exoplanets, go to Zooniverse's planethunters.org website and sign up - it's really that easy! You don't need a Ph.D. or experience in science to participate. You will learn to examine the Kepler telescope's time-series graphical data images to spot possible exoplanet candidates while helping these organizations sift through the immense amounts of data that are still left. One college student, two years ago, discovered a new planet! Others have discovered new planets in 'group' research projects.