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A Plethora of Planets

March 12, 2011 |  by  |  Science

While a flock of robotic probes flit throughout the Solar System, treading intrepidly on alien soil, splashing radiantly through cometary comae, and dancing gaily with a myriad of moons across the floor of Saturn’s rings, one diminutive spacecraft trails behind the Earth, tagging along in well-trodden footsteps, falling further and further behind, as if the last, forgotten member of a scout troop who is distracted by the intricacy of the everyday that others ignore.

And while a fleet of telescopes sneak furtive glances at the exotic treasures of the cosmos, hunting hidden wonders among the bleakness of space, collecting jeweled specimens of the celestial menagerie, and capturing spectacular collisions of galaxies at the precise moments of impact that occurred billions of years before, that persistent spacecraft stares forever at a single part of the sky, reporting not with images, but in the inaccessible language of machines, focusing only on the imperceptible twinkling of a familiar neighbourhood of stars, as if peering into a stereoscopic image, waiting for magic to leap out of the mundane.

Kepler

The Kepler Spacecraft

Despite its pedestrian journey and its mundane vigil, the Kepler spacecraft is nevertheless telling us a new and awing story of our universe, a story which finally begins to answer the question that humanity has for so long asked of the heavens: “Are we alone?”

These first tantalizing discoveries have been the stuff of nightmares, monstrous gas giants capable of dwarfing into submission Jupiter, the king-god of our solar system

Kepler’s mission is to find alien planets, worlds whose very existence we have long imagined, even expected, but never known with certainty. Until very recently, there was debate as to how many, and even if, other stars in the heavens could be home to planets like our own.

In a universe where rocky worlds are rare, precious quirks of stellar formation, notable for their physical improbability more than their features, the likelihood of life becomes remote, chances of its detection from Earth unlikely, our hopes of ever finding an extra-solar civilization absurd.

If however the laws governing our universe favour the formation of terrestrial worlds, if our sun, with its handful of planets and its gross of moons, each unimaginably varied and wonderfully unique, turns out to be exceedingly ordinary among the stars, then the sheer number of suns in our galaxy alone causes the boundless imagination of man to fall short in its ability to grasp the number and variety of worlds that can play host to life.

For the first time in human history, observation is replacing speculation with regard to extra-solar planets. In the past twenty years, evidence of planets beyond our solar system has been mounting, worlds once only dreamt of popping into reality, slowly at first, but at an ever-accelerating pace.

However, these first tantalizing discoveries have been the stuff of nightmares, monstrous gas giants capable of dwarfing into submission Jupiter, the king-god of our solar system, hellish worlds orbiting far too close to their sun, ‘rocky’ only on technicality, their molten surfaces glowing malevolently, inhospitable planets upon which no Earth-like life could imaginably hold purchase.

Scorched World

A Scorched World

Before Kepler, the known exoplanets were scattered throughout the sky, rare, inhospitable, and ultimately disappointing. While it is possible to imagine living things that may thrive in the clouds of a gas giant, perhaps even possible to imagine biochemistry which could occur on the dark side of a tidally-locked lava world, while possible, such imagined life stirs within us no imagination. We yearn to find Earth-like worlds that may host Earth-like life, fertile grounds upon which our knowledge of biology can apply, can allow our imagination to cultivate, in the soil of reality, the seeds of the familiar and alien environments from our science fiction.

Most prized among exoplanet trophies is such an Earth-like planet, a rocky world like our own, which orbits its star in the ‘goldilocks zone’, not so far that its water must be locked eternally in granite-hard ice, but not too near where its surface is scorched dry, any water searing steam among the atmosphere.

Kepler’s Mission

The primary mission objective of Kepler is to find these extra-terrestrial earths, to catalog them within a rather pedestrian portion of our galactic neighbourhood, and to extrapolate their numbers among our entire galaxy and the universe.

Keplers Search Space

Kepler's Search Space

After its launch in March of 2009, Kepler began its hunt for exoplanets in May, staring unblinkingly at a small portion of the sky near the constellation Cygnus, observing over 155,000 stars simultaneously, a silent sentinel, watching, with infinite patience, for the infinitesimal evidence of planetary transits.

Kepler Star Field

Kepler's Star Field

Detecting planets orbiting stars, let alone classifying them, is extraordinarily difficult, the star’s brightness far over-shining that of its planets, their incredible distance shrinking the entire system down to a pinpoint, the whole endeavour like peering at a distant car at night, a car so far away that it could perhaps even be a motorcycle, staring at its headlights, trying to identify, and to catalog, the insects flitting past the light.

For every Earth-like trophy that Kepler bags, hundreds more escape capture, camouflaged, their planes of orbit just out of alignment.

While some have been directly, albeit crudely, imaged by very powerful telescopes, the vast majority of exoplanets have been identified through inference, by their measurable and much more easily detectable effects on their host stars. Several successful methods of detection have been developed, Kepler using the Transit Method, a method which measures the dips in the brightness of a star as a planet transits, crosses in front of the star, shading part of its light.

The Transit Method

This transit method involves watching the change in a star’s brightness as a planet crosses in front, amazingly detecting not only the amount of light blocked by the planet’s eclipse, but also measuring the change of light as a result of the differing phases of that planet as seen from Earth. This technique is illustrated beautifully by the following NASA animation, which was reposted by Phil Plait of Bad Astronomy, an astronomer, science advocate, and purveyor of critical thinking par excellence, as part of his excellent article on Kepler and the transit method.

Kepler measures the light curve of its observed stars, the fluctuations in brightness of the stars over time, fluctuations which are sometimes muted, allowing easier detection of the effects of a planetary transit on that brightness, but which are often noisily varied, serving to hide the ever-so subtle dips and valleys of an exoplanet eclipse.

In a terrific act of crowdsourcing science, the people at planethunters.org, knowing that the human ability for pattern recognition in many ways supersedes that of computers, have created a website which allows armchair astronomers to actually participate in the hunt for exoplanets, to help identify the transit features among the actual light data curves of the stars that Kepler is watching.

Planet Hunters

Participate in the detection of exoplanets at planethunters.org

Still, there are two major hurdles for Kepler to overcome in its hunt for extra-solar Earths, obstacles which limit both the number of planets that may be found and their rate of detection.

The first obstacle is that the transit method can only work if the orbits of extra-solar systems are almost perfectly aligned with us, only if the exoplanets actually cross the tiny space directly between us and their stars. NASA describes geometric probability of transits using examples from our own Solar System, revealing that for every 212 systems which may host an Earth-like world in a similar orbit, we could expect to see only one transit its star.

Kepler’s engineers developed a beautifully simple solution to this obstacle, developing a spacecraft which watches 155,000 stars at once, overcoming long odds through a brute force attack, as if a gambler getting thousands of throws of the dice for the cost of one.

With such stacked odds, Kepler is guaranteed a successful hunt. Most important to remember is that, for every Earth-like trophy that Kepler bags, hundreds more escape capture, camouflaged, their planes of orbit just out of alignment.

The second challenge affects the rate at which exoplanets can be discovered via the transit method. More than one transit must be observed before a planet can be suspected, the planet needing to cross its star twice before its orbital distance can be determined, a third crossing required before cementing its likelihood as a planet.

Assuming again an Earth orbiting a star in Kepler’s field of view, because we may have just missed the most recent transit, we can’t expect to see two transits until after nearly three years of observation, with a third transit not appearing until after the fourth year.

While nothing can be done to accelerate the rate at which Earth-like worlds in similar orbits can be detected, detections which aren’t expected until 2012-2013, Kepler can sooner detect exoplanets orbiting closer to, and therefore faster around, their host stars.

Mission Results

On February 2nd, 2011, NASA released the second quarter of Kepler mission results, adding to the first, shortened, quarter of mission data already released to the public, results together that include only observations made during the first four months of the mission, between May and September 2009.

Because the timeframe of observation is quite short, we know that the only exoplanet candidates that could have been observed were those which transit rather frequently, and therefore orbit quite closely to their stars, most less than half the distance that Earth orbits the Sun.

Kepler-Planet-Candidates---2011.02.01

Transiting Planet Candidates before/after Kepler's Q1 and Q2 Results

Nevertheless, after four months of looking in this limited search area, Kepler has identified over 1,200 planet candidates, including 50 found orbiting in the ‘goldilocks zone’ of their host stars. While these are currently called “candidate planets” pending verification by secondary source, it is expected that most, if not all, are actually extra solar worlds.

After just a few short months of observation, Kepler has glimpsed the truth: planets are everywhere.

The following animation, created by my friend Jer Thorp, data-artist-in-residence at the New York Times and writer of the delightful and enlightening blprnt.blog, visualizes what the planet candidates might look like if they were all to orbit our Sun. The video begs to be viewed fullscreen in HD, promising to well reward readers who oblige.

Most of the 50 planet candidates in the habitable zone of their stars are much larger than Earth, gas giants the size of Neptune, Jupiter, even larger, a handful of those found have been super-earth sized, 1.25-2x Earth’s radius, some of which are likely rocky, and one of these goldilocks candidates is even slightly smaller than the Earth, a rocky world that could very conceivably be home to liquid water, even possibly life.

Although that Earth-sized rocky world orbits in the habitable zone of its star, a distance range where the temperatures could support liquid water, the star it orbits is relatively quite cool, the ‘habitable’ planet needing to orbit frighteningly close at just 1/20th the distance that Earth orbits the Sun, its ‘year’ only nine days, its similarities to Earth tenuous at best.

While none of those planets quite serve as a mirror for Earth, many of the Neptune and Jupiter-sized planets could themselves be home to dozens of moons. If Jupiter were in our Sun’s habitable zone, many of its moons could support water, one of which, Europa, would be an ocean planet only a quarter the size of Earth, but home to about twice the volume of water. With what Kepler has already delivered in its first four months, the dreamers among us certainly have plenty of fodder for their imaginings while we all wait for a true Earth-like analog.

Projections

While it will be a year or two more before we expect to hear of a truly Earth-like world in an Earth-like orbit around a Sun-like star, the early results from Kepler are remarkable. Before Kepler, extra-solar planets were rare, scattered among the sky, but after just a few short months of observation, Kepler has glimpsed the truth: planets are everywhere.

Kepler-Planet-Candidates-in-FOV

Planet Candidates in Kepler's Field of View before/after 4 Months

The animation above beautifully illustrates the amount by which Kepler has increased our understanding of exoplanet frequency, but still the picture it paints is a poor shade of reality, the planets indicated being diluted three-fold by various circumstances.

If one is to get a true idea of the number of planets that may exist in the photo above, one must first understand what factors limit the displayed exoplanets:

  1. Kepler has only been watching for four months, limiting substantially the number of yellow dots that will be present after several more years of observation.
  2. Due to the infrequency of perfectly aligned orbits, each dot in the picture above represents dozens, even hundreds of similar planets in the same field of view.
  3. While Kepler points at about 4.5 million stars in our galactic neighbourhood, only a tiny fraction, about 155,000, shine brightly enough or stably enough to have been selected for observation.

We see a mirror reflecting the image of dreamers, staring at their own skies, wondering about us.

The Kepler team did statistical analysis of the planets they found during these first few months, analyzing the mathematical likelihood that each type of candidate planet could have been found, projecting that likelihood against an assumption that every star Kepler is watching has a planet of exactly that type, ultimately coming up with an absolute maximum number of that type of planet that could be found.

By comparing this maximum number of each exoplanet type that could be detected against the number that actually were detected, Kepler scientists arrived at a good estimate of the frequency for those planet types across all stars in Kepler’s field of view. These results are highly encouraging to those rooting for exoplanets.

The frequency results published to date, limited only to planets which orbit at less than half the distance that Earth orbits, suggest that fully 6% of the observed stars have an Earth-sized planet orbiting within that distance, nearly 7% have super-Earth-sized planets (1.25-2x the size of Earth), with a total of 34% having planets of some kind.

So, the statistics indicate that 34% of the observed stars have at least one planet which orbits at a distance near or within the orbit of Mercury. Applying that against the 155,000 stars being observed suggests over 53,000 planets hugging close to the stars that Kepler is observing, with another 1.5 million close-in around the 4.5 million stars in Kepler’s field of view. The mind boggles when one thinks of how many star-hugging planets there may be among the 100-400 million stars in our galaxy.

Of course, our own Solar system has but one small world orbiting at that distance, with almost 200 more planets and moons falling outside the range of those frequency estimates, their existence suggestively insinuating an abounding horde of exoplanets standing at the threshold of our view, patiently awaiting the light of added observation to reveal their multitude.

A Plethora of Planets

With all of the above in mind, one is justified in imagining a veritable zoo of exoplanets, a glittering panoply of worlds, their variety and their innumerability defying true comprehension. Knowledge of this vast host of worlds is finally coming to light, slowly are the details emerging from our observations, details which confirm and surprise our wildest dreams.

The mind champs and stamps, accustomed to roaming free through familiar pastures filled with daydreams of other-worldly worlds, feeling for the first time the reins of knowledge on unbounded imagination, reins which at first curb the eagerness to explore, but which will soon be used to gallop our minds further and faster than ever before, leading us on a journey filled with unfamiliar and unimaginable truths.

On the forefront of discovery, indeed even able to participate in the science itself, we are all living in a wondrous age of breakthrough, a renaissance of our knowledge of the universe, a time when the history of science is occurring before our eyes, the future textbooks recording a second chapter under the name Kepler, a chapter in which the dates are familiar and colourful to us, filled with the richness of our own lives, the period destined to become lore, to be dreamt of by our children’s children.

When we look now to the heavens, we no longer see a canopy of pinprick stars over our Earth, an untouchable canvas filled only by the details of our imagination, we now see a sky teeming with real worlds of all shapes and sizes, we see a community of planets of which we are a part, we see a mirror reflecting the image of dreamers, staring at their own skies, wondering about us.    


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