"Curioser and curioser," said Alice as she explored the weirdness of a Wonderland world, and like her we search the Wonderland of a starlit sky above our planet in an effort to understand who we are by searching for that which is beyond ourselves. By asking the profound question of whether or not we are alone in an unimaginably vast Universe--that we have barely begun to explore--we are really trying to understand who we are in the mysterious cosmic scheme of things. The quest to discover life on other worlds has long been the Holy Grail of curious planet-hunters, and the treasure trove of exoplanets that have been discovered over the past generation, has certainly tickled our imaginations by presenting us with a cosmic Wonderland filled with weird worlds--as well as worlds that are eerily like the familiar planets of our Solar System, inhabiting the family of our own Sun. In February 2017, astronomers using NASA's infrared Spitzer Space Telescope, announced that they have discovered the first known system around a solitary star, that hosts seven Earth-size exoplanets. A trio of these planetary Wonderlands are firmly situated in the comfortable habitable zone around their parent stars, where life-loving liquid water may exist. Life as we know it depends on the presence of liquid water.
This remarkable discovery sets a new record for the greatest number of habitable zone exoplanets detected in the family of a solitary, distant star beyond our Sun. All seven of these alien planets could have liquid water under the right atmospheric conditions. However, the chances of this are highest with the trio of worlds in the habitable zone.
"This discovery could be a significant piece in the puzzle of finding habitable environments, places that are conducive to life. Answering the question 'are we alone' is a top science priority and finding so many planets like these for the first time in the habitable zone is a remarkable step forward toward that goal," commented Dr. Thomas Zurbuchen in a February 22, 2017 NASA Press Release. Dr. Zurbuchen is an associate administrator of NASA's Science Mission Directorate in Washington.
The Quest
Historically, the quest to discover distant planets, dwelling within the alien families of stars beyond our own Sun, proved to be difficult. The discovery of the first batch of exoplanets a generation ago clearly represents one of humanity's greatest achievements. Detecting a giant planet, such as our Solar System's gaseous behemoth, Jupiter, has been compared to observing light skipping off a small moth flying near the 1,000-watt light bulb of a shining street lamp--when the observer is miles away.
The original technique used by astronomers back in 1995--the Doppler Shift method--favored the discovery of giant planets orbiting around their parent-stars in close, roasting orbits. However, as time passed, technology improved to the point that ever smaller and smaller exoplanets have been detected. Many astronomers think that Earth-size alien planets are probably common inhabitants of our Galactic neighborhood.
The smaller the exoplanet, the more difficult it is to detect. For example, if an alien astronomer, belonging to an advanced technological civilization, went on the hunt for other worlds in distant regions of our Milky Way Galaxy, it would have a hard time spotting our dim, rocky, little blue planet. Our Earth would appear only as a faint and insignificant speck lost in space. In fact, our planet is very well-hidden from prying alien vision because the brilliant glare of our much larger Star, the Sun, overwhelms it.
The first scientific discovery of an exoplanet was in 1988--but the first confirmed detection came in 1992 with the discovery of some very weird and inhospitable worlds in orbit around a dense stellar relic called a pulsar--the lingering remains of a massive star that lost its "life" in the violent fireworks of a supernova blast, that tore the original progenitor star to pieces. Astronomers detected the first exoplanet in orbit around a still "living" star, like our own Sun, in 1995. As of March 1, 2017, 3,586 exoplanets, inhabiting 2,691 planetary systems, have been discovered--and 603 multiple planetary systems have also been confirmed.
Since 2004, the European Southern Observatory's (ESO's) High Accuracy Radial velocity Planet Searcher (HARPS) 3.6 meter telescope, has successfully spotted approximately 100 alien worlds, and since 2009, NASA's Kepler Space Telescope has discovered more than two thousand. Kepler has also spotted a few thousand candidate planets, of which only about 11% may prove to be false-positives. Planet-hunting astronomers think that about 1 in 5 stars similar to our Sun are circled by an "Earth-sized" planet situated comfortably in the habitable zone of their stellar parent. If there are 200 billion stars dancing around in our Milky Way, it may be that there are 11 billion potentially habitable Earth-sized planets in our Galaxy. This already impressive number could rise even further if planets orbiting the numerous red dwarf stars are included in the count. Red dwarf stars are the smallest, as well as the most abundant, true stars inhabiting our Galaxy. Red dwarfs are even smaller than our small Sun, and they can potentially "live" for trillions of years on the hydrogen-burning main-sequence of the Hertzsprung-Russell Diagram of Stellar Evolution.
The least massive alien world known is Draugr (PSR B1257+12 A or PSR B1257+12 B), which is clearly the runt of the known exoplanet litter, at only twice the mass of Earth's Moon. In dramatic contrast, the most massive known planet listed on the NASA Exoplanet Archive is dubbed DENIS-P J082303.11-491201 b, and it is approximately 29 times the mass of Jupiter. However, according to some definitions of a planet, this hefty distant world is too massive to be a planet and may really be a type of stellar failure called a brown dwarf. Brown dwarfs are objects that are probably born the same way as their true stellar kin--from the collapse of a dense blob embedded within a giant, cold, dark molecular cloud--but have not managed to reach the necessary mass to ignite their nuclear-fusing furnaces.
There are exoplanets that hug their parent-star in such tight, roasting orbits that they take only a few hours to circle it--and there are others that are so far away from their stellar parent that they take thousands of years to make a single orbit. Indeed, some exoplanets are so far from their host star that it is difficult for astronomers to determine whether they really are gravitationally tied to it. Almost all of the remote worlds, that belong to the families of stars beyond our Sun, are denizens of our own Milky Way--but there have also been detections of a few intriguing potential extragalactic exoplanets. The closest exoplanet to Earth is Proxima Centauri b, which is a "mere" 4.2 light-years away from our planet, and is in orbit around Proxima Centauri, the closest neighboring star to our Sun.
Of course, the discovery of such an abundant treasure trove of distant exoplanets has triggered significant scientific interest in the search for life beyond our own planet--with a special focus on planets that circle their star within its habitable zone. However, the search for life beyond Earth also must take into consideration a wide range of additional factors in determining the suitability of a planet for hosting life as we know it.
In addition, there are also so-called rogue planets, which do not orbit any star at all, but roam lost and alone through interstellar space without a stellar family to call their own. Astronomers tend to consider these solitary worlds separately, particularly if they are gas giant planets like our own Solar System's Jupiter and Saturn, in which case they are often classified as sub-brown dwarfs. The rogue planets that roam our Galaxy possibly number in the billions--or even more!
Seven Earth-Size Worlds Inhabit The Family Of A Distant Star
At approximately 40 light-years (235 trillion miles) from our planet, the newly discovered seven worlds are considered to be relatively close neighbors of our Solar System.
This richly-endowed exoplanet system is named TRAPPIST-1, for The Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile. In May 2016, astronomers using TRAPPIST announced their discovery of a trio of planets in the system. Assisted by several ground-based telescopes, including the ESO's Very Large Telescope (VLT), Spitzer confirmed the existence of two of the trio, and also went on to discover five additional planets. This increased the number of known planets in this system to seven.
Using new data derived from Spitzer, the team of astronomers made a precise measurement of the sizes of the seven exoplanets, and went on to develop the first estimates of the masses of six of them. This enabled the team to estimate their density.
Based on their densities, all of the TRAPPIST-1 alien worlds are considered to be rocky--like our Earth. Additional observations will help astronomers determine whether they are rich in water--and possibly also show if any of them could potentially have liquid water streaming on their surfaces. The mass of the seventh and most distant exoplanet has not yet been estimated. However, the astronomers think that it could be an icy, "snowball-like" world--but further observations are necessary.
Dr. Michael Gillon noted in the February 22, 2017 NASA Press Release that "The seven wonders of TRAPPIST-1 are the first Earth-size planets that have been found orbiting this kind of star. It is also the best target yet for studying the atmospheres of potentially habitable, Earth-size worlds." Dr. Gillon is lead author of the paper, describing the new discovery, published in the February 22, 2017 issue of the journal Nature. Dr. Gillon is also the principal investigator of the TRAPPIST Exoplanet Survey at the University of Liege, in Belgium.
In March 2017, astronomers using NASA's planet-hunting Kepler Space Telescope, reported that they have also been observing the TRAPPIST-1 system since December 2016.
During the period of December 15, 2016 to March 4, 2017, the highly successful Kepler spacecraft, currently operating as the K2 mission, gathered data on the ultracool dwarf star's minuscule alterations in brightness as a result of transiting planets. Transiting events occur when a planet floats in front of the glaring face of its parent-star, thus causing a small dip in its apparent brightness.
These additional observations are expected to help astronomers refine the earlier measurements of six of the planets, determine more precisely the orbital period and mass of the more elusive seventh, and learn more about the magnetic activity of the small, cool parent-star.
The observation period, known as K2 Campaign 12, provides 74 days of monitoring the TRAPPIST-1 system. This is the longest, nearly continuous set of observations of this system yet, and it provides astronomers with an opportunity to further observe the gravitational dance of the seven Wonderland worlds--and also hunt for distant exoplanets that may have remained undiscovered in this very fertile system.
Dr. Michael Haas noted in a March 8, 2017 NASA Press Release that "We were lucky that the K2 mission was able to observe TRAPPIST-1. The observing field for Campaign 12 was set when the discovery of the first planets orbiting TRAPPIST-1 was announced, and the science community had already submitted proposals for specific targets of interest in that field. The unexpected opportunity to further study the TRAPPIST-1 system was quickly recognized and the agility of the K2 team and science community prevailed once again." Dr. Haas is science office director for the Kepler and K2 missions at NASA's Ames Research Center in California.
The additional refinements to the earlier measurements of the known planets, and any additional planets that may be spotted in the K2 data, will help astronomers plan for follow-up studies of the TRAPPIST-1 Wonderland worlds using NASA's upcoming James Webb Space Telescope.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, journals, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, "Wisps, Ashes, and Smoke," will be published soon.
By
Judith E Braffman-Miller
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Seven Earth-Size Worlds Inhabit The Family Of A Distant Star
