The planets of Kepler-37 compared to the Solar System's inner worlds (Image: NASA/Ames/JPL-Caltech).
Good things come in small packages--A Proverb
The search for planets orbiting stars beyond our Sun has been a difficult quest, and their ultimate discovery arguably represents one of humanity's greatest accomplishments. Spotting a planet the size of Jupiter, circling a distant star dwelling in our Milky Way Galaxy, has been compared to seeing light zipping off a speck of dust close to a 1,000-watt light bulb when the observer is miles away. The first extrasolar planet circling a Sun-like star was discovered in 1995--and it was enormous because the original method used by planet-hunters to discover extrasolar planets--the so-called Doppler Shift method--favoured the detection of giant planets circling fast and close to their parent stars (hot Jupiters). However, as technologies have improved, smaller and smaller extrasolar planets have been detected by tenacious planet-hunters. In fact, astronomers have discovered that Earth-size extrasolar planets are common! In February 2013, planet-hunters announced the discovery of the smallest extrasolar planet yet, circling a star like our own Sun! Smaller than the planet Mercury, this bewitching mini-world is only a little bigger than Earth's own Moon!
Dr. Carl Sagan wrote in Scientific American back in 1975 that "Our Galaxy is one of at least billions and perhaps hundreds of billions, of galaxies. Our particular Sun and its surrounding planets constitute no more than one example that surely must be repeated innumerable times in the vastness of Space and Time."
Sagan wrote these words over a generation ago, but the very first planets discovered orbiting stars beyond our Sun were not discovered until twenty years later. It was not until 1995 that Sagan's prediction was verified by observation. Because of discoveries made over the last two decades, astronomers have now concluded that planets orbiting other stars not only exist, but do so in great numbers, and with great diversity. We now know of the existence of more than 800 extrasolar planets existing in our Galaxy alone, and this most likely represents only the tip of the iceberg--a very, very big iceberg!
Planet-hunters have succeeded in discovering many, many more planets beyond our own Solar System than the eight major planets that dwell within it. As Thomas R. McDonough noted back in 1989, in his book, Space: The Next Twenty-Five Years: "We live in a Universe so vast that most people cannot begin to comprehend its size." Indeed, the immense super clusters of galaxies--the largest visible structures in our Universe--are composed of enormous populations of thousands of galaxies that are strung out approximately 10 to the twenty-fourth power miles from one end to the other (that is, 10 followed by 23 zeroes). Earth dwells peacefully at one end of a super cluster chain. The light now reaching our planet from the opposite end began its long, treacherous journey 300,000,000 years ago, or about the same time humanity's ancestral species first began their momentous Great Crawl out of the ancient waters of the Earth, eventually to evolve into land-dwelling creatures.
The smaller the extrasolar planet, the more difficult it is to detect. For example, if an intelligent, alien astronomer, dwelling on a comfortable extrasolar planet circling a benevolent, life-loving star, decided to peer into the remote corners of the Milky Way in search of planets orbiting other stars, it would have a very hard time discovering our own very small, dim world. Earth is tiny and insignificant as it swims in the vast sea of Space and Time, and our "tiny dust mote"--lost in Space--would be hidden by the blinding glare of our much more brilliant Sun.
A Mini-Mercury Lost In Space
Astronomers on Earth, peering into the remote corners of our Galaxy, have discovered the smallest extrasolar planet yet--one that is only a little bit bigger than Earth's Moon. They announced their discovery in February 2013 in the journal Nature.
Dr. Geoffrey Marcy, of the University of California at Berkeley, is one of the original planet-hunters. Dr. Marcy noted to the press on February 20, 2013 that "This new discovery raises the spectre that the Universe is jam-packed, like jelly beans in a jar, with planets even smaller than Earth." Dr. Marcy was not a member of the team that discovered the mini-planet.
The little planet was spotted by astronomers using NASA's Kepler space telescope. This smallest of all known extrasolar planets is part of a planetary system dwelling around a star similar to our own. The parent star of this distant planetary system, which is about 210 light-years from Earth, and is located in the constellation Lyra, has been dubbed Kepler-37. The smallest planet, Kepler-37b, is smaller than the planet Mercury in our own Solar System, and it measures in at about one-third the size of Earth.
Kepler-37b has two larger sister planets, and all three members of the trio were discovered by planet-hunters using Kepler. The Kepler mission was designed to discover Earth-sized planets dwelling within the so-called habitable zone of their stars. The habitable zone is that "Goldilocks" region of a planetary system--not too hot, not too cold, but just right--for liquid water to exist on the surface of an orbiting planet. Where there is water, there is the possibility--though not the promise--of life as we know it to exist.
The star, Kepler-37, belongs to the same class as our own Sun. However, it is a bit cooler, as well as smaller. The trio of planets circle Kepler-37 at less than the distance of Mercury (the innermost planet in our Solar System) to the Sun. The mini-planet, Kepler-37b, is a roaster--it is very hot, and inhospitable to life as we know it. The little world circles its star every 13 days at less than one-third Mercury's distance from the Sun, and its surface temperature is estimated to be more than 800 degrees Fahrenheit. This hellish surface is hot enough to melt zinc. The two other sister-planets, Kepler-37c and Kepler-37d, circle their star every 21 days and 40 days, respectively.
It is obvious that planet-hunters will have to search elsewhere in our Galaxy for life as we know it. The mini-world orbits its star much too closely, and is far too sizzling, to host delicate living organisms. Discover, Dr. Thomas Barclay, Kepler scientist at the Bay Area Environmental Research Institute in Sonoma, California, was exhilarated when he first spotted the teeny world, and he told the press in February 2013 that for days he kept reciting the line from the movie Star Wars: "That's no moon." It took over a year for an international team of astronomers to verify that the mini-world was really a planet.
Dr. Barclay commented in a February 20, 2013 NASA Jet Propulsion Laboratory (JPL) Press Release that: "We uncovered a planet smaller than any in our Solar System orbiting one of the few stars that is both bright and quiet, where signal detection was possible. This discovery shows close-in planets can be smaller, as well as much larger, than planets orbiting our Sun."
The team of planet-hunters used data obtained from Kepler, which simultaneously and continuously observes and measures the brightness of over 150,000 stars every half-hour. When a potential extrasolar planet transits (passes in front of the glaring face of its parent star), a percentage of the light emitted by the parent star is blotted out. This results in a tiny dip in the brightness of the starlight, suggesting the transiting planet's size relative to that of its glowing parent star.
The entire system circling Kepler-37 would fit cosily within the orbit of Mercury, which orbits our Star in 88 days.
Dr. Barclay commented to the press on February 20, 2013 that "When we first found exo-planets, they were all much larger than anything we have in the inner Solar System. We didn't know of anything that was smaller. This is the first time we've been able to probe the smallest range, smaller than anything we have in our Solar System."
This article is dedicated to the memory of my Uncle Donald, the poet.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, magazines, and journals. 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.
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Calling all Guest Bloggers! Could you write a great post about astronomy, astrophysics and cosmology like Judith's?