Ever since the first alien world, circling a distant Sun-like star, was spotted almost twenty years ago, the discovery of an Earth-like exoplanet has been the Holy Grail of planet-hunting astronomers. No such Earth-analog has as yet been identified, and so the search continues.
“We only know of one star that hosts a planet with life, the Sun. Finding a planet in the habitable zone around a star like our Sun is a significant milestone toward finding truly Earth-like planets,” Dr. Thomas Barclay said in an April 18, 2013 NASA Jet Propulsion Laboratory (JPL) Press Release. Dr. Barclay, a scientist with NASA’s planet-hunting, Kepler Space Telescope, is at the Bay Area Research Institute in Sonoma, California, and lead author of a paper describing the discovery of the Kepler-69 planetary system that was published in the Astrophysical Journal. The system contains two planets: Kepler-69b and Kepler-69c. Kepler-69c was classified as a “super-Earth”. A “super-Earth” is usually defined as an exoplanet with a mass between 1 and 10 Earth-masses.
At the time of Kepler-69c’s discovery, it was thought to be comfortably orbiting its star at the Goldilocks distance where liquid water could exist. This fortunate location–which is not too hot, not too cold, but just right–is termed a star’s habitable zone. Where there is liquid water, there is the possibility, though not the promise, of life. In addition to possessing liquid water on its surface, a habitable exoplanet must also sport a life-friendly atmosphere and a well-behaved parent star.
The search for exoplanets, circling other stars beyond our Sun, has proven to be an extremely difficult quest, and their ultimate discovery almost a generation ago clearly represents one of humanity’s proudest moments. There are, as of this writing, more than 800 confirmed exoplanet discoveries–but there are very likely billions more as yet undiscovered alien worlds inhabiting our Milky Way Galaxy! Some of the confirmed alien worlds are eerily familiar, resembling the planets of our own Solar System. Others, however, are so bizarre that they are not like anything astronomers ever thought could exist.
Since the first discovery of an exoplanet orbiting a Sun-like star back in 1995, the discoveries keep rushing in at a breathtaking pace. Planet-hunters keep improving their techniques, as they carefully study the accumulated data collected by instruments both on Earth and in Space. The biggest batch of alien worlds to be confirmed–in the foreseeable future–should be netted by NASA’s unfortunate Kepler Space Telescope. Kepler successfully accumulated a vast number of both confirmed and potential exoplanet discoveries, before it became crippled in May 2013, when the second of its four orientation-maintaining reaction wheels ceased to function.
The Kepler-69 System
In April 2013, astronomers announced that Kepler had spotted two new planetary systems. One of the then newly discovered systems was the Kepler-69 system, while the other was the Kepler-62 system. The Kepler-62 system sported five planets: 62b, 62c, 62d, 62e and 62f. Three planets inhabiting the two newly discovered systems were designated “super-Earths”: Kepler-62e, Kepler-62f, and Kepler-69c.
Kepler-69c is calculated to be about 70% larger than the size of our planet. However, astronomers remain uncertain about the composition of Kepler-69c, even though it is known to sport an orbit that circles its star every 242 days– a distance comparable to the orbit of the planet Venus around our own Sun.
The Kepler-69 exoplanets’ parent star belongs to the same stellar class as our Sun, termed G-type. It is approximately 93% the size of our Sun and 60% as luminous. The Kepler-69 system is located about 2,700 light-years from Earth in the constellation Cygnus.
“The detection and confirmation of planets is an enormously collaborative effort of talent and resources, and requires expertise from across the scientific community to produce these tremendous results. Kepler has brought a resurgence of astronomical discoveries and we are making excellent progress toward determining if planets like ours are the exception or the rule,” Dr. William Borucki noted in the April 18, 2013 JPL Press Release. Dr. Borucki is Kepler science principal investigator at NASA’s Ames Research Center at Moffett Field, California. Dr. Borucki is also lead author of the Kepler-62 paper published in April 2013 in the journal Science.
Although Kepler-69c was originally thought to be a “super-Earth,” happily dancing around its star in the comfortable Goldilocks region where liquid water could exist, astronomers now think that this may not be the case. Kepler-69c is more likely a hostile world, like Venus in our own Solar System. Venus is a near-Earth-size ball of hell; an inhospitable world pelted by corrosive sulfuric acid raindrops, stricken by violent volcanic eruptions, and so hot on its eerie, creepy, glowing-red surface that it could melt lead. Indeed, Venus literally has the hottest surface of any planet in our Solar System! If it were possible for an Earthling to stand on Venus, the Sun would be seen to rise in the West, travel slowly across the sky, and then set in the East–the precise opposite of what occurs on our own planet. Venus rotates backwards in comparison to the other planets of our Sun’s family. If observed from high above its north pole, Venus would appear to be rotating clockwise. Surface pressure on Venus is comparable to being 900 meters under water! The hell-like planet is also as dry as a bone; a truly horrific example of the “runaway greenhouse effect” in action–life could not possibly evolve on Venus!
Although the newer measurements indicate that Kepler-69c is more likely a “super-Venus” than a “super-Earth”, the planetary status alteration is largely only a reflection of the difficulties arising over how to define the habitable zone surrounding a star.
“There are a lot of unanswered questions about habitability,” Dr. Lucianne Walkowicz said in an October 21, 2013 statement to the press. Dr. Walkowicz, an astrophysicist, is a Kepler science team member at Princeton University.
“If the planet gets zapped with radiation all the time by flares from its parent star, the surface might not be a very pleasant place to live. But on the other hand, if there’s liquid water around, that makes a really good shield from high-energy radiation, so maybe life could thrive in the oceans,” Dr. Walkowicz continued to explain.
A close examination of the planet’s chemistry now suggests that Kepler-69c actually skirts its star just outside the habitable zone’s innermost edge.
“[M]olecules in a planet’s atmosphere will absorb a certain amount of energy from starlight and radiate the rest back out. How much of this energy is trapped can mean the difference between a turquoise sea and erupting volcanoes,” NASA announced in a follow-up press release issued in June 2013.
The astronomers also studied the parent star’s energy output, as well as Kepler-69c’s orbit around it, in order to make their later determination. However, the astronomers still find it difficult to say for certain whether or not the planet is in the habitable zone around its star. It is still necessary to observe the planet’s atmosphere, but it is difficult for telescopes available today to detect “signatures” of oxygen, water, methane, or carbon dioxide, that could hint at the presence of life.
Even though the upcoming James Webb Space Telescope, scheduled for launch in 2018, can study exoplanet atmospheres, it was constructed to spot planets that are much larger than Earth. Determining the nature of Kepler-69c’s atmosphere may have to wait for a still more sensitive telescope, according to NASA,
This research was published in the June 20, 2013 issue of Astrophysical Journal Letters.