Nature Vs Nurture: The Strange Case Of The Exoplanet “Cousins”
By Judith E Braffman-Miller
Nature versus nurture refers to a long-standing debate among scientists who are trying to find out if human behavior is determined by the environment or is merely the result of a person’s genes. Planets and people can have a lot in common, and the atmospheres of a duo of hot Jupiter exoplanets is a case in point. These two worlds serve as examples of how nature versus nurture operates when it comes to these two “cousin” exoplanets. In a one-of-a-kind experiment, planet-hunting astronomers used NASA’s Hubble Space Telescope (HST) to observe the hot Jupiter “cousins”, and because these two distant, gaseous and broiling worlds are virtually identical in both size and temperature, circling their nearly identical parent-stars at the same distance, the astronomers thought that their atmospheres would also be alike. What they found surprised them–one of these kindred worlds is cloudier than the other, and the difference between these distant worlds is now a delightful mystery just waiting to be solved by curious planetary scientists who are trying to understand why this difference exists between two such closely related worlds.
Lead scientists Dr. Giovanni Bruno of the Space Telescope Science Institute (STSI) in Baltimore, Maryland, explained in a June 5, 2017 STSI Press Release that “What we’re seeing in looking at the two atmospheres is that they’re not the same. One planet–WASP-67b is cloudier than the other–HAT-P-38b. We don’t see what we’re expecting, and we need to understand why we find this difference.”
The planetary scientists used HST’s Wide Field Camera 3 to observe the two “cousin” exoplanets’ spectral fingerprints, which measure chemical composition. “The effect that clouds have on the spectral signature of water allows us to measure the amount of clouds in the atmosphere. More clouds mean that the water feature is reduced,” Dr. Bruno added.
“This tells us that there had to be something in their past that is changing the way these planets look,” he continued to explain.
From a historical perspective, the hunt for distant alien worlds, located within the families of stars beyond our own Sun, proved to be a difficult endeavor. The discovery of the first exoplanets a generation ago clearly represents one of humanity’s greatest accomplishments. Spotting a giant planet, such as our own Solar System’s banded behemoth, Jupiter, has been compared to observing light skipping off a gnat that is flying in front of the 1,000-watt light bulb of a street lamp–when the observer is 10 miles away.
The smaller the exoplanet, the harder it is to discover. For example, if an alien astronomer, belonging to a technologically advanced civilization, went on the hunt for other planets in remote regions of our Milky Way Galaxy, it would have a difficult time finding our small planet. This is because our Earth would appear as only a faint and insignificant speck in the vastness of space. Indeed, our planet is very well hidden from prying alien astronomers because the glare of our Star overwhelms it.
The first detection of an exoplanet occurred back in 1988. However, the first confirmed discovery came in 1992, with the detection of some bizarre and hostile planets circling a dense, city-sized stellar corpse called a pulsar. Pulsars are the lingering relics of massive stars that have perished in the terrible fury of a supernova explosion. This furious, fatal, final blaze of glory marks the violent and catastrophic end of the star-that-was.
Astronomers detected the first exoplanet in orbit around a still “living” star, like our own Sun, back in 1995. However, this historic discovery left confusion in its wake. The newly discovered alien world, dubbed 51 Pegasi b, was unlike anything planetary scientists thought could exist. 51 Peg b is a hot Jupiter–a giant gaseous world, like our Solar System’s Jupiter, that closely hugs its parent-star in a roasting orbit that is much closer to its stellar parent than Mercury’s orbit around our Sun. Before the discovery of 51 Peg b, most astronomers thought that giant gaseous planets could only exist much farther away from their stars–comparable to Jupiter’s distance from our Sun. Jupiter is located in the cold outer region of our Solar System.
The original technique used by astronomers back in 1995–the Doppler Shift method–favors the discovery of giant planets circling around their parent-stars in close, broiling orbits. The Doppler Shift method looks for a tiny wobble induced on a star by an orbiting planet–the larger the planet, the greater the wobble, and the easier it is for planet-hunting astronomers to spot.
As of June 1, 2017, 3,610 exoplanets, inhabiting 2,704 planetary systems, have been discovered–and 610 multiple planetary systems have also been verified. Since 2004, the European Southern Observatory’s (ESO’s) High Accuracy Radial velocity Planet Searcher (HARPS) 3.6 meter telescope, has detected approximately 100 exoplanets, and since 2009, NASA’s Kepler Space Telescope has discovered over two thousand. Kepler has also spotted a few thousand candidate planets, out of which only about 11% may prove to be false-positives. Planet-hunting astronomers estimate that about 1 in 5 stars similar to our Sun are orbited by an “Earth-sized” planet located in the habitable zone surrounding their star. The habitable zone of a star is that Goldilocks region where temperatures are not too hot, not too cold, but just right for water to exist in its life-sustaining liquid phase. Where liquid water exists, life can potentially evolve as well. If there are 200 billion stars inhabiting our Galaxy, it may be that there are 11 billion potentially habitable Earth-sized worlds in our Milky Way. This already huge number could rise even further if planets circling the numerous, long-lived red dwarf stars are included in the estimate. Red dwarf stars are the smallest, coolest, and most abundant true stars dwelling in our Galaxy. Red dwarfs are even smaller than our own small Sun, and they can potentially remain on the hydrogen-burning main-sequence of the Hertzsprung-Russell Diagram of Stellar Evolution for trillions of years. For this reason, it is generally thought that there are (as yet) no red dwarf relics inhabiting the Cosmos. This is because our Universe is a “mere” 13.8 billion years old, and no red dwarf has had enough time to die since the Big Bang.
The least massive exoplanet discovered so far is Draugr (PSR B1257+12A or PSR B1257+12B), which weighs-in at only twice the mass of our planet’s Moon. In contrast, the most massive known exoplanet is DENIS-P J082303.11-491201 b, and it is about 29 times more massive than Jupiter. However, according to some definitions of a planet, this extremely large world is too massive to be a planet, and may be a type of failed star called a brown dwarf. Brown dwarfs are relatively small distant worlds that probably form the same way as their true stellar kin, but never manage to attain the mass necessary to light their nuclear-fusing fires. These stellar failures are really a pretty purple-pink color called magenta, and they are born as a result of the collapse of a dense pocket embedded within the swirling, undulating folds of a cold, giant molecular cloud–just like their more successful stellar relatives.
Some exoplanets cling closely to their parent-star in such tight, broiling orbits that they require only a few hours to complete a single orbit. However, there are other alien planets that take thousands of years to circle their star. Indeed, some exoplanets are so far from their parent-star that it is sometimes very hard for astronomers to determine whether they are really bound to it gravitationally. Almost all of the exoplanets discovered so far are inhabitants of our own Milky Way Galaxy, but there have also been detections of a handful of intriguing, but still unconfirmed, extragalactic exoplanets. The nearest exoplanet to Earth is dubbed Proxima Centauri b, which circles Proxima Centauri, the closest star to our own Sun. Proxima Centauri b is “only” 4.2 light-years from Earth.
There is also a heavy population of so-called rogue planets, which do not belong to the family of any star at all, but wander through the wilderness of interstellar space without a parent-star to call their own. Alas, these lonely, solitary alien worlds were probably once members of a planetary system, but were rudely evicted by the gravitational nudges of sibling worlds, or by the gravitational disruption caused when a traveling star passed too close to their own stellar parent. Astronomers tend to consider these lonely worlds separately, especially if they are gas-giant planets. If this is the case, these rogue planets are frequently classified as sub-brown dwarfs. The rogue planets that roam our Milky Way may number in the billions.
Nature Vs. Nurture: The Strange Case Of The Exoplanet “Cousins”
The two mismatched “cousin” exoplanets–one cloudy and one clear–circle around their yellow dwarf stars once every 4.5 Earth days. Both exoplanets hug their parent-star tightly, much more closely than Mercury hugs our Sun. However, long ago, the planets likely migrated inward toward the glaring fires and searing-heat of their stellar parent from the more distant regions where they were born.
It is possible that one planet formed differently from the other as the result of a different set of circumstances. “You can say it’s nature versus nurture. Right now, they appear to have the same physical properties. So, if their measured composition is defined by their current state, then it should be the same for both planets. But that’s not the case. Instead, it looks like their formation histories could be playing an important role,” explained study co-investigator Dr. Kevin Stevenson in the June 5, 2017 the STSI Press Release.
The clouds of this distant duo of searing-hot Jupiter-like gas-giants are not like the clouds we see on Earth. Instead, these very alien clouds are likely alkali clouds. This means that they are probably made up of molecules such as sodium sulfide and potassium chloride. The average temperature on each of these broiling planets is over 1,300 degrees Fahrenheit.
The two exoplanets are also tidally locked. This means that they always show the same side facing their stellar parent. The two worlds have an extremely hot day-side and a cooler night-side.
The team of astronomers have only just begun to learn what factors are important in causing some exoplanets to be cloudy, in contrast to others that are clear. In order to gain a better understanding of what the planets’ mysterious pasts may have been like, scientists will need future observations with the HST and the soon-to-be-launched James Webb Space Telescope.
The team’s results were presented on June 5, 2017 at the 230th meeting of the American Astronomical Society in Austin, Texas.
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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