A Distant Warm Neptune’s Surprising Atmosphere
By Judith E Braffman-Miller
The weird, wonderful worlds that exist in the distant families of stars beyond our Sun, have presented more than a few bewitching, bewildering, and bothersome surprises for planet-hunting astronomers to solve. While the discovery of new exoplanets occurs on an almost daily basis today, it was only a generation ago that the very first confirmed batch of remote planets in orbit around an alien star were first detected–and they were a bizarre bunch, circling a city-sized, dense little stellar corpse called a pulsar. A pulsar is a newborn neutron star–the lingering relic of a massive star that has perished in the fiery fury of a supernova explosion–leaving behind this dense little tattle-tale testimony of its former stellar existence. In May 2017, a team of astronomers announced their discovery of yet another strange world belonging to the family of a faraway star. This study, combining observations from NASA’s Hubble (HST) and Spitzer space telescopes, reveals that a distant exoplanet dubbed HAT-P-26b possesses a primitive atmosphere made up almost entirely of hydrogen and helium. Located far, far away, about 437 light-years from Earth, Hat-P-26b circles a star approximately twice as old as our own 4.6 billion year old Sun.
This analysis is one of the most detailed studies to date of a warm Neptune–which is a planet that is approximately the same size as or own Solar System’s Neptune, but hugs its own roiling parent-star much more closely than Neptune does our Sun. In dramatic contrast, our own Solar System’s Neptune is the most distant of the eight major planets from our Star, and it is a frigid, but nonetheless beautiful, blue banded gaseous world–an ice giant alien planet dwelling in the frozen perpetual twilight of our Solar System’s outer limits. Here, in this dusky, dimly-lit region, our own Sun hangs suspended in the sky, appearing to be only an especially large star floating in a dark sea along with countless others of its own stellar kind.
The quartet of giant gaseous planets inhabiting the outer regions of our Solar System–Jupiter, Saturn, Uranus, and Neptune–twirl around our Sun in this distant murky realm. Jupiter and Saturn are enormous balls of gas, perhaps containing only very small solid cores hidden beneath heavy and extremely thick atmospheres–although some planetary scientists have proposed that the enormous, majestic duo of gas-giant planets contain no solid surfaces at all. Uranus and Neptune are ice-giant planets that do contain large icy-rocky cores, and comparatively thin gaseous atmospheres compared to the duo of enormous gas-giants. Uranus and Neptune are giant planets, but they are not nearly as enormous as Jupiter and Saturn. Of the quartet of giants, Neptune is the smallest.
It is almost certain that the duo of ice-giants inhabiting our Solar System were not born where we see them today. Uranus is 19 Astronomical Units (AU) from our Sun, while distant Neptune is about 30 AUs away from the light and warmth of our Star. One AU is equivalent to the mean distance between our Earth and the Sun, which is about 93,000,000 miles. The accretionary processes that formed full-sized planets in our young Solar System functioned much more slowly farther from our Sun, where Uranus and Neptune are now situated. The protoplanetary accretion disk made up of gas, ice, and dust was too thin in this distant region to enable planets of this gigantic size to form as rapidly as they would in warmer, denser regions of the disk closer to our Star.
The astronomers who made the recent observations of HAT-P-26b‘s atmosphere found that it is relatively clear of clouds, and also displays a strong water signature. However, this alien planet is not a water world. The observations of HAT-P-26b’s atmosphere represents the best measurement of water to date on an exoplanet of this size.
The detection of an atmosphere, with this particular composition on an exoplanet, has important implications for the way scientists explain the birth and evolution of planetary systems. In contrast to the duo of ice giant worlds in our own Solar System, HAT-P-26b is thought to have been born either closer to its parent star or later in the development of its planetary system–or both.
“Astronomers have just begun to investigate the atmospheres of these distant Neptune-mass planets, and almost right away, we found an example that goes against the trend in our Solar System. This kind of unexpected result is why I really love exploring the atmospheres of alien planets,” commented Dr. Hannah Wakeford in a May 12, 2017 NASA Press Release. Dr. Wakeford is a postdoctoral researcher at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the study published in the May 12, 2017 issue of the journal Science.
A Brief History Of The Hunt
As of May 1, 2017, 3,608 confirmed exoplanets are listed in the Extrasolar Planets Encyclopedia. This treasure trove of distant worlds now includes a few alien planets that were actually discovered in the late 1980s, but were only first confirmed more recently. The first published discovery of an exoplanet to receive subsequent confirmation was made in 1988 by the Canadian astronomers Dr. Bruce Campbell, Dr. G.A.H. Walker, and Dr. Stephenson Yang of the University of Victoria and the University of British Columbia. Even though the three Canadian astronomers were cautious about claiming an exoplanet detection, their radial-velocity observations indicated that a distant planet orbits the star Gamma Cephei.
On January 9, 1992, radio astronomers Dr. Aleksander Wolszczan and Dr. Dale Frail announced the discovery of two of the strange worlds in orbit around the pulsar PSR 1257+12. This discovery was confirmed, and it is usually considered to be the first definite detection of exoplanets. Additional observations confirmed a third pulsar planet in 1994. These pulsar planets are believed to have been born from the bizarre remnants of the supernova that produced the pulsar, in a second round of planet formation–or, alternatively, to be the lingering, remaining rocky cores of doomed gas giants disrupted by the supernova blast. The lingering rocky cores of these ill-fated giants eventually decayed into their current orbits, where they now reside, bathed in deadly showers of radiation flowing out from all that is left of their parent-star.
On October 6, 1995, Dr. Michel Mayor and Dr. Didier Queloz of the University of Geneva in Switzerland announced the first definite discovery of an exoplanet circling a main-sequence star like our own Sun. The discovery was made at the Observatoire de Haute-Provence, and its detection heralded the modern era of exoplanetary discovery. The planet, named 51 Pegasi b (51 Peg b, for short), is a hot Jupiter–which is a giant gaseous planet circling its parent-star fast and close in a roasting orbit. The discovery of 51 Peg b surprised astronomers because theories of planetary formation at that time suggested that giant planets could only be born much farther from their stars.
Some Like It Hot
The first exoplanets to be confirmed–the pulsar planets and the hot Jupiter, 51 Peg b–proved to be only the tip of the iceberg–a very big iceberg. In fact, planet-hunting astronomers have learned by now to expect the unexpected. The cornucopia of alien worlds that have been discovered over the past generation revealed exoplanets that are sometimes eerily similar to the planets inhabiting our Solar System, while others are definitely eerily alien–so alien, in fact, that before they were detected, astronomers never even dreamed that such bizarre worlds could exist.
A hot Neptune is a type of planet sporting a mass similar to that of our own Solar System’s ice-giants Uranus and Neptune. However, these warm worlds orbit much closer to their parent-stars than our Solar System’s distant frigid duo circle our Sun. Normally, hot Neptunes orbit their star at the truly searing-hot distance of less than 1 AU. The very first hot Neptune to be found with certainty was Gliese 436 b back in 2007–an alien world dwelling about 33 light-years from Earth. More recent observations have revealed that there may be a much larger possible population of hot Neptunes within our own Milky Way Galaxy than previously thought.
Because they hug their parent-stars so closely, hot Neptunes have a much greater chance of transiting in front of the glaring face of their stellar parent as observed from a farther outlying point, than exoplanets of the same mass that are in larger orbits. This increases the chances of discovering them by transit-based observation techniques.
Transiting hot Neptunes that have so far been discovered include Gliese 436 b and HAT-P-11b, which were both observed by NASA’s highly successful planet-hunting Kepler Space Telescope. Gliese 436 b (GJ 436b) was the first hot Neptune to be detected with certainty in 2007. However, the exoplanet named Mu Arae c (HD 160691) detected in 2004 might also be a hot Neptune, but it has not been definitely confirmed as such.
A Distant Warm Neptune’s Surprising Atmosphere
In order to study HAT-P-26b’s atmosphere, the astronomers relied on data that had been obtained using the transit method–when the distant alien world floated in front of the face of its fiery parent star. During a transit, a fraction of the emitted starlight gets filtered through the planet’s atmosphere, which absorbs some wavelengths of light but not others. By studying how the signatures of the starlight change as a result of this filtering, scientists can work their way backward to determine the chemical composition of the atmosphere.
In the case of HAT-P-26b, the team of astronomers pooled data obtained from four transits measured by HST, in addition to two that were observed by Spitzer. Together, those observations covered a wide range of wavelengths ranging from yellow light through the near-infrared region of the electromagnetic spectrum.
“To have so much information about a warm Neptune is still rare, so analyzing these data sets simultaneously is an achievement in and of itself,” explained study co-author Dr. Tiffany Kataria in the May 11, 2017 NASA Press Release. Dr. Kataria is of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.
The study provided an exact measurement of water. This enabled the astronomers to use the water signature to estimate HAT-P-26b’s metallicity. Astronomers determine the metallicity as an indication of how gifted the planet is in all atomic elements that are heavier than hydrogen and helium. This is because the information will provide them with important clues about how a planet formed. The Big Bang birth of our Universe, almost 14 billion years ago, only manufactured the lightest atomic elements–hydrogen, helium, along with traces of lithium and beryllium. All atomic elements heavier than helium are termed metals by astronomers–and all of the metals in the Cosmos were produced either in the nuclear-fusing furnaces of stars–that progressively fused the atoms of lighter atoms into heavier ones (stellar nucleosynthesis)–or, alternatively, in the powerful, fiery blast of a supernova explosion that heralds the demise of a massive star.
In order to compare planets according to their metallicities, astronomers use our own Sun as a reference point. This technique has been compared to describing how much caffeine beverages contain by comparing them to a cup of coffee. Our own Solar System’s banded behemoth Jupiter has a metallicity approximately 2 to 5 times that of our Sun. In the case of Saturn–our Solar System’s second largest gas giant planet–it is about 10 times as much as the Sun. These comparatively low values indicate that the gas-giant duo are composed almost entirely of hydrogen and helium.
The ice-giants Neptune and Uranus, in contrast, are smaller than the gas-giants but more richly endowed with the heavier elements–the metals. Neptune and Uranus have metallicities approximately 100 times that of our Sun. Therefore, for the quartet of giant outer planets in our Sun’s family, the trend is that the metallicities are lower for the bigger planets.
Astronomers believe that this happened because, as our Solar System was first forming, Neptune and Uranus were born in a region toward the outer limits of the gigantic disk of gas, dust, and debris that whirled around the baby Sun. Basically, this rather complicated process of planetary formation can be described in more or less simple terms. Neptune and Uranus would have been badly bombarded by an invading crashing horde of rampaging icy debris that was heavily laden with metals. In contrast, Jupiter and Saturn–born in the warmer and more brightly-lit region of the primordial accretion disk–would have not been subjected to a similar heavy bombardment of crashing icy debris.
However, Dr. Wakeford and her colleagues discovered that HAT-P-26b travels to the beat of a different drum. The astronomers determined its metallicity to be only approximately 4.8 times that of our Sun–much closer to the value for Jupiter than for the more metal-rich Neptune.
Dr. David K Sing, of the University of Exeter in the UK, and the second author of the paper, noted in the May 11, 2017 NASA Press Release that “This analysis shows that there is a lot more diversity in the atmospheres of these exoplanets than we were expecting, which is providing insight into how planets can form and evolve differently than in our Solar System. I would say that has been a theme in the studies of exoplanets. Researchers keep finding surprising diversity.”
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, journals, and magazines. 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.