A Magnifying Glass In Space Reveals An Icy Alien Planet
By Judith E Braffman-Miller
“This space we declare to be infinite… In it are an infinity of worlds of the same kind as our own.”
Giordano Bruno (1584)
In the 16th century the Italian astronomer, mathematician, and philosopher, Giordano Bruno, became one of the first to propose that the stars we observe in our night sky are really fiery celestial objects similar to our Sun–and are likewise accompanied by their own retinue of planets. However, it was not until 1992 that the first batch of planets, circling a star beyond our own, were discovered–and they were genuine “oddballs” in orbit around a type of stellar corpse called a “pulsar”. Pulsars are very young neutron stars, the sad, dense, city-sized relics of massive stars that recently perished in the brilliant blast of a supernova explosion. The pulsar planets were the very first hint that planets existing in orbit around distant stars may be weird worlds that bear little or no resemblance to the planets inhabiting our own Solar System. In April 2017, astronomers announced that they had found yet another “oddball” distant world–another bizarre planet among the thousands of weird, wonderful, and sometimes eerily familiar worlds that have been discovered over the past generation. Sporting a mass similar to Earth, and orbiting its star at the same distance we orbit our Sun, it is a planetary “iceball”.
This bewitching world of ice is much too cold to be inhabited by life as we know it, because its parent-star is extremely faint. However, this discovery contributes to our scientific understanding of the often strange planetary systems that exist beyond our own Sun’s family.
“This ‘iceball’ planet is the lowest-mass planet ever found through microlensing,” commented Dr. Yossi Shvartzvald in an April 26, 2017 NASA Jet Propulsion Laboratory Press Release. Dr. Shvartzvald is a NASA postdoctoral fellow based at the JPL, located in Pasadena, California, and lead author of a study published in the April 26, 2017 issue of the Astrophysical Journal Letters.
Magnifying Glasses In Space
Microlensing is a technique that aids in the detection of distant objects by using background stars as magnifying glasses. When a foreground star travels precisely in front of a brilliant background star, the gravity of the foreground star focuses the light emanating from the background star, making it appear brighter. If there is a planet orbiting the foreground star, it may cause an additional blip in its parent-star’s brightness. In the case of the “snowball” exoplanet, the blip only lasted for a few hours. Astronomers using this technique have discovered the most distant known exoplanets from Earth. Furthermore, this technique can spot low-mass planets that are considerably farther from their parent-stars than Earth is from the Sun.
The term gravitational lensing itself refers to the path that traveling light takes when it has been deflected. It occurs when the mass of a foreground object warps, bends, and distorts the light of an object situated in the background. The traveling light does not need to be entirely visible light–it an be any form of radiation. As a result of lensing, beams of traveling light that normally would not be seen are bent in such a way that their paths wander towards the observer. Conversely, beams of light can also be bent in such a way that they wander away from the observer. There are different types of gravitational lenses: strong lenses, weak lenses, and microlenses. The differences between these three distinct forms of gravitational lenses has to do with the position of the background object that is sending its light out into space, the foreground lens that is distorting the light, and the position of the observer. The mass or shape of the foreground gravitational lens can also play an important role. Therefore, the foreground object determines how much light emanating from the background object will be distorted, and also where this light will wander.
Albert Einstein’s Theory of Special Relativity (1905), describes a Spacetime that is often compared to an artist’s blank canvas. The artist paints points and lines on this marvelous canvas which represents the stage where the universal drama is being played out–but it does not play a role in the drama itself. The great achievement linking the stage with the drama came a decade later with Einstein’s Theory of General Relativity (1915). According to General Relativity, Space itself becomes a star player in the drama. According to the play’s plot, Space tells mass how to move, and mass tells Space how to curve. Spacetime is as flexible as a trampoline, onto which children toss a heavy ball. The ball represents a massive object–for example, a star. The heavy ball creates a dimple in the flexible fabric of the trampoline. If the children then playfully toss marbles onto the stretchy fabric, the marbles will travel curved paths around the “star”–as if they were real planets in orbit around a real star. If the heavy ball is taken away, the marbles then travel straight paths on the fabric of the trampoline, because there is no dimple on the stretchy fabric to bend their paths. The stage and the drama are united, and it will last for as long as the main players exist.
The Theory of General Relativity predicts that heavy concentrations of mass in the Universe will warp traveling light like a lens, thus magnifying celestial objects situated behind the mass when observed by astronomers on Earth. The very first gravitational lens was detected back in 1979, and lensing now provides a new tool for astronomers to use in order to observe the Cosmos soon after its primordial birth about 14 billion years ago.
When the path that wandering light takes is far from the mass, or if the mass is not particularly large, weak lensing occurs–and the background object is only slightly distorted. In contrast, when the background object is positioned almost exactly behind the mass, strong gravitational lensing can occur, smearing out extended foreground objects–such as galaxies or galaxy clusters. However, the strong lensing of small, point-like objects often produces multiple images–such as an Einstein cross–dancing a dazzling display around the lens.
The idea that extrasolar planets exist has been contemplated for centuries. However, until about a generation ago, there was no way to detect them–or even to estimate how frequently they occur, or even to determine how similar (or dissimilar) they might be to the planets of our Sun’s familiar family.
The idea that planets could exist around stars beyond our own Sun, was mentioned by Sir Isaac Newton in the 18th century in the General Scholium that concludes his Principia. While making a comparison to our Sun’s family of planets, Newton wrote: “And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One.”
In 1952, more than 40 years before the first hot Jupiter planet was discovered, the Russian astronomer Otto Struve (1897-1963) wrote that there is no particular reason why planets could not hug their parent-stars much more closely than the quartet of inner planets inhabiting our own Solar System hug our Sun. Struve went on to propose that Doppler spectroscopy and the transit method could spot “super-Jupiters” in close orbits around their star.
Indeed, the first planet to be discovered circling a star similar to our own Sun, was a hot Jupiter–dubbed 51 Pegasi b (51 Peg b, for short). The discovery of this enormous gas-giant planet, back in 1995, created both considerable joy, as well as considerable confusion, among planet-hunting astronomers. This is because it was previously thought that giant, gaseous planets like 51 Peg b could not orbit their stars in extremely close, “roasting” orbits–that carried them much closer to their star than Mercury’s orbit around our Sun. 51 Peg b was discovered by astronomers using the Doppler method–that looks for an extremely tiny wobble induced by a planet on its parent-star. This method favors the discovery of massive planets in tight, roasting, star-hugging orbits.
Claims of exoplanet detections have been made by many frustrated astronomers since the nineteenth century. For example, some of the earliest involved the binary star 70 Ophiuchi. In 1855, William Stephen Jacob at the East India Company’s Madras Observatory announced that he had detected orbital anomalies that made it “highly probable” that there was a “planetary body” lurking in this system.
The first scientific discovery of an extrasolar planet was in 1988. However, the first confirmed detection did not come until 1992. As of April 1, 2017, there have been 3,607 exoplanets detected inhabiting 2,701planetary systems and 610 multiple planetary systems that have been confirmed.
The European Organization For Astronomical Research (ESO) High Accuracy Radial Velocity Planet Searcher (HARPS) (since 2004) has detected approximately a hundred exoplanets, while NASA’s Kepler space telescope (since 2009) has discovered a few thousand candidate distant worlds. Approximately 11% of these newly discovered candidates may be false positives. On average, there is at least one planet per parent star in our Galaxy, with many of them orbited by multiple planets.
The least massive known exoplanet is Draugr (PSR B1257+12A or PSR B1257+b), which is only about twice the mass of Earth’s Moon. The most massive known exoplanet listed on the NASA Exoplanet Archive is DENIS-P J082303.1-491201 b, which is approximately 29 times more massive than our own Solar System’s planetary behemoth, Jupiter. However, because DENIS-P J082303.1-491201 b is so massive, according to some definitions of “planet”, it may be classified as a type of failed star known as a brown dwarf. Brown dwarfs probably are born the same way as their more successful stellar kin–as the result of the collapse of a particularly dense blob, embedded within the ruffling, swirling, undulating folds of a cold, dark, giant molecular cloud. However, brown dwarfs never manage to attain sufficient mass to ignite their stellar fires.
There are exoplanets that hug their parent-stars so closely that they require only a few hours to complete one orbit–while there are others so far from their star that they take literally thousands of years to complete a single orbit. Indeed, some exoplanets are so far from their stellar parent that it is difficult to determine whether they are really bound gravitationally to their star. Almost all of the exoplanets discovered so far dwell within our own Milky Way Galaxy, but there are also a handful of potential detections of extragalactic planets far, far away. The closest known exoplanet to us is Proxima Centauri b, situated a “mere” 4.2 light-years from Earth, and in orbit around the nearby star Proxima Centauri, which is the closest star to our Sun.
The discovery of exoplanets has intensified scientific interest in the search for extraterrestrial life.
A Magnifying Glass In The Sky Reveals A Distant Icy World.
However, the newly discovered “iceball” world is not likely to host life as we know it. Called OGLE-2016-BLG-1195Lb, it is nevertheless of great value to astronomers in their quest to discover the distribution of planets in our Milky Way. One important unanswered question is whether there is a difference in the frequency of planets in our Galaxy’s central bulge when compared to its disk. The disk is a pancake-like region encircling the bulge. OGLE-2016-BLG-1195Lb inhabits our Milky Way’s disk, as do a duo of exoplanets that had been previously discovered through microlensing conducted by NASA’s Spitzer Space Telescope.
“Although we only have a handful of planetary systems with well-determined distances that are far outside our Solar System, the lack of Spitzer detections in the bulge suggests that planets may be less common toward the center of our Galaxy than in the disk,” explained Dr. Geoff Bryden in the April 26, 2017 JPL Press Release. Dr. Bryden is an astronomer at JPL, and co-author of the study.
For the new study, astronomers were alerted to the initial microlensing event by the ground-based Optical Gravitational Lensing Experiment (OGLE) survey, managed by the University of Warsaw in Poland. The authors of the study used the Korea Microlensing Telescope Network (KMTNet), operated by the Korea Astronomy and Space Science Institute, and Spitzer. The telescopes were used in order to track the event from Earth and in space.
KMTNet is composed of three wide-field telescopes: one in Australia, one in Chile, and one in South Africa. When the astronomers from the Spitzer team received the OGLE alert, they realized that it could be indicating the discovery of a new exoplanet. The microlensing event alert was made only two hours before Spitzer’s targets for the week were to be finalized.
With both KMTNet and Spitzer keeping a watchful eye on the event, astronomers benefited from having two vantage points from which they could study the objects being targeted. It was if two eyes separated by a great distance were observing it. Possessing data from these two perspectives, the astronomers were able to spot the planet with KMTNet and calculate the mass of both the exoplanet and its parent star using Spitzer data.
“We are able to know details about this planet because of the synergy between KMTNet and Spitzer,” commented Dr. Andrew Gould in the April 26, 2017 JPL Press Release. Dr. Gould is professor emeritus of astronomy at Ohio State University in Columbus, and a study co-author.
OGLE-2016-BLG-1195Lb is almost 13,000 light-years away from Earth, and circles a star that is so small and dim astronomers aren’t even certain that it is really a star. It could really be that unfortunate runt of the stellar litter, a total failure as a star–a brown dwarf. This particular possible star is a “mere” 7.8 percent the mass of our own Sun, and it is precariously poised right on the border between being a true star or a stellar failure, whose nuclear fusing fires did not ignite because it never grew hot enough to generate energy through that process.
There is an alternative suggestion that it may be an ultra-cool dwarf star, which is a true star, despite its relatively petite size. In fact, it could be a little star very similar to TRAPPIST-1, which Spitzer and ground-based telescopes recently revealed to be the stellar parent of seven Earth-size worlds. Those seven distant planets all closely orbit their parent-star, hugging it even more closely than Mercury does our Sun. All seven of these intriguing worlds could potentially possess liquid water. The presence of liquid water indicates the possiblility, though not the promise, of the existence of life as we know it. However, OGLE-2016-BLG-1195Lb, at the Earth-Sun distance, is extremely cold. This is because its stellar parent is a very faint star, and this planetary “iceball” is likely to be more frigid than Pluto in our own Solar System–indicating that any water that may exist on this world would be frozen. A planet would need to orbit much closer to the small, faint star to be gifted with sufficient light to maintain liquid water on its surface.
Ground-based telescopes that are available today are not able to detect smaller planets than this distant, frozen “iceball” using the microlensing method. A very sensitive space telescope would be required in order to discover smaller worlds in microlensing events. NASA’s upcoming Wide Field Infrared Survey Telescope (WFIRST), scheduled to launch in the mid-2020s, will have this capability.
Dr. Shvartzvald commented in the April 26, 2017 JPL Press Release that “One of the problems with estimating how many planets like this are out there is that we have reached the lower limit of planet masses that we can currently detect with microlensing. WFIRST will be able to change that.”
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, magazines, and newspapers. Although she has written on a variety of subjects, 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.