The Mysterious Dance Of Four Distant Worlds

The first discoveries of alien planets, orbiting distant stars beyond our own Sun, proved to be shocking–strange wonderland worlds that astronomers originally believed could not exist. However, more than twenty years later, after the passage of an entire generation, the discovery of exoplanets has become almost routine–merely “business as usual”. Still, the more recent discoveries of faraway brave new worlds, the planetary children of stars beyond our own Sun, can manage to carry with them some bewildering, bewitching, and bothersome surprises for the curious astronomers who hunt for them. In May 2016, a team of astronomers announced their bizarre discovery of a quartet of exoplanets belonging to the Kepler-223 stellar system and these four weird alien worlds seem to have little in common with the familiar outer planets of Earth’s own Solar System–Jupiter, Saturn, Uranus, and Neptune. Also, in May 2016, astronomers made the announcement that NASA’s Kepler mission has verified 1,284 new alien planets–the single largest treasure trove of exoplanets to date!

The new study reveals that the Kepler-223 system is caught in an orbital configuration that the quartet of gaseous, giant denizens of our own Solar System managed to escape from billions of years ago. The four distant alien worlds seem to waltz together in lock step, trapped in a strange, exotic dance.

Each member of the distant quartet of gaseous, puffed-up worlds is far more massive than our own planet, and all four of them circle very, very close to their parent-star–much closer than Mercury’s distance to our own Star. The exoplanets’ orbits are also stuck together in a very precise pattern, which is what raises the question of whether the four puffy gas giants of our own Solar System somehow managed to break free of a similar configuration in the distant past.

“Exactly how and where planets form is an outstanding question in planetary science. Our work essentially tests a model for planet formation for a type of planet we don’t have in our own Solar System,” noted Sean Mills in a May 11, 2016 University of Chicago Press Release. Mills, a graduate student in astronomy and astrophysics at the University of Chicago, is the study’s lead author.

Because the orbital configuration is so different from the one in our own Solar System, Mills continued to explain, there’s a big debate about how such planets form, how they got there, and why Earth’s Solar System evolved the way it did.

The Kepler-223 star system was first observed several years ago by the Kepler spacecraft, and it is generally considered to be unusual. The four planets inhabiting this rare type of system are all miniature Neptunes, tenderly nested close to the parent-star they are orbiting. Locked in a unique resonance that has lasted for billions of years, for every three orbits of the outermost planet, the second orbits four times, the third six times and the innermost eight times.

Orbital resonances are not uncommon occurrences. For example, our own Solar System’s dwarf planet Pluto circles our Star twice during the same period that Neptune completes three orbits. However, a four planet resonance like the one observed in the Kepler-223 system is considered to be a rarity.

A team of astronomers from the University of Chicago and the University of California at Berkeley, reported the discovery of the dancing quartet of distant planets online in the May 11, 2016 edition of the journal Nature. Study co-author, Dr. Howard Isaacson, a Berkeley research astronomer, noted in a May 11, 2016 UC Berkeley Press Release that the Kepler-223 system can help astronomers understand how our own Solar System and other star systems (discovered during the past few decades) evolved. In particular, it could help answer the important question of whether planets remain in their place of birth, or instead migrate elsewhere–moving either farther from or closer to their parent-stars over the passage of billions of years.

“Basically, this system is so peculiar in the way that it’s locked into resonances that it strongly suggests that migration is the method by which the planets formed–that is, migrating inward closer toward the star after forming farther out,” Dr. Isaacson continued to explain.

NASA’s Kepler mission has revealed a number of alternative scenarios for how planets are born and then migrate elsewhere in planetary systems that are very different from our own.

“Before we discovered exoplanets, we thought that every system must form like ours. Thanks to Kepler, we now have hot Jupiters, many planets that are closer to their star than Mercury or in between the size of the Earth and Neptune. Without the discovery of exoplanets, we would not have known that the Earth is something of an outlier,” Dr. Isaacson added.

When the very first hot Jupiters were discovered more than 20 years ago, they were generally thought to be “oddballs”. Hot Jupiters are gigantic gas-giant planets–like our own Solar System’s behemoth Jupiter–that cling to their parent-stars in searing-hot, close-in orbits. Because we have nothing like hot Jupiters in our own familiar Solar System, astronomers erroneously viewed them as “oddballs”. However, as increasing numbers of these distant, alien worlds were detected–along with smaller exoplanets that also hug their parent-stars fast and close in broiling orbits–our own Solar System began to look like the true nonconformist stellar system.

The orbital configuration of our own Solar System appears to have evolved since its birth 4.6 billion years ago. The four dancing denizens of the Kepler-223 system are considered to be much older than the planets of our Sun’s family. However, the distant exoplanets have somehow managed to keep their orbital configuration for a much longer time.

The quartet of Kepler-223 planets are considerably larger than our Earth, and they are thought to consist of a solid core encased within a thick blanket of gas. The four worlds orbit their parent-star in periods that range from only seven to 19 days. Astronomers term such exoplanets “sub-Neptunes”, and they are the most common class of planets known in our Milky Way Galaxy.

Planets, such as those observed in the Kepler-223 system, are in resonance when, for example, every time one of them orbits its parent-star once, the next one goes around twice. Large Galilean moons orbiting the planet Jupiter were the first bodies discovered displaying resonance.

Kepler-223’s duo of innermost planets are in a 4:3 resonance. The second and third are in a 3.2 resonance, and the third and fourth are in a 4:3 resonance.

“This is the most extreme example of this phenomenon,” commented study co-author Dr. Daniel Fabrycky in the May 11, 2016 University of Chicago Press Release. Dr. Fabrycky is an assistant professor of astronomy and astrophysics at the University of Chicago.

Dr. Isaacson is part of the California Planet Search team and he obtained a spectrum of Kepler-223 in 2012 using the high-resolution echelle specrometer (HIRES) spectrometer on the Keck-1 10-meter telescope poised at the summit of the dormant Mauna Kea volcano in Hawaii. The spectrum showed a star very similar in mass and size to our own Sun–except it is much older. This distant star is over 6 billion years old, while our Sun is a comparatively youthful 4.6 billion years old.

“You need to know the precise size of the star so you can do the dynamical and stability analysis, which involve estimates of the masses of the planets. The Keck telescope is absolutely critical in this regard,” Dr. Isaacson commented in the May 11, 2016 Berkeley Press Release.

The University of Chicago graduate student, Sean Mills, and his collaborators then went on to use brightness data derived from the Kepler telescope to study how the quartet of exoplanets block the starlight and alter each other’s orbits–thus inferring the planets’ sizes and masses. The team of astronomers next performed numerical simulations of planetary migration that could have caused the system’s current structure.

Some stages of the planet-birthing process can involve violent smashup events. However, during other stages, baby planets can evolve from gaseous disks (planetary accretion disks) in a gentle and smooth way–which is likely the way the sub-Neptunian exoplanets of the Kepler-223 stellar system were born.

“We think that two planets migrate through the disk, get stuck and then keep migrating together; find a third planet, get stuck, migrate together; find a fourth planet and get stuck,” Mills explained in the May 11, 2016 University of Chicago Press Release.

That process differs dramatically from the one that astronomers think resulted in the formation of the four inner, rocky terrestrial planets of our Sun’s family: Mercury, Venus, Earth, and Mars. The relatively small quartet of terrestrial planets inhabiting our Solar System are generally thought to have been born in their current orbital locations.

Mills went on to explain that our Earth probably formed when Mars- or Moon-sized bodies smacked into one another in a chaotic and violent process. When planets are born this way their final orbital periods are not near a resonance.

However, many astronomers suspect that our Solar System’s four giant gaseous denizens of the outer limits probably migrated substantially during their early formative years. Our own gigantic quartet may have been booted out of resonances that once were similar to those observed in the Kepler-223 system–possibly after having enounters with numerous traveling asteroids and the small planetary building blocks termed planetesimals.

The resonance observed in the sub-Neptunian planets of the Kepler-223 system could have been established within a few 100,000 years, because each one of the four planets in turn migrated close enough to the others to get snared. The astronomers suspect there were special circumstances that enabled the resonance to persist for 6 billion years.

“These resonances are extremely fragile. If bodies were flying around and hitting each other, then they would have dislodged the planets from the resonance,” Dr. Fabrycky noted in the same press release. However, Kepler-223’s alien planets somehow managed to escape unharmed from this chaotic cosmic shooting gallery of crashing primordial worlds.

Other processes, including tidal forces that can flex unfortunate baby planets, may also result in the destruction of resonances.

“Many of the multi-planet systems may start out in a chain of resonances like this, fragile as it is, meaning that those chains usually break on long timescales similar to those inferred for the Solar System,” Dr. Fabricky added.

The Hunt For Alien Planets

The distance to even the closest star beyond our own Sun is vast. Our closest stellar neighbor in space is a triple star system known as Alpha Centauri, which is four light-years away–or 24 trillion miles! The distance separating stars in interstellar space is so vast that it is measured in light-years. Light travels in a vacuum at 186,000 miles per second. This means that one light-year is the equivalent of six trillion miles! There is no known signal in the Universe that can travel faster than light in a vacuum, and therefore light sets a universal speed limit.

Back in 1992, Dr. Alexander Wolszczan of Pennsylvania State University made history with his discovery of the first batch of alien planets in orbit around a star other than our Sun. After having made very careful observations of radio emissions emanating from a compact millisecond pulsar, with the drab telephone number name of PSR B1257+12, Dr. Wolszczan came to the realization that it was being circled by several bizarre beasts of the planetary zoo. PSR B1257+12 is a type of dense stellar relic –the lingering cinder of a “dead” massive star–situated about 1,300 light-years from Earth in the Virgo Constellation.

A pulsar is a small, dense object, a mere 12 miles in diameter. Pulsars are actually the burned-out, collapsed relic core of what was once a doomed massive main-sequence (hydrogen-burning) star on the Hertzsprung-Russell Diagram of stellar evolution. The doomed massive erstwhile star ended its life, after consuming its necessary supply of fuel, by blowing itself apart in the raging, flaming fury of a supernova blast. Pulsars are wildly whirling young neutron stars, fresh from the funeral pyre of a supernova’s final blaze of glory. These strange stellar remnants can contain an almost unimaginable 1,000,000 000 tons of stellar material that is squeezed by gravity into a Seattle-size volume. One teaspoon full of neutron star stuff can weigh as much as a family of elephants.

In 1995, two Swiss astronomers, Dr. Michel Mayor and Dr. Didier Queloz of the Geneva Obervatory made the historic announcement that they had detected the first exoplanet in orbit around a Sun-like main-sequence star. This great discovery left bewilderment in its wake. This is because the gigantic exoplanet, named 51 Pegasi b, or 51 Peg b, for short, clung to its parent-star–51 Pegasi–in a very close, searing-hot orbit. In fact, 51 Peg b, a gas-giant planet similar to our own Solar System’s Jupiter, broils in its orbit at a mere 4,300,000 miles from its star–circling its stellar-parent every 4.2 days. 51 Peg b was the very first hot Jupiter exoplanet to be discovered, but many more of its alien kind were spotted later in other observations. Before this first hot Jupiter was discovered, it was generally thought that such enormous gas-giant worlds could only exist much farther away from their stars.

In October 1995, two astronomers from the United States, Dr. R. Paul Butler and Dr. Geoffrey W. Marcy, confirmed the Swiss team’s historic discovery using the Lick Observatory’s three-meter telescope poised at the summit of Mount Hamilton in California.

Launched on March 7, 2009 from Cape Canaveral, Florida, the Kepler Space Telescope was designed to detect Earth-like alien planets belonging to stars beyond our Sun. As its name indicates, the quartet of distant exoplanets composing the Kepler-223 system was discovered by the Kepler Space Telescope.

In May 2016, it was announced that Kepler has verified 1,284 new exoplanets–the single largest discovery of planets to date.

“This announcement more than doubles the number of confirmed planets from Kepler. This gives us hope that somewhere out there around a star much like ours, we can eventually discover another Earth,” commented Dr. Ellen Stofan to the press on May 10, 2016. Dr. Stofan is chief scientist at NASA Headquarters in Washington.

Analysis was performed on Kepler’s July 2015 planet candidate catalog, which identified 4,302 potential planets. For 1,284 of the candidates, the probability of being a planet is greater than 99 percent–the minimum required to earn the status of “planet”. An additional 1,327 candidates are more likely than not to be actual planets, but they do not meet the 99 percent threshold and will need additional study. The remaining 707 are probably some other astrophysical phenomena. The analysis also validated 984 candidates previously verified by other techniques.

Dr. Paul Hertz, Astrophysics Division Director at NASA Headquarters, told the press on May 10, 2016 that “Before the Kepler Space Telescope launched, we did not know whether exoplanets were rare or common in the Galaxy. Thanks to Kepler and the research community, we now know there could be more planets than stars. This knowledge informs the future missions that are needed to take us ever-closer to finding out whether we are alone in the Universe.”

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