“Second star to the right, and straight on ’till morning.”
— Captain James T. Kirk, ‘Star Trek VI: The Undiscovered Country’ (1991).
It is unclear which types of planets did the fictional Enterprise crew discover around the star that their captain was referring to. But in real life, a new research being published by European astronomers, using data from NASA’s Kepler space telescope, points toward a star that is home to a planetary system very similar in architecture to our own.
The Kepler telescope was constantly aimed for years at a certain region of the sky containing 150,000 stars. During that time, it made huge contributions to the field of exoplanet research, discovering 167 confirmed planets to date and another 3,538 candidates that await confirmation. Many of the planets being discovered from the space telescope or other Earthbound observatories were found to be members of multi-planetary systems. Yet all of these alien solar systems turned out to be vastly different in structure to our own: from systems replete with gas giant planets boiling away by their very close proximity to their host star to systems harboring planets with long elongated orbits. More and more, our Solar System was beginning to look like a strange peculiarity, a rare example in the galaxy. And astronomers were trying to figure out how these many different systems came to be, running counter to the predictions of planetary formation theories. But it seems that the Universe may have decided to put some of our troubles at ease, presenting us with a solar system that is more compatible with our pre-conceived notions of how things should be.
A team consisting of astronomers at the German Aerospace Center (DLR) collaborating with European colleagues published a study presenting the discovery of new planets and confirming previously observed candidates around the star KOI-351 (where KOI stands for “Kepler Object of Interest”). The star (also known as Kepler-90 and KIC 11442793) is a Sun-like dwarf, approximately 2,500 light-years away in the constellation Draco. KOI-351 had been observed before by Kepler and has also been an object of study by amateur astronomers using the citizen science project’s Planet Hunters website. Three candidate planets had already been observed in 2012 by professional astronomers working with the Kepler mission, and another one was reported this year by the general public using the Planet Hunters website. The recent announcement by the European team confirms the presence of the previously observed candidates and adds three more planets to the list, making KOI-351 the first ever seven-planet system to be discovered by the Kepler telescope, using the transit detection method.
Extrasolar multiplanetary systems are nothing new. But what’s significant about KOI-351 is that its structure resembles very much that of our own Solar System. The first five planets are a mix of Earth-sized and Super Earth-sized worlds, with the outer two falling into the Jupiter-size category. “We cannot stress just how important this discovery is. It is a big step in the search for a ‘twin’ to the Solar System, and thus also in finding a second Earth,” says Juan Cabrera, an astrophycisist at the DLR and lead author of the new study.
The first two planets, b and c, have a size of 1.3 and 1.2 times that of Earth, with orbital periods of seven and nine days around their star, respectively. These two Earth-sized planets are thought to be rocky in composition, just like the terrestrial planets in our Solar System. Moving outward from the star, the planets d and e are Super Earths, with sizes 2.7 and 2.9 times that of our planet, respectively. Planet d completes an orbit around its star every 60 days, which is comparable to the orbital period of Mercury. Planet e completes one orbit every 91 days. The following planet f is another Super Earth-type planet, with a size 2.9 times bigger than Earth, revolving once every 124 days. The KOI-351 planetary list concludes with planets g and h, which are huge, Jupiter-size planets. Planet g, being 8 times bigger than Earth, has the same size as Saturn, and planet h, being 11 bigger than Earth, is similar to Jupiter. It’s interesting to note that planet g completes one orbit every 210 days, which is somewhat similar to Venus’, and planet h once every 331 days, having an orbit similar to Earth’s.
Although the planetary characteristics unveiled in the study are intriguing, there’s little more that can be said about the planets’ properties. Because of the transit detection method used, which detects the dimming of light when a planet happens to cross the face of its star, only the approximate sizes of the planets could be inferred. Learning about the planets’ compositions requires a reading of their masses, something that the transit method can’t provide. Other methods, like radial velocity measurements, are needed, which the research team didn’t have available. Yet, even by their size determination, astronomers can be fairly confident that the small inner planets are terrestrial in nature, and the outer giants are gaseous like Jupiter and Saturn.
Another very intriguing fact about the KOI-351 system is its compact nature and dynamic orbital resonances. Indeed, all seven planets orbit their star in tightly compact orbits, with only the outer one, planet h, having a distance of 1 Astronomical Unit from its star. (1 A.U. is the mean distance between the Earth and the Sun). With such compact orbits, one would think that the whole planetary system would be inherently unstable, leading to an ultimate collapse. Yet the answer to the system’s stability comes in the form of orbital resonances. “The resonances of the planetary orbits are another interesting feature of this system,” says Szilárd Csizmadia, a member of the research team. “Resonances also play an important role in the Solar System; for example, the moons of Jupiter. So KOI-351 is a gold mine for all researchers investigating planetary formation and the stability of multi-body systems.”
The three inner moons of Jupiter—Io, Europa, and Ganymede—are locked in a 4:2:1 orbital resonance, also known as a “Laplace resonance.” That means that for every full orbit Ganymede makes around Jupiter, Europa makes two and Io makes four. These dynamic interactions between the moons are caused by the moons’ mutual gravities, helping their orbits to remain stable. In a similar fashion, in the KOI-351 system the two innermost planets, b and c, are caught in a 5:4 resonance. The planets d, e, and f show signs of being caught in a Laplace resonance similar to the three Galilean moons, and the outermost planets, g and h, show signs of a 5:8 resonance.
The compact nature of these planets means that most of them orbit on the inside of their star’s habitable zone (the range of orbits where liquid water could be sustained on a planet’s surface). The only one lying inside the habitable zone is the outermost planet h, which circles its star every 331 days. Being a Jupiter-sized, more likely gaseous world, planet h could harbor moons that could have conditions suitable for life. Although out of the reach of current observing techniques, exo-moons around other exoplanets could be discovered by the next generation of space-based planet-hunting telescopes being proposed or constructed at the moment.
The field of exoplanet discovery is one of the most productive in astronomy. The worlds around other stars being discovered, constantly beckon us to explore them, unravel their mysteries, and try to understand our relevance and place within this Universe we share with them—and some time in the future, possibly attempt to visit them.
As captain Jean-Luc Picard would say: “Make it so!”