“Towards the end of the second day, we found ourselves so close to the pole that when the disk of the blue sun had almost completely disappeared beyond the horizon, the crimson tinge of the clouds in the opposite direction heralded the rising of the red sun. The vast blackness of the ocean and the empty sky above it were then filled with a blindingly fierce clash between hard colors aglow like metal, glistening with poisonous green and subdued hollow flames of crimson, while the ocean itself was rent with the glare of two counterposed disks, two furious fires, one mercuric and one scarlet;”
— Stanisław Lem, “Solaris” (1961)
We’re all familiar with the notion of multiple planets around a star, but what about the other way around? Descriptions of binary and multiple star systems that shine down on distant alien worlds have graced many science fiction stories for decades; yet, as the advent of exoplanetary research has shown, these imaginative setups are far from fictional. More than 100 extrasolar worlds, out of the thousands that have been discovered to date, have been found to revolve around binary, triple, or quadruple star systems. Now, a team of astronomers in the U.S. has recently added two more to the list, by raising the number of stars inside two previously detected exoplanetary systems.
Prior to the discovery of the first exoplanets more than 20 years ago, the general consensus between astronomers had been that the formation of planets around binary stars—let alone multiple star systems—constituted a highly improbable scenario. Conventional wisdom held that the combined gravitational interactions between the closely packed stars in such systems would prevent the tiny planetesimals inside any surrounding protoplanetary disks from coalescing into full-fledged planets. Even if planetary formation were to occur, established theoretical models predicted that this would result in planets with highly unstable orbits which would eventually cause a dynamic collapse of the whole planetary system in a short amount of time. For this reason, the first discoveries during the early 1990s of exoplanets around binary stars initially caught many astronomers by surprise. There were clearly many physical processes in the Cosmos that allowed for the formation of such systems, which theoretical models hadn’t accounted for. Since then, astronomers have been able to discover dozens more exoplanets around multiple star systems, indicating that the latter are indeed far from rare. Now, a new study from a team of U.S. astronomers, which was recently published in The Astronomical Journal, comes to give an interesting twist on this fascinating field of astrophysics, by adding more stellar companions inside two nearby star systems that were already known to host exoplanets.
The discoveries come courtesy of the Robo-AO, a fully automated laser adaptive optics system which is mounted on the Palomar Observatory’s historic 60-inch telescope, in California. With its unprecedented ability to conduct many dozens of high-contrast, high-angular resolution observations per hour, this innovative instrument can image hundreds of astronomical objects per observing session, providing astronomers with the ability to resolve and study very close pairs of binary stars in visible wavelengths with a projected angular separation as small as 0.12 arcseconds. One of Robo-AO’s primary science missions, which was partially funded by NASA in November 2014 under the agency’s Exoplanets Research Program, is the complete survey of the thousands of candidates and confirmed exoplanets that have been detected to date by NASA’s planet-hunting Kepler space telescope, with the goal of uncovering additional hidden stellar companions to these planets’ host stars that might have gone undetected and helping astronomers to characterise the overall abundance of planet-bearing multiple star systems in the galaxy.
To that end, the researchers, led by Dr. Lewis Roberts Jr., an astronomer at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., studied the nearby star systems 30 Arietis and HD 2638, which are located approximately 136 and 160 light-years away, in the direction of the constellations Aries and Cetus respectively. Both of these systems were already known to host exoplanets, which had been previously detected from independent radial velocity observations conducted by ground-based telescopes during the last decade. More specifically, the 30 Arietis system harbors a massive gas giant that has a estimated minimum mass approximately 10 times that of Jupiter and a highly inclined, tight orbit with a period of 335 days. This massive hot Jupiter revolves around 30 Arietis B, a relatively young F-type star slightly larger than the Sun, which in turn is gravitationally tied to 30 Arietis A, a distant pair of spectroscopically similar binary stars that are located some 1,670 Astronomical Units away from 30 Arietis B. Through a series of high-resolution observations with Robo-AO between 2012 and 2013, Roberts’ team was able to detect a fourth stellar companion, elevating 30 Arietis to a quadruple star system. The fourth star, named 30 Arietis C, was found to form a close pair with the planet-hosting 30 Arietis B, with a calculated distance between them of approximately 22.3 Astronomical Units, which is a bit larger than the Sun-Uranus distance in our Solar System. Follow-up observations with the PALM-3000 adaptive optics system, which is mounted on Palomar’s 200-inch Hale telescope, confirmed the 2+2 hierarchical nature of the 30 Arietis system, where both binaries orbit the system’s common center of mass. Its discovery also firmly established 30 Arietis as only the second quadruple star system with an orbiting exoplanet to be detected to date, following the discovery of PH1b (better known as Kepler-64b) by the Planet Hunters citizen science project in 2013, which was found to orbit the four-star system Kepler-64.
If we consider terrestrial sunsets as being a majestic sight, then the view from the exoplanet around 30 Arietis would be nothing short of spectacular, according to the researchers. The distant binary pair of 30 Arietis A would look like just one small star that would be visible even in the daylight, while the newly discovered 30 Arietis C would appear as a bright star next to 30 Arietis B which in turn would dominate the sky with its Sun-like appearance. “Star systems come in myriad forms,” comments Roberts. “There can be single stars, binary stars, triple stars, even quintuple star systems. It’s amazing the way Nature puts these things together.”
A similar celestial view would await anyone looking up from the gaseous surface of HD 2638 b, the infernal hot Jupiter that is locked in a tight, circular orbit around the Sun-like star HD 2638, completing one full revolution at just under four days from a distance of 0.044 AU away. HD 2638 itself was already a known distant binary pair of metal-rich, Sun-like stars with an estimated age of 3 billion years. By utilising the superior capabilities of the Robo-AO adaptive optics system, Roberts’ team was able to discover a third companion to the system, HD 2638 C, at a distance of just 28 AU from the primary star, or approximately the Sun-Pluto distance in our Solar System. Subsequent observations with the PALM-3000 allowed the researchers to better study the newly found companion star, confirming that their projected separation in the sky is a really close one.
The discovery of the new stellar companions in both the 30 Arietis and HD 2638 systems indicates that the former probably play a more active role in the formation and dynamicall evolution of planetary systems with hot Jupiters, through the mechanisms of planetary migration.
“It has long been thought that hot Jupiters formed beyond the snow line and migrated to their current locations,” write the researchers in their study. “There are a number of theories on what caused that migration. One suggested mechanism is that interaction between the protoplanetary disk and the planet causes the planet to migrate inwards. A second method is planet-planet scattering that puts exoplanets into eccentric orbits that are then tidally circularized. Another method is Kozai migration, where a stellar companion on an inclined orbit induces exoplanets into eccentric orbits. The primary star will then tidally circularize the planet … It is quite probable that all of these methods occur in nature, but we do not know which is the dominant mechanism.”
The results of the new study come on the heels of a similar one also conducted concurrently by the members of the Robo-AO science team of the star nearby star HD 8673, which lies 118 light-years away, in the direction of the constellation Andromeda. In similar fashion to the 30 Arietis and HD 2638 systems, the researchers detected an M-type red dwarf companion to the main star, which also hosts a supermassive hot Jupiter, further strengthening the case for a correlation between massive gas giants and multiple star systems. What’s more, a growing number of studies have indicated that since at least half the stars in the Milky Way are members of binary and multiple star systems, they should also host as many exoplanets as single stars.
“There have been a number of surveys to measure the duplicity rates among the exoplanet hosts,” writes Roberts’ team. “These surveys have revealed several interesting features. The frequencies of exoplanets among single stars and components of wide binaries (semi-major axis larger than 100 AU) are indistinguishable. Also, the population of exoplanets in wide binaries is essentially the same as the population around single stars, indicating that wider binaries have minimal impact on dynamics of the exoplanets. The population of exoplanets in binaries with semi-major axis smaller than 100 AU is statistically different than those orbiting single stars. There are fewer exoplanets in tight stellar binary systems, but the exoplanets tend to be more massive … HD 2638 BC and 30 Ari BC both have projected separations of less than 30 AU and add to the rather small number of systems with such low separations. The stellar companions have the potential to have been a key part of the dynamical evolution of the exoplanets … These systems strengthen the link between massive planets and binary stars. The astrometry of these binaries should continued to be monitored … With a long enough span of observations, an orbit can be computed, which will provide a more complete picture of the nature of the dynamical interaction between the binary companion and the exoplanet.”
To that end, Roberts’ team plans to continue observations with the Robo-AO system, as well as similar, even more advanced ones that are planned to go online in the near future. “At the moment, Robo-AO is the only instrument that can give us the necessary combination of resolution and efficiency,” says Dr. Christoph Baranec, an astronomer at the University of Hawaiʻs Mānoa Institute for Astronomy. “Once we discover something interesting with Robo-AO, we can follow-up with the ‘Formula 1’ systems, like PALM-3000 or the SCExAO system at the Subaru Telescope in Hawaii, to obtain the absolute sharpest images possible. Additionally, we’re planning to bring a new, more powerful Robo-AO system to the University of Hawaii 2.2-m telescope to leverage the pristine skies of Mauna Kea, Hawaii. We’ll use it for even larger surveys and follow-up observations of asteroids and supernovae discovered by ATLAS (Asteroid Terrestrial-impact Last Alert System), on Mauna Loa and Haleakala.”
Besides the discoveries regarding the 30 Arietis, HD 2638, and HD 8673 systems, Robo-AO has also been instrumental in the characterisation of the recently discovered ancient exoplanetary system Kepler-444, as well as in the confirmation of the exoplanets that have been detected by the Kepler space telescope’s extended mission, named K2, proving its worth in the field of exoplanetary research. Its continuing operation will undoubtedly help astronomers to uncover even more stellar companions in exoplanetary systems that up till now have remained elusive, and ultimately shed more light on the processes behind their formation and overall evolution. “About four percent of solar-type stars are in quadruple systems, which is up from previous estimates because observational techniques are steadily improving,” comments Dr. Andrei Tokovinin, an astronomer at the Cerro Tololo Inter-American Observatory in Chile and member of Roberts’ team.
Perhaps even more importantly, these studies will help to once more showcase that the Cosmos is full of unmatched beauty, structure, and order, to which even our most imaginative speculations pale in comparison.