The discovery of yet another exoplanet orbiting outside of the habitable zone of its star hardly seems newsworthy these days, given the routine nature of new exoplanet findings. Having already discovered thousands of alien worlds within the Milky Way galaxy showcasing a huge diversity in mass, size, orbital characteristics, and possible habitability, astronomers are now focused on finding a true “Earth analog”: a terrestrial planet in an Earth-type orbit, around an Earth-like star. Working toward that goal, four collaborating international teams of astronomers have recently announced the discovery of a seemingly inconspicuous cold terrestrial exoplanet around a binary red dwarf star system. What makes this discovery significant, however, is that this newly found alien world is the first to have an orbit of approximately 1 Astronomical Unit, or AU, from its host star, which is the same distance between the Earth and Sun, indicating that such Earth-type orbits might indeed be common in other exoplanetary systems as well.
Even though red dwarf stars are the most abundant stellar population in the Milky Way galaxy, possibly comprising up to 80 percent of the galaxy’s total number of stars, they had systematically been ignored for years by astronomers in their search for exoplanets. With masses and sizes ranging from approximately 1/10th to 1/3rd that of the Sun and a metallicity (abundance in chemical elements heavier than helium) substantially lower than that of Sun-like stars, red dwarfs were seen as very poor candidates for forming and maintaining any exoplanetary systems. In addition, the lower luminosity of red dwarf stars made them very difficult targets for study. Yet the advent of observational exoplanetary research during the last two decades with a new generation of advanced ground- and space-based instruments of high-sensitivity has revealed a multitude of planets and planetary systems around red dwarf stars, leading scientists to calculate that there might be billions of exoplanets in the Milky Way, many of which potentially habitable, orbiting these previously neglected, inconspicuous stars.
The presence of planets around multiple star systems was also considered an unlikely scenario. Previous theoretical models of planetary formation had suggested that the combined gravitational interactions of these closely bound together stars would either disrupt the formation of any protoplanetary disks around them or would lead to planets with dynamically unstable orbits. For this reason, and despite the fact that most stars in the galaxy are part of binary or multiple star systems, the discovery of many dozens of planets around such stars, like Kepler-16b, Kepler-47b and c, Gliese 667C, and Alpha Centauri Bb, has come as a surprise to many astronomers. “Binary systems were largely ignored before,” says Dr. David Trilling, Assistant Professor of Astronomy at the Northern Arizona University’s Department of Physics and Astronomy, who led a research team in 2006 which discovered stable protoplanetary disks around dozens of binary stars, using NASA’s Spitzer Space Telescope. “They are more difficult to study, but they might be the most common sites for planet formation in our galaxy. There appears to be no bias against having planetary system formation in binary systems. There could be countless planets out there with two or more suns.”
The newest addition to the list of exoplanets around binary stars comes from a new study partially funded by NASA and recently published in the July 4 issue of the journal Science by four international research teams led by Dr. Andrew Gould, Distinguished Professor of Mathematical and Physical Sciences at the Ohio State University’s Department of Astronomy. As part of the Optical Gravitational Lensing Experiment, or OGLE, a Polish astronomy research project which utilizes the 1.3-m Warsaw University Telescope located at the Las Campanas Observatory in Chile for the study of stellar microlensing events, the astronomers discovered a terrestrial exoplanet in April 2013 around a red dwarf star located approximately 3,000 light-years away, using the gravitational microlensing method. Contrary to the radial velocity and transit methods which are more widely used for exoplanet detection, the gravitational microlensing method can only be used in the case when the light from a distant, background star is magnified by the gravitational field of a closer, foreground star that happens to pass in front, as seen by our line of sight here on Earth. In the case of two stars without planets, the background star’s brightness will increase as the foreground star passes in front of it and then decrease as the latter moves away, in a predictable way during a period of days or weeks, producing a well-defined light curve. If the foreground star happens to have any planets orbiting it, these will distort and dim the light from the background star in a noticeable way as well, which will help astronomers measure some of their basic properties, like their mass and orbital period. Since both stars must be exactly aligned for this method to work, such exoplanet microlensing events are extremely rare and are more readily observed during wider microlensing astrophysical surveys which monitor hundreds of millions of stars, like the ongoing OGLE-IV survey at the Las Campanas Observatory.
The newly discovered exoplanet, named OGLE-2013-BLG-0341LBb, was found during such a microlensing event, when its host star, a red dwarf, passed in front of a more distant star, located approximately 20,000 light-years away in the constellation Sagittarius. During this event, the astronomers observed a small “dip” in the brightness of the background star lasting one day, as the foreground star crossed in front of it. By observing this small dip in the star’s brightness, the astronomers realised that the object causing it was a planet heading toward the direction of its host star as it moved in its orbit. “Before the dip, this was just another microlensing event,” says Dr. Andrew Gould, lead author of the study. “It’s really the new OGLE-IV survey that made this discovery possible. They got a half-dozen measurements of that dip and really nailed it.” As the microlensing event progressed in the weeks that followed, the combined gravity of the star-planet system magnified the light of the background star, while astronomers were observing the event from multiple observatories around the world, in New Zealand, Australia, Israel, and Chile. Yet they were met with an unexpected surprise: The foreground star caused a sudden eruption in the background star’s light (which in astronomical jargon is called a “caustic crossing”), a phenomenon that signaled the presence of an additional stellar object in the foreground star’s vicinity. With all the data gathered by the multiple observatories studying this caustic crossing microlensing event, the astronomers were able to reveal the true nature of the foreground red dwarf as being a binary member of a two-star system of red dwarfs bound together by their mutual gravity, with the distance between them being approximately 10 to 15 AU—greater than the orbit of Saturn in our Solar System.
More interestingly, the light curve of this caustic crossing was also found to be distorted in a way that perfectly matched the “dip” in brightness that revealed OGLE-2013-BLG-0341LBb’s presence a few weeks earlier, thus allowing astronomers to make detailed measurements of its mass and orbit. “Even if we hadn’t seen the initial signature of the planet, we could still have detected it from the distortion alone,” says Gould. “The effect is not obvious. You can’t see it by eye, but the signal is unmistakable in the computer modeling.” These observations have shown that the newly discovered exoplanet has a mass no more than twice that of Earth and an orbit of approximately 90 million miles, or 0.9 AU, from its star—just shy of the Earth’s distance of 1 AU from the Sun. Even though this puts OGLE-2013-BLG-0341LBb’s well outside of its host star’s habitable zone, rendering it a frozen world with temperatures that could be as low as -352 degrees Fahrenheit (-213 Celsius), it nevertheless indicates that Earth-type terrestrial exoplanets in such Earth-type orbits might, according to astronomers, be quite common in the galaxy.
But how common? In their attempt to answer this question, Dr. Gould’s research team pondered another one: If every star that would undergo any detectable microlensing events were members of similar binary star systems that hosted terrestrial exoplanets like OGLE-2013-BLG-0341LBb, how many of them could be detected? “If all stars with magnitudes lower than 18.5 that undergo microlensing events had such planets, we would detect approximately 0.04% of them,” concludes the research team in their study. “During the three years that OGLE-IV has issued [microlensing] alerts, it detected [approximately a thousand such] events in its high-cadence fields, which implies an expectation of 0.4 such planets. This would be compatible with survey results showing that Earths and super-Earths are the most common type of planet orbiting stars with a wide range of masses and with predictions from microlensing based on more massive planets orbiting low-mass stars. Second, this result shows that terrestrial planets can exist relatively far (approximately 1AU) from their hosts, even if the latter have relatively nearby binary companions [approximately 20 AU away], thus providing empirical test of models of terrestrial planet formation in such close binaries. Third, when combined with the radial velocity detection of a terrestrial planet orbiting very close (0.04 AU) to Alpha Centauri Bb, which is a solar-type star, it shows that terrestrial planets can form in binaries with diverse properties in terms of host mass and planet-host separation. Although OGLE-2013-BLG-0341LBb was discovered in a search of approximately a thousand microlensing events and Alpha Centauri Bb resulted from intensive observations of a single system, the expected yield in each case (if all stars had similar planets) was roughly unity.”
Furthermore, the discovery of OGLE-2013-BLG-0341LBb showcases the value of the gravitational microlensing method for exoplanet detection. Contrary to the radial velocity method, which is best suited for discovering exoplanets in very close orbits around their host stars, gravitational microlensing can reveal the presence of planets in much wider Earth-type orbits. “This greatly expands the potential locations to discover habitable planets in the future,” says Dr. Scott Gaudi, Associate Professor at the Ohio State University’s Department of Astronomy and co-author of the study. “Half the stars in the galaxy are in binary systems. Now we know that with gravitational microlensing, it’s actually possible to infer the existence of a planet—and to know its mass, and its distance from a star—without directly detecting the dimming due to the planet. We thought we could do that in principle, but now that we have empirical evidence, we can use this method to find planets in the future.”
On our path toward finding a true “Earth-analog” alien world somewhere in the vast expanses of the Milky Way galaxy, the discovery of OGLE-2013-BLG-0341LBb is an important step forward, showing us that terrestrial planets in orbits of approximately 1 AU from their host stars isn’t something exclusive to our Solar System only. Even though the newly discovered OGLE-2013-BLG-0341LBb orbits well outside of its star’s habitable zone, such a planet around a Sun-like star would be right in the middle of it. And that gives us more confidence that such “Earth twins” are indeed out there. And that finding them can only be a matter of time.