One defining scientific revolution of our generation is the discovery of thousands of exoplanets around other stars, which has transformed our view of the Solar System from being the only one in existence in a vast and immense Universe, to being just one between millions or even billions in our home galaxy alone. This plurality of worlds has forced scientists and non-scientists alike to ask the next big question: How many of them harbor planets that could sustain life? In the absence of hard evidence, this topic has been the subject of a multitude of theoretical studies throughout the years, with many of them often reaching a variety of different conclusions. A new research based on data from NASA’s Hubble Space Telescope offers a new insight into this fascinating subject, by presenting evidence that many of the extrasolar worlds that have been previously deemed as being potentially habitable, might actually not fit the bill.
In the search for alien worlds beyond our Solar System, no other observatory has had such a huge impact than that of NASA’s Kepler space telescope. Having already entered its sixth year of successful operations, Kepler forever changed the field of exoplanetary research with its ground-breaking discoveries of thousands of candidate and confirmed exoplanets to date. Extrapolating on this treasure trove of data, astronomers have reached the conclusion that our Milky Way galaxy alone most probably harbors hundreds of billions more, with a large fraction of them being potentially habitable. In turn, the concept of exoplanet habitability itself is intrinsically linked with the notion of the “habitable zone,” which defines the region around a star within which conditions would be just right for any existing exoplanets to exhibit life-friendly conditions, mainly mild temperatures that would allow for the presence of liquid water. In our own Solar System, the Earth is right in the middle of the Sun’s habitable zone, which has allowed for the emergence and evolution of life as we know it. But what about planets around other stars? Could we find alien worlds that share life-friendly conditions similar to Earth? A series of past AmericaSpace articles has explored such a possibility for a varying set of exoplanets, including terrestrial-type worlds around single and multiple red dwarf star systems, nearby terrestrial exoplanets more than 10 billion years-old, as well as hypothesised “super-habitable” worlds far from the habitable zone of their host stars.
One key characteristic of habitable zones is that they are not fixed in place, but they are very dynamic in nature, constantly being shaped by a host of other factors like stellar flux and stellar evolution, the mass and size of the exoplanets themselves, as well as their atmospheric conditions. For instance, the Sun is at present 30 percent brighter than what it was during the early history of the Solar System, causing the latter’s habitable zone to move outward with time. In a few billion years when the Sun’s luminosity will have increased further still, the inner region of the habitable zone will have moved beyond the current orbit of Mars, leaving our own planet uninhabitable.
But what about double and multiple star systems? Contrary to the Sun, which is a solitary star, the bulk of our galaxy’s stellar population resides in such systems. It was previously thought that the existence of planets around such systems was an unlikely scenario, because the multiple gravitational interactions of the closely packed stars in these systems were believed to be a great inhibitor to planetary formation. Yet, as the Kepler space telescope has shown in recent years, exoplanets are the norm not only for single stars like the Sun, but for multiple ones as well. Nearly half of the more than 4,000 exoplanet candidates detected by Kepler to date have been found inside such binary and multi-star systems, while various studies based on these findings by Kepler have argued that nearly half of all the galaxy’s total of binary and multi-star systems must indeed harbor exoplanets. If so, what are the chances of finding any potentially habitable worlds around those stars? In recent years, a growing list of exoplanets discovered inside such systems have been shown to lie inside their stars’ habitable zone, like Kepler-16b inside the Kepler-16 binary star system and the planets of the Gliese 667 triple star system—just to name a few.
A defining factor that can affect the prospects of exoplanet habitability inside multiple star systems is the nature of the planetary orbits themselves. If the stars in such systems form close pairs, they would create one common habitable zone for all orbiting planets, whereas if the companion stars have wide separations, each one would have its own habitable zone which in turn would be affected by the properties of their neighboring stellar companions, like mass, size, and luminosity. For these reasons, many scientists have argued that the prospects of habitability inside such systems is probably very low.
A new study, recently published at The Astrophysical Journal, comes to further reassess the potential habitability for a sample of exoplanets inside multiple star systems, by providing evidence that some of these alien worlds which were previously thought to harbor conditions that were favorable for life are in fact much more inhospitable. More specifically, the study’s research team, which was led by Kimberly Cartier, a graduate student at the Pennsylvania State University’s Department of Astronomy & Astrophysics, conducted a detailed photometric analysis and high-resolution imaging of a total of 22 Kepler exoplanet candidates with NASA’s Hubble Space Telescope, in order to better determine the properties of the exoplanet candidates and their respective host stars. The researchers focused on three of the most promising planetary systems in their sample: Kepler-296, KOI-2626, and KOI-3049. The latter two are multiple red dwarf systems comprised of three and two stellar companions respectively and harbor one exoplanet candidate each, while Kepler-296, which harbors a total of five exoplanets (two confirmed and three candidates), was originally thought to be a single red dwarf star.
The analysis by Cartier’s team, with the help of Hubble’s superior observing capabilities, showed that Kepler-296 is a close binary system of two red dwarf stars instead that were unresolved from the Kepler space telescope. With the help of previously gathered data from Kepler, as well as from the results of simulations and computational analysis, the researchers determined that due to the binary nature of the Kepler-296 system, the exoplanet Kepler-296 d (the third planet in the system), which was previously thought to lie inside the system’s habitable zone, was actually much farther in. In contrast, the system’s other two outer confirmed exoplanets (Kepler-296 e and f) were repositioned inside the habitable zone, due to the binary system’s recalculated stellar flux levels. “[Previous studies] had determined that Kepler-296 d was in the Habitable Zone of the assumed single star,” write the researchers in their study. “Using our stellar solutions for Kepler-296, Kepler-296 d is not habitable around either star, and in fact falls significantly interior to the Habitable Zone of either star. The outermost planet in the system (Kepler-296 f) now falls comfortably within the Habitable Zones of both the primary and the secondary stars. Kepler-296 e also falls just barely interior to the Habitable Zone of the secondary, but the uncertainty on the effective stellar flux at that planet makes it another likely habitable candidate.”
Another important factor for habitability, besides an exoplanet’s location in respect to its star’s habitable zone, is its size. Astronomers theorise that exoplanets with a radius up to 1.6 times that of our home planet have most probably retained a rocky composition, which renders them potentially habitable. If on the other hand they are bigger than this upper limit, they most probably belong in the mini-Neptune class of gaseous alien worlds. The main reason for classifying the exoplanets of Kepler-296 as potentially habitable in the first place was due to the fact that previous studies had estimated that their sizes were slightly larger than Earth’s. Yet the recent study by Cartier’s team, aided by Hubble’s high-resolution imaging, have readjusted upward the sizes of the Kepler-296 exoplanets, as well as those of the other two systems, KOI-2626 and KOI-3049. “Habitable planets in the canonical sense must not only have the capability for liquid water on the surface, but also have a solid surface on which that water can exist,” write the researchers. “In short, the planets must be rocky and not gaseous. Using radial velocity measurements coupled with Doppler spectroscopy, high-resolution imaging, and asteroseismology, [previous studies] measured the radii and masses for 65 planet candidates and concluded that only planets with radii less than 1.5 Earth radii are compatible with purely rocky compositions. Planets larger than that must have a larger fraction of low-density material, e.g. hydrogen, helium and water. Our updated planetary radii indicate that none of our potentially habitable planets (Kepler-296Af, Kepler-296Bf, Kepler-296Be, KOI-2626 A.01, KOI-2626 B.01, and KOI-2626 C.01) are small enough to have purely rocky compositions and thus are not habitable in the canonical sense.”
Even though these findings may seem disappointing, Cartier’s team acknowledges the fact that their results are the product of a limited data set and that future observations could help further reassess the habitability in the stellar systems examined, as well as in other similar ones in the galaxy. “We cannot exclude the possibility of a very massive yet rocky planet like Kepler-10c as we lack radial velocity measurements needed to calculate the planetary masses and densities directly,” conclude the researchers. “Even if Kepler-296Af, Kepler-296Bf, Kepler-296Be, KOI-2626 A.01, KOI-2626 B.01, and KOI-2626 C.01 remain too large to be rocky, the possibility of habitable exomoons would remain.”
Exoplanetary research is an ever-changing and dynamic field of study, one where past assumptions are replaced with new ones as more detailed data become available with time. It’s worthy to keep in mind that our understanding of exoplanetary formation and evolution advances with each new finding, as the last 20 years of exoplanetary discovery have demonstrated. Since the time of the first discoveries of giant gaseous hot-Jupiters, we have progressed to the point where we can detect and characterise terrestrial alien worlds, some of which have been found to be even smaller than Earth. In similar fashion, our continuing search of distant worlds around other suns will eventually allow us to make the long-awaited first discoveries of far-away, alien pale blue dots, somewhere inside the vast expanses of the Milky Way.