|Dr. Suman Satyal|
More than 3800 exoplanets (planets that are outside our Solar System) have been discovered to date. This number is low compared to the known ~300 billion stars in our home galaxy – Milky Way, where each star has a probability of hosting one or more planets. Nevertheless, given the fact that the first exoplanet, 51 Pegasi b, was discovered only 23 years ago and the majority of the exoplanets were discovered in the past 10 years, this exponentially growing discovery rate is a great success. So, what triggered this discovery rate? Well, part of this is due to the advancement in the detection technology. The state-of-the-art ground-based and space-based telescopes have gotten bigger and better than ever. The humungous success of Kepler Space Telescope, which is now credited for the discovery of more than 2,600 exoplanets before being put to sleep after 9 years in service, has motivated the community for even more systematic searches. Recently, another space telescope called TESS (Transiting Exoplanet Survey Satellite) was launched to search for exoplanets orbiting 200,000 brightest stars in the sky and has already discovered one planet and several other planetary candidates. In addition, here on the Earth, Mauna Kea in Hawaii and the La Silla observatories house some of the best ground-based telescopes.
Two major detection techniques are widely used in the search of exoplanets – Transit and Radial Velocity. In the transit method, a small dip in the stellar flux is measured when an orbiting planet eclipses that star. Therefore, every time a planet orbits around a star, that is once every year, a periodic dip is seen in the light curves, which are then fitted to extract other planetary orbital parameters to confirm its existence. A sample light curve is shown in Fig. 1.
A star-planet system always orbits about their barycenter, the center of mass of one or more bodies. Depending on the size of the planet, the barycenter can be within the stellar diameter or can be well outside of it. In either case, the star orbiting around the barycenter, as seen from the Earth, causes the starlight to get blue-shifted and red-shifted. Blueshift occurs when the star orbits towards the Earth and the redshift occur when it orbits away. By measuring these shifts, which produces a sinusoidal curve, planetary parameters such as its orbital period, the distance from the star and its minimum mass (m.sini) can be calculated. This is called the Radial Velocity method and the curves derived from this method – RV curve. Fig. 2 shows the RV curve of the first exoplanet, 51 Pegasi b. The nomenclature of the exoplanets follows a simple rule: Star name followed by lower case alphabets b, c, d, etc. Hence, 51 Pegasi is the star name and b means the first discovered planet in this system. The next planet (if any) will be named 51 Pegasi c. The not-yet-named stars get their names from the parent telescopes.
Bigger planets that are closer to their host star are easier to detect because they create large and prominent depth in the light curves as well as a large amplitude oscillations in the RV curves. In addition, it is favorable if the inclination angle (i) of the line of sight (as seen from the Earth) is at 90 degrees with the planet-star rotational plane. If this angle and the planetary mass gets smaller, it becomes harder to get the desired signal for the given telescope sensitivity. This seems to be the primary challenge for the recently discovered exoplanet orbiting a nearby star called Barnard’s Star, even after collecting the data for 20 years. Multiple researchers from various international institutes were involved in this search and their finding was published in the Nature Letters [I. Ribas et al. Nature 563, 365-368 (2018)].
Barnard’s star is the second closest star from the Solar System (5.9 light years) following the triple star system of Alpha Centauri (4.3 light years) and lies in the constellation of Ophiuchus. It is an M-type red dwarf star with a mass of 0.163 Solar-mass, a radius of 0.178 Solar-radii and effective temperature 3278 degrees, Kelvin. It is much smaller and less luminescent than the Sun, but much more active due to its unpredictable flare activities. The reported planet candidate in the system, Barnard’s star b (also called GJ 699 b) is a super-earth with a minimum mass of 3.2 Earth-mass, orbital period of 233 days, and orbits at an average distance of 0.404 au from its star. One of the caveats of the RV method is that it only provides the minimum mass (m.sini). Therefore, without knowing the inclination angle (i) discussed earlier, the actual planetary mass cannot be calculated. This angle can be computed by using the transit method or the direct imaging, but neither was applied for this system (probably due to the underlying technical challenges). This means, the actual mass of the Barnard’s Star b can be much bigger than the reported 3.2 Earth-mass. As discussed earlier, the RV method favors large planets; however, the researchers for the first time have managed to use this technique to find such a small planet. This is another breakthrough in the detection techniques in recent years. The RV curve for the planet was obtained using eight different telescopes, which are listed in Fig. 3 along with their respective data points.
So, does the discovery of Barnard’s star b has some significance to our quest of finding life outside the Solar System, or is it just another statistics in the exoplanet family? It is probably the latter. We are closer than ever to find a planet that is similar to the Earth in terms of its size, distance from its host and the host itself being similar the Sun-like star. A list of such potentially habitable exoplanets can be found in www.phl.upr.edu. The distance of the exoplanets in the list extends from 4 LY to as far as 1200 LY. The near Earth-like exoplanet would be of more value to us because the interstellar travel (in a long shot) would be easier and more practical. Barnard’s star is a better candidate-system for this purpose as this is the second closest planet from the Solar System, which is located only 5.9 LY away. This distance may sound long enough for the travel technology we have, but with respect to the size of the Milky Way galaxy, which has the diameter of 100,000 LY, the planet is our second neighbor (after Proxima Centauri b) at the arm’s length.
On the other hand, this planet may not be habitable by the Earth’s standard. The criteria for the habitability and the habitable zone is another big topic (for another day), and highly debatable, in the exoplanetary science. Based on the Solar System, a general definition of a habitable zone has been proposed – a region around a star where water can exist in the liquid state. Then, if a planet has a size similar to that of the Earth (hence the right amount of gravity) and orbits within the stellar habitable zone, it is likely to host life, the life as we know on the Earth. For solar-type stars, this distance is about 1 au (the Earth-Sun distance). Barndard’s Star b lies well outside of the habitable zone in the snow line, with a temperature close to -170 degree centigrade and if any water does exist, it should be in the frozen state. Hence, making it a less probable candidate to host life. The other challenge for the life in this planet is the star itself, which is highly active and engulfs the planet with high-energy radiation. This active nature of red dwarf (or M dwarf) stars, in general, creates a challenging and hostile environment for any planets to host life. The insolation from these stars are so low compared to the Sun that the habitable zone lies extremely close to their surface. Thus, any planet in the habitable zone is likely to be tidally locked which results in the extreme variation of temperature in the planetary surface not favoring the Earth-like life.
It is a very exciting time for those who are interested in astronomy and astrophysics or exoplanets in particular. From not knowing what is beyond our horizon to traveling across the Solar System and looking through the galaxy into the universe and be able to comprehend its vastness is the greatest achievement of the humankind. We may not experience the interstellar travel in our lifetime, but we will definitely have set a foundation stone for it. In the next decade or so, we will have found thousands of exoplanets, some within the stellar habitable zone. We shall also have made a major breakthrough in space technologies and constructing spaceships. Then, we will enter a space era.
In my personal opinion, we are definitely not alone in the Universe. I believe that life does exist out there. It may take one year, or it may take hundreds of years to find the extra-terrestrial beings. On the other hand, we may not even find it at all due to our limitations in the technology. The life on earth may come to extinction (the asteroid strikes, the climate change, or some other epidemics) years before we could probe deeper into the other habitable worlds. The other possibility is that the terrestrial beings may find us even before we can make it to the Galilean moons. In either case, I believe that life does exist in the Universe. We cannot find it does not mean it is not out there.
May the force be with us.
(Dr. Suman Satyal is an exoplanetary scientist at the Department of Physics, University of Texas at Arlington, Arlington, TX, 76019, USA and a member of the scientific advisory committee at Nepal Astronomical Society-NASO).