The closest stars to the Earth are also some of the best studied celestial objects. In Astrophysics, distance makes all the difference. The closer an object is, the more light we will be able to glean from it. Stars come in different types (masses, chemical compositions, ages), but the closest example of each type will be a critical object to study. In this article we review five fast facts about the closest systems to the Earth and what they mean for astrophysical studies.
Jackie Faherty, astrophysicist, Senior Scientist with the American Museum of Natural History and Editor at dailygalaxy.com. Jackie was formerly a NASA Hubble Fellow at the Carnegie Institution for Science. Aside from a love of scientific research, she is a passionate educator and can often be found giving public lectures in the Hayden Planetarium. With over 100 peer-reviewed articles in astrophysical journals, Faherty has been an invited speaker at universities and conferences across the globe. Jackie received the 2020 Vera Rubin Early Career Prize from the American Astronomical Society, an award that recognizes scientists who have made an impact in the field of dynamical astronomy, and the 2021 Robert H Goddard Award for science accomplishments.
The closest star system to the Sun is called the Alpha Centauri system. It’s composed of three stars and at least two exoplanets (possibly more). The name of the system is derived from the constellation it resides in — the “Centaurus”. This close-by triple can be found in the southern hemisphere’s sky. Of the three stars, only two of them could be seen by the naked eye. The closest of the three, named “Proxima Centauri” is a very low mass star and has a brightness just under the ability of the human eye to perceive it. That star is at just over 4.2 light years away from the solar system. That means that even if you could travel at the speed of light — the fastest thing we know of in the Universe — it would still take you just over 4 years to arrive at the star system. The other two stars are more like the Sun and they are tightly orbiting each other (Proxima is a more distant companion). There have been countless studies of these stars in search of planets they might harbor. For a time, it was thought that Alpha Centauri hosted at least one potentially rocky world around it. The techniques required to confirm such a world operate at the cutting edge of what is possible in Astrophysics and the planet eventually was deemed a systematic issue with the data. In 2016 the star Proxima Centauri was found to have at least one potentially rocky world around it. In 2019 a second exoplanet candidate was also declared. Astronomers are now designing future instrument and telescope ideas with the concept in mind that we would like to image these newly found worlds to characterize the conditions which might give hints at how life might thrive nearby.
If you look at a list of the 100 closest stars to the Sun, you will see a pattern. Astronomers have developed a system for characterizing stars by taking their light, breaking it up with a prism, and then examining the fingerprints of elements that imprint themselves into the starlight. The spectral classification scheme is labeled with letters with the highest mass stars called O and B stars, and the lowest mass stars called M dwarfs. Our Sun, a G star, is a middle of the range type of star. If you were to look at the stellar class for the 100 closest stars, you would see the letter “M” repeated over and over again. That is because the M-type stars dominate the Galaxy by number. They are the most common type of star and it should come as no surprise that they also dominate our neighborhood. In fact the very closest star to the Sun, Proxima-Centauri, is an M-type star and the most common of the M’s (an M5 type star). Famous M dwarfs near the Sun include Barnard’s star (the second closest system to the Sun), Wolf 359 which was famously used in an episode of Star Trek the Next Generation, and Teegarden’s Star as well as Ross 128 which both host potentially rocky worlds that Astronomers are studying in detail.
The closest stars to the Sun are not a static list of objects. We are not moving through the Galaxy with a family of objects all keeping the same distance from each other. Given that the entirety of the Galaxy is in motion, stars are constantly moving closer and further from each other. If we take the motions of stars and run them forward and backward in time, we find that our solar neighborhood has looked very different in the past and will look very different in the future. For instance, 70,000 years ago a low mass star called Scholtz’s star (which happens to be orbited by a brown dwarf) flew through the Oort cloud of our solar system. In just over one million years, a star called Gliese 710 will make a close encounter and reach as close as ~16,000 AU of our Sun. That’s going to be CLOSE! If we are all still around and active in Astrophysics at that time, then we might have an up close chance to study the worlds that the star Gliese 710 has around it (we have to find them first).
One of the most intriguing aspects of our solar neighborhood is that we are still uncovering new objects within it. You might expect that it’s pretty hard to hide a star within a few lightyears of the Sun given how advanced our Astrophysical technology has become. And you’d be right that it’s hard to hide a star. But it isn’t hard to hide a brown dwarf. In 2012 and then again in 2013, the list of the top five closest systems to the Sun was turned on its head as two new brown dwarf systems were discovered. In 2012, two brown dwarfs closely orbiting each other at just over 6.5 light years were found using NASA’s Wide Field Infrared Survey Explorer (WISE). They are the third closest system to the Sun. Then in 2013 the coldest compact source ever found outside our solar system was discovered — nicknamed WISE 0855– at just over 7.2 light years making it the fourth closest system to the Sun. Searches are still ongoing for Backyard Worlds and there is a citizen science project (backyardworlds.org) that you can participate in which invites anyone and everyone to the search party.
Image credit: telescopic view of Proxima Centauri star via Shutterstock License