In the vastness of interstellar space, stars are separated by great distances. Indeed, the closest star to our own Sun is really a triple stellar system named Alpha Centauri, and it is about 24 trillion miles from Earth–or 4 light years. This means that Alpha Centauri is over 266,000 times more distant from us than our Sun, and because no known signal in the Universe can travel faster than light in a vacuum, it sets something of a universal speed limit. Light takes a mere eight minutes to travel to Earth from our own Star, but it requires four years to get to Alpha Centauri. A spacecraft sent from our planet to this distant stellar system would need 40,000 years to finally arrive at its destination. In December 2017, a team of astronomers from Yale University in New Haven, Connecticut, announced that they have taken a fresh look at the Alpha Centauri star system, and have found new ways to narrow the hunt for habitable planets there. Because of this, they hope to find an answer to one of the most profound mysteries of our own existence on Earth: Are we alone?

According to the new study, led by Yale’s Dr. Debra Fischer and graduate student Lily Zhao, there may be small, as-yet-undiscovered. Earth-like planets hiding in Alpha Centauri, that have been overlooked. In the meantime, the study ruled out the existence of a number of larger planets lurking in the system that had emerged in earlier models.

“The Universe has told us that the most common types of planets are small planets, and our study shows these are exactly the ones that are most likely to be orbiting Alpha Centauri A and B,” Dr. Fischer commented in a December 18, 2017 Yale University Press Release. Dr. Fischer is a leading expert on exoplanets, who has devoted decades of research to the important quest of finding the Holy Grail of an Earth analog in the distant family of a star beyond our own Sun.

A duo of brilliant sister stars, Alpha Centauri A and B, are sibling members of a binary star system. Both stellar sisters orbit each other at about the same distance that Uranus circles our Sun in our Solar System’s outer kingdom. A third stellar sibling, named Proxima Centauri, is much fainter than its two sparkling sister stars, and is loosely bound gravitationally to them.

Alpha Centauri B wanders around our Sun in an elliptical (football-shaped) orbit, that takes it 11 astronomical units (AU) from Alpha Centauri A every 80 years. One AU is equal to the mean distance between Earth and Sun, which is about 93,000,000 miles. Little Proxima Centauri, which is a faint red dwarf, is about 15,000 AU, or almost one-fourth of a light-year away, from its two more brilliant sisters. Red dwarf stars are the smallest true stars in the Universe, the most abundant, and the longest-lived.

The Exoplanet “Holy Grail”

The Holy Grail of planet-hunting astronomers is centered on their quest to find a small, rocky world, with a sloshing water ocean, circling a distant star that is like our Sun. Where there is liquid water, there is the possibility–though by no means the promise–of the existence of life as we know it. Therefore, planet-hunting astronomers search, with great dedication, for lovely little blue planets in orbit within the habitable zones of their parent-sunlike stars. The habitable zone of a star is that comfortable “Goldilocks” region that is not too hot, not too cold, but just right for water to exist in its life-sustaining liquid phase.

Alpha Centauri A and B appear to the unaided eye as a single speck of light, thus forming the brightest star in the southern constellation of Centaurus, as well as the third brightest star in the entire sky, outshone only by Sirius and Canopus.

Alpha Centauri A sports 1.1 times the mass and 1.519 the luminosity of our Sun, while Alpha Centauri B is both cooler and smaller, at about 0.907 times solar mass, with about 0.445 times our Sun’s visual luminosity. During the duo’s 79-91-year orbit around their common center, the distance between them varies from nearly the distance between Pluto and our Sun (35.6 AU) to the separation between Saturn and our Sun, which is significantly closer at 11.2 AU.

Smaller, fainter Proxima Centauri is situated somewhat closer to us. Indeed, this little red dwarf is only about 4.24 light-years from our Star, even though it is not visible to the unaided human eye. The separation of Proxima from its two larger sibling stars–Alpha Centauri A and B–is approximately 13,000 AU. This distance is equivalent to about 480 times the size of Neptune’s orbit around our Sun. The ice-giant planet Neptune is the outermost planet from our Sun. Proxima Centauri b, an Earth-sized exoplanet, was discovered in 2016.

The Alpha Centauri system is located 24.9 trillion miles from Earth–which makes it very close to us by star standards.

“Because Alpha Centauri is so close, it is our first stop outside our Solar System. There’s almost certain to be small rocky planets around Alpha Centauri A and B,” Dr. Fischer commented in the December 18, 2017 Yale University Press Release.

Planet-hunting astronomers have been discovering exoplanets since 1995. Some of these distant alien worlds bear an eerie resemblance to the planets within our own Sun’s family, while others are unlike anything astronomers once thought could exist in nature. But, despite the obvious desire of discovering exotic distant worlds beyond our own Star, a familiar planet that looks like our own still remains the coveted first prize.

Studies have revealed that multiple planet systems–similar to our own Solar System–are not uncommon in the cosmic scheme of things. Where one planet hides, there are likely to be other planets hiding as well. Astronomers have been detecting ever smaller and smaller exoplanets since the historic discovery over a generation ago of a giant alien world in orbit around a sunlike star. That very first discovery of a bizarre planet, 51 Pegasi b (51 Peg b), heralded the existence of an unforeseen group of brave new worlds dubbed Hot Jupiters. Hot Jupiters are enormous exoplanets that orbit their parent-stars, fast and close, in roasting, searing-hot orbits. 51 Peg b swiftly circles its stellar parent, 51 Pegasi, at a broiling distance–but until this huge planet was discovered, astronomers did not even suspect that such weird beasts, inhabiting the planetary zoo, could possibly exist. Before 1995, astronomers generally thought that giant planets could only form at colder, greater distances from their stars. This is because the magnificent quartet of gaseous, giant planets, inhabiting our own Solar System–Jupiter, Saturn, Uranus, and Neptune–exist in the cold, remote outer limits of our Sun’s family. It was generally assumed that other solar systems would be similar to our own.

An Earth-mass exoplanet would be about 150 times smaller than the enormous roaster 51 Peg b. The original successful method used by planet-hunting astronomers, the Doppler shift method, favors the discovery of giant planets traveling in orbits close to their parent-stars. This is because that method searches for a subtle tattle-tale “wobble” of the host star, suggesting there is a planet pulling on it gravitationally. Smaller planets, that travel in orbits that are further from their stars, do not produce much of a “wobble” on their stellar parents.

Since 1995, planet-hunting astronomers have devised a variety of other techniques in addition to the “wobble” method to discover ever smaller and smaller exoplanets, as well as greatly improved methods of analysis.

Alpha Centauri was the name originally given to this multiple stellar system that appears to be a solitary star when viewed by the unaided eye–rather than the triple stellar system that it really is. Together, the bright visible duo of larger stars, that create the binary system, carry the name Alpha Centauri AB. The designation AB-C refers to the orbit of the small, faint Proxima around the central binary, which is the distance between the center of gravity and the more distant dim stellar sibling.

Astroseismic research, chromospheric activity, and the star’s rotation (gyrochronology), are apparently consistent with the Alpha Centauri system being of approximately the same age–or slightly older than–our Sun, with typical ages being between 4.5 and 7 billion years.

Hunting For Hidden Small Worlds In Alpha Centauri

The new findings of the Yale team are based on data derived from more advanced spectroscopic instruments at observatories located in Chile: CHIRON, a spectrograph designed by Dr. Fischer’s team; HARPS, designed by a team of astronomers from Geneva, Switzerland; and, UVES, which is a component of the Very Large Telescope Array (VLA). “The precision of our instruments hasn’t been good enough, until now,” Dr. Fischer commented in the December 18, 2017 Yale University Press Release.

The astronomers constructed a grid system for the Alpha Centuri stellar trio and asked, based on the spectroscopic analysis, “If there was a small, rocky planet in the habitable zone, would we have been able to detect it?” Frequently, the answer to that question came back as a resounding “No.”

Ms. Zhao, who is the study’s first author, concludes that for Alpha Centauri A, there might still be orbiting planets that are smaller than 50 Earth masses. For Alpha Centauri B there might be orbiting planets that are smaller than 8 Earth masses; for Proxima Centauri, there might be orbiting planets that are less than 50% of Earth’s mass.

The study also eliminates the possibility of the existence of a number of larger planets lurking undiscovered in Alpha Centauri. Zhao noted that this eliminates the chance of giant Jupiter-sized alien worlds launching an invading horde of asteroids that might blast into or alter the orbits of the smaller, Earth-like planets.

“This is a very green study in that it recycles existing data to draw new conclusions. By using the data in a different way, we are able to rule out large planets that could endanger small, habitable worlds and narrow down the search area for future investigations,” Zhao continued to explain in the Yale Press Release.

This new information will help astronomers prioritize their observations in the quest to discover additional alien planets in the system, the Yale researchers added. Furthermore, the continuing effort by Dr. Fischer and others to improve spectroscopic technology will enable astronomers to better understand and identify the composition of the distant planetary children of other stars beyond our Sun.

The paper describing this study is published in the December 18, 2017 issue of the Astronomical Journal. Study co-authors are Dr. John Brewer and Dr. Matt Giguere of Yale and Dr. Barbara Rojas-Ayala of Universidad Andres Bello in Chile.