Second exoplanet with four stars discovered

© Karen Teramura/UH IfA

Artists conception of the 30 Ari star system. In the foreground is the primary star about which the massive exoplanet orbits. The primary's newly-found binary partner, a red dwarf, can be seen in the upper left and the secondary binary system can be seen to the upper right.

For only the second time, an exoplanet living with an expansive family of stars has been revealed.

The exoplanet, which is a huge gaseous world 10 times the mass of Jupiter, was previously known to occupy a 3-star system, but a fourth star (a red dwarf) has now been found, revealing quadruple star systems possessing planets are more common than we thought.

"About four percent of solar-type stars are in quadruple systems, which is up from previous estimates because observational techniques are steadily improving," said co-author Andrei Tokovinin of the Cerro Tololo Inter-American Observatory in Chile.

The whole 4-star family is collectively known as 30 Ari, located some 136 light-years from Earth — in our interstellar backyard. The exoplanet orbits the primary star of the system once every 335 days. The primary star has a new-found binary partner (which the exoplanet does not orbit) and this pair are locked in an orbital dance with a secondary binary, separated by a distance of 1,670 astronomical unit (AU), where 1 AU is the average distance between the Earth and sun.

The new star discovery was made by the Robo-AO adaptive optics system, developed by the Inter-University Center for Astronomy and Astrophysics in India and the California Institute of Technology (Caltech), and the PALM-3000 adaptive optics system, developed by NASA's Jet Propulsion Laboratory in Pasadena, Calif., two instruments at the Palomar Observatory in San Diego, Calif.

The discovery of planets in systems like 30 Ari raises some important questions about planetary formation in multi-star systems and, as this particular system is so extreme, astronomers doubt that the massive exoplanet, nor its hypothetical system of moons, could support life (as we know it).

© NASA/JPL

The four stars and one planet of the 30 Ari system are illustrated in this diagram. This quadruple star system consists of two pairs of stars: 30 Ari B and 30 Ari A. A gas giant planet (red) orbits one of the stars in 30 Ari B about once a year. New observations led by NASA's Jet Propulsion Laboratory in Pasadena, California, identified the fourth star in the system (green); the three others stars and the planet were previously known. This is the second quadruple star system known to host a planet.

Although undoubtedly rare, this is the second exoplanet discovered in a quadruple star system. The first, KIC 4862625, was discovered by a citizen science project that scours data from the Kepler space telescope to seek out exoplanetary transit signals. But the fact that two such systems have been discovered, when only a couple of thousand exoplanets have been discovered so far, hints that exoplanet formation can occur in some pretty unlikely places.

"Star systems come in myriad forms. There can be single stars, binary stars, triple stars, even quintuple star systems," said Lewis Roberts of JPL. "It's amazing the way nature puts these things together." Roberts is the lead author of the study to be published in the Astronomical Journal.

Most stars in our galaxy are known to exist in multi-star systems and astronomers are currently trying to understand how they came to be this way. Were the stars gravitationally bound at birth inside their stellar nurseries? Or did they capture one another some time in their travels around the Milky Way? Recent unrelated research hints that multi-star systems may be born that way.

Now, with the increasing number of exoplanet discoveries in binary and multi-star systems, we are quickly realizing that many of our science fiction notions of far-off alien worlds are now actually modern science fact. One such world is Luke Skywalker's homeworld Tatooine from "Star Wars: A New Hope", where, at sunset, two stars of a binary pair dip low on the horizon.

But how would this multi-star system look from the vantage point of the exoplanet in 30 Ari? According to a NASA JPL news release, "the four parent stars would look like one small sun and two very bright stars that would be visible in daylight. One of those stars, if viewed with a large enough telescope, would be revealed to be a binary system, or two stars orbiting each other."

In this research, another exoplanet in a triple-star system called HD 2638 is also detailed. In that system, where a third star has just been confirmed, the "hot-Jupiter" exoplanet has a roller coaster 3-day orbit around its star.

How multi-star systems affect the evolution of planets is of key interest to Roberts' team — these two multi-star systems play host to at least one massive planet, boosting evidence that planets inside multi-star systems can have their orbits dramatically modified by the crazy orbital dynamics such systems possess, although the newly-discovered star in 30 Ari does not seem to be impacting the orbit of its neighboring exoplanet.

These systems also appear to boost the mass of exoplanets. Only through the discovery of more examples such as 30 Ari and HD 2638 can we understand the driving physics of massive exoplanet evolution.