A tilted “Tatooine planet” whose two suns aren't stars at all

Abstract

An indirectly detected planet on a highly inclined orbit about a pair of brown dwarfs shows that, when it comes to exoplanet diversity, anything goes.

The bonanza in recent years of discoveries of extrasolar planets by the thousands, fueled in large part by NASA’s Kepler and Transiting Exoplanet Survey Satellite (TESS) missions, has vastly expanded astronomers’ understanding of the broad diversity of exoplanetary systems. Of the nearly 6000 confirmed exoplanets, only about 15% are in systems similar to our own Solar System, with a mix of inner terrestrial and outer gas giant planets on stable orbits about a central star. The vast majority are in systems that are quite different, with types of planets entirely foreign to our Solar System—including so-called super-Earths, mini-Neptunes, and hot Jupiters—and/or whose orbital characteristics are very different from our Solar System’s mostly circular and coplanar orbits.

A MOST EXOTIC CASE

The most unusual exoplanetary systems, representing perhaps 10% of known cases, have extreme orbits that are highly eccentric or highly non-coplanar or orbit unusual types of stars that are very different from our own Sun. The system presented by Baycroft et al. (1)—dubbed 2M1510—provides strong, if indirect, evidence for the existence of one of the most exotic types of exoplanetary systems yet found.

The 2M1510 system combines a set of characteristics that are individually unusual and that, in combination, make it a first of its kind. The most unusual characteristics of the 2M1510 system include the following:

1) The planet (estimated to have a mass ranging from that of a mini-Neptune to a Saturn) orbits not one star but two—which is to say, it orbits around a binary star system. Exoplanets orbiting individual stars that are members of binary star systems are not especially rare; some 230 are currently known. What is rare is planets orbiting all the way around the entire binary star system—the two stars a kind of double sun at the center of the planet’s orbit; only 15 other circumbinary planets are known. These are the so-called “Tatooine planets,” similar to Luke Skywalker’s home planet in the original Star Wars.

2) Most known exoplanets and all of the planets in our own Solar System are on orbits that are coplanar with the central star’s spin orientation. In the case of a circumbinary planet, a coplanar orbit would be in the same plane as that of the stars’ orbit about each other, and all of the previously found circumbinary planets orbit in this way. In contrast, the 2M1510 planet is on a polar orbit, meaning that the plane of its orbit is perpendicular to the stars’ orbit, making it one of a kind.

3) Technically, the two stars at the center of the 2M1510 circumbinary orbit are not stars at all. They are both a type of object known as a brown dwarf, a “stillborn star” that never had enough mass to ignite nuclear fusion—a star’s defining characteristic—while also having too much mass to be considered a planet. Neither planet nor star, brown dwarfs have a narrow range of masses of about 15 to 70 Jupiters (the two objects at the center of 2M1510 each weigh about 35 Jupiters, making them definitive brown dwarfs). Only about 30 or 40 binary brown dwarf systems are known.

4) The system has not two brown dwarfs, but three, in total. The third brown dwarf orbits the entire circumbinary planetary system at a rather large distance of about 250 astronomical units (for reference, Earth orbits the Sun at a distance of 1 astronomical unit, and the most distant planet in our Solar System orbits at 30 astronomical units; the planet in 2M1510 orbits the central binary at a distance of only 0.06 astronomical units). Only one other trinary brown dwarf system was previously known (2).

5) The two brown dwarfs at the center of the circumbinary planetary system exhibit periodic eclipses as our vantage on the system from Earth has us viewing the brown dwarfs’ orbit edge-on, such that one partially blocks our view of the other every time around. Only one other eclipsing binary brown dwarf system was previously known (3).

Together, the evidence strongly suggests that 2M1510 is the first known trinary brown dwarf system with a central pair of eclipsing brown dwarfs that host a circumbinary planet on a polar orbit.

FORTUNATE GEOMETRY AND TIMING

What is more, it is only because of fortunate geometry and timing that the planet was found. The circumbinary planet’s orbit happens to be entirely in the plane of the sky as seen from Earth; in other words, the planet’s motion is entirely “side to side” as seen by us, so the brown dwarfs’ gravitational reflex motion is also entirely side to side and therefore produces no telltale shifting of the light spectrum (the so-called Doppler effect, by which many exoplanets are discovered).

The circumpolar orbit leads to an effect known as retrograde precession, in which the orientation of the brown dwarfs’ elliptical orbit slowly turns (precesses) in the direction opposite (retrograde) to their orbital motion. This retrograde precession could itself only be detected because of the favorable orientation that allows us to see the brown dwarfs periodically eclipse, providing a kind of clock (4) whose ticking slips, with each subsequent eclipse occurring slightly later than the last one.

We can only witness eclipses when the brown dwarfs’ elliptical orbit happens to be oriented longwise directly toward Earth, which it does at the present time. However, due to the orbit’s precession (caused by the circumbinary planet), it usually does not point toward Earth. The eclipses are becoming less detectable with time, and in about 95 years, eclipses will cease to be observable from our direction in space at all (Fig. 1).

Fig. 1. Fortuitous alignment, for now.

(Left) The two brown dwarfs at the center of the 2M1510 system are presently observed to undergo eclipses as they orbit each other; these periodic eclipses provide a kind of clock whose slow but steady lag (referred to as negative precession) likely signals the presence of a circumbinary planet on a polar orbit about the two brown dwarfs. (Right) The precession of the brown dwarfs’ orbit will cause the eclipses to cease, as seen from Earth, in about 100 years, at which point detection of the circumbinary planet would be much more difficult. Illustration credit: Ashley Mastin/Science Advances.

Astrophysically rare systems are oftentimes more than curiosities, especially when their rarity is due to a combination of unusual physical phenomena. Such systems can serve as laboratories for the study of physics that may be difficult to study in other ways. The one previously known brown dwarf eclipsing binary, called 2M0535-05, is a case in point. That system distinguished itself immediately by exhibiting an unexpected inversion of temperature with mass (5)—the more massive brown dwarf in that system is cooler than its lower-mass sibling—contrary to the prediction of all standard theoretical models of stars and brown dwarfs.

A series of follow-on investigations (6) revealed that the 2M0535-05 system represented an extreme case of what is now understood to be a general phenomenon in which a star’s magnetism changes the physical balance at the star’s surface, resulting in a decrease in the star’s surface temperature and an increase in its size, a physical effect now referred to generically as magnetic temperature suppression and radius inflation. One extremely rare system taught us generally about an aspect of the physics of magnetism that applies to all stars.

The 2M1510 system may be no different. Theoretical investigations into the possibility of circumbinary planets on stable polar orbits (7), and on the possibility of detecting them specifically through retrograde precession (4), now seem prescient.

Although a theoretical basis exists for understanding the dynamical behavior of the 2M1510 system and why a system like it can exist as a stable configuration, we need to learn how a circumbinary planet moves into such a polar orbit. One known example (8) of a circumbinary ring of protoplanetary gas and dust in a polar configuration exists; a planet that forms from that material will find itself in a polar circumbinary orbit. However, this example only begs the question of how a protoplanetary ring can get into a polar orbit.

TRIUNE DEUS EX MACHINA

Enter the triune deus ex machina. Trinary star systems have unique conditions that permit certain complex dynamical behaviors. In general, such systems evolve toward ever more stable configurations—a system of three gravitationally interacting objects in a triangle configuration is the least stable of all—leading to a rapid evolution of the system in which two of the siblings cast out the third but experiencing a dynamical kick in return that also brings the two into tighter quarters.

Such hierarchical trinaries may be the primary mechanism (9) for creating tight central binaries, like the eclipsing pair of brown dwarfs at the center of the 2M1510 system. Perhaps it was the casting out of the distant third brown dwarf in the system that gave the circumbinary planet the kick it—or its preceding protoplanetary ring—needed to get so tilted.

The discovery of the 2M1510 system also demonstrates the utility of using multiple clever techniques to understand the full diversity of planetary system types and architectures. Already, researchers have used the European Space Agency’s Gaia satellite to detect planets by the miniscule side-to-side reflex motions of their stars (10). In the coming years, we can expect to see the next planet-hunting boom from NASA’s Roman Space Telescope and its planned exoplanet gravitational microlensing survey, expected to find tens of thousands of Earth-like planets on Earth-like orbits, and in a manner that is even more direct than precession or reflex motion methods.

What new rarities are we likely to discover? As the 2M1510 system and other rare systems like it suggest, it appears that, unless the laws of physics forbid it, Nature will do it, at least once. If a combination of unlikely conditions and forces had to come together to explain it, the real question may not be why, but why not?