While our Sun prefers to go solo, many other stars are parts of binary systems, with a pair of stars gravitationally bound to each other. In some cases, the stars are far enough apart that planets can form around each of them. But there are also plenty of tight binary systems, where the stars orbit each other at a radius that would place them both comfortably inside our Solar System. In these systems, exoplanets tend to be found at greater distances, in orbits that have them circling both stars.

On Wednesday, scientists described a system that seems to be neither of the above. It is a tight binary system, with a heavy central star that's orbited by a white dwarf at a distance two to three times larger than Earth's orbit. The lone planet confirmed to be in the system is squeezed in between the two, orbiting at a distance similar to Earth's distance from the Sun. And, as an added bonus, the planet is orbiting backward relative to the white dwarf.

Orbiting ν Octantis

The exosolar system is termed ν Octantis (or Nu Octantis), and its primary star is just a bit heavier than our Sun (1.6 solar masses). It's orbited by a far dimmer companion that's roughly half of our Sun's mass, but which hasn't been characterized in detail until now. The companion's orbit relative to the central star is a bit lopsided, ranging from about two astronomical units (AU, the typical Earth-Sun distance) at its closest approach to roughly three AU at its farthest. And, until yesterday, the exact nature of the companion star was not clear.

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© NASA/Goddard Space Flight Center

These types of exoplanets are some of the most common in our galaxy - but they've remained mysterious until now.

What does the "typical" exosolar system look like? We know it's not likely to look like our own Solar System, given that our familiar planets don't include entire classes of planets (Hot Jupiters! Mini-Neptunes!) that we've found elsewhere. And our discovery methods have been heavily biased toward planets that orbit close to their host star, so we don't really have a strong sense of what might be lurking in more distant orbits.

A new study released on Thursday describes a search for what are called "microlensing" events, where a planet acts as a gravitational lens that magnifies the star it's orbiting, causing it to brighten briefly. These events are difficult to capture, but can potentially indicate the presence of planets in more distant orbits. The researchers behind the new work find indications that there's a significant population of rocky super-Earths that are traveling in orbits similar to that of Jupiter and Saturn.

Lenses go micro

The two primary methods we've used to discover exoplanets are called transit and radial velocity. In the transit method, we simply watch the star for dips in the light it sends to Earth, which can be an indication of a planet orbiting in a way that it eclipses a small fraction of the star. For radial velocity, we look for red- or blue-shifts in the light received from the star, caused by a planet tugging the star in different directions as it orbits.

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© Harvard-Smithsonian,Center for Astrophysics/D. A. Aguilar