Type Ia supernovae are critical tools in astronomy, since they all appear to explode with the same intensity, allowing us to use their brightness as a measure of distance. The distance measures they've given us have been critical to tracking the expansion of the Universe, which led to the recognition that there's some sort of dark energy hastening the Universe's expansion. Yet there are ongoing arguments over exactly how these events are triggered.

There's widespread agreement that type Ia supernovae are the explosions of white dwarf stars. Normally, these stars are composed primarily of moderately heavy elements like carbon and oxygen, and lack the mass to trigger additional fusion. But if some additional material is added, the white dwarf can reach a critical mass and reignite a runaway fusion reaction, blowing the star apart. But the source of the additional mass has been somewhat controversial.

But there's an additional hypothesis that doesn't require as much mass: a relatively small explosion on a white dwarf's surface can compress the interior enough to restart fusion in stars that haven't yet reached a critical mass. Now, observations of the remains of a supernova provide some evidence of the existence of these so-called "double detonation" supernovae.

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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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