STEREO Looks at the Sun; Finds Planets

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The primary mission of the twin STEREO probes is to explore the 3-dimensional makeup of our Sun. Each craft carries a variety of instruments. One of them, the Heliospheric Imager (HI), doesn’t look directly at the Sun, but rather, explores a wide field near the Sun in order to explore the physics of coronal mass ejections (CMEs), in particular, ones aimed at the Earth. But while not focusing on solar ejections, the HI is free to make many other observations, including its first detection of an extrasolar planet.

As the Heliospheric Imager stares at the space between the Earth and Sun, it has made many novel observations. The device first opened its shutters in 2006 the instrument has observed the interaction of CMEs with the atmosphere of Venus, the stripping of a tail of a comet by a CME, atomic iron in a comet’s tail, and “the very faint optical emission associated with so-called Corotating Interaction Regions (CIRs) in interplanetary space, where fast-flowing Solar wind catches up with slower wind regions.”

The spacecraft allows for long periods of time to stare at patches of sky as the satellites precede and follow Earth in its orbit. The spacecraft is able to take pictures roughly every 40 minutes for almost 20 days in a row giving excellent coverage. As a result, the images taken have the potential to be used for detailed survey studies. Such information is useful for conducting variable star studies and a recent summary of findings from the mission reported the detection of 263 eclipsing variable stars, 122 of which were not previously classified as such.

Another type of variable star observed by the STEREO HI, was the cataclysmic sort, in particular, V 471 Tau. This red giant/white dwarf binary in the Hyades star cluster is a strong source of interest for stellar astrophysicists because the system is suspected to be a strong candidate for a type Ia supernova as the red giant dumps mass onto its high mass, white dwarf companion. The star system is extremely erratic in its light output and observations could help astronomers understand how such systems evolve.

Although planetary hunting is at the very edge of the capabilities of the HI’s limitations, eclipses caused by planet sized objects are feasible for many of the brighter stars in the field of view as dim as approximately 8th magnitude. Around one star, HD 213597, the STEREO team reported the detection of an object that seems too small to be a star based on the light curve alone. However, follow up studies will be necessary to pin down the object’s mass more accurately.