An astral wonder

This morning many Finns dragged themselves out of bed at the crack of dawn to gaze at the sky. The planet Venus’ procession across the disc of the Sun was most clearly visible in the Arctic region, eastern Asia and Australia. A glimpse of Venus could be seen in Helsinki as well.

An astral wonder

For astronomers on the hunt for exoplanets, planetary transits in our own solar system are a special occasion indeed.

“It is a unique opportunity to test the methods used for hunting and identifying exoplanets in our own test laboratory, that is, our own solar system,” explains astronomer and docent Maarit Mantere.

Sadly, transits are rare. Venus’ next such appearance is more than a century away, and in Finland the next opportunity to observe one will be as late as in 2247.

A key method for searching for exoplanets is to monitor dips in the stars’ light curves. Some of them are caused by orbiting planets. However, these partial stellar eclipses are difficult to sift out from among other phenomena that cause drops in brightness.

Transits of known planets are excellent material for testing how well we can identify planets and sort out other causes for dimming, referred to as “noise”.

The worst cause of interference are star spots.

“In many other stars, spots are a great deal larger than our own sun spots,” Mantere explains.

Mantere’s research group in Kumpula focuses on stars’ magnetic cycles as well as on star spots, i.e., spots on the star’s photosphere caused by gas cooler than the ambient temperature.

“This transit of Venus coincided almost exactly with the maximum activity of the Sun’s magnetic cycle,” the scholar asserts.

“With maximum sun spot activity, we can test methods for eliminating interference particularly well.”

Mantere herself monitored the transit in Haikko, where she is participating in the biennial Astronomers’ Days organised by the Finnish Astronomical Society.

To help her in the process she had a small Dobson telescope borrowed from the Metsähovi Observatory, in addition to which all scholars were equipped with solar viewing glasses that allow direct observation of the Sun without burning one’s eyes.

Unfortunately the sky was cloudy and Venus could not be seen in Haikko. Skywatchers in Kaivopuisto Helsinki had more luck. The clouds parted to give them a glimpse of the planet.

Text:Virve Pohjanpalo
Photo: Veikko Somerpuro
Translation: Language Services of the University of Helsinki
University of Helsinki, digital communications

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