Students discover planet orbiting rapidly rotating

hot star
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: 08 December, 2008

Three undergraduate students from Leiden University have discovered the first extrasolar planet orbiting a fast-rotating hot star.

Artist impression of the planet OGLE-TR-L9b circling its host star in about 2.5 days at a distance just three percent of the Earth-Sun distance from its star. The star itself is the hottest star with a planet ever discovered. Image: ESO/H. Zodetp.

As part of an undergraduate research project, Meta de Hoon, Remco van der Burg and Francis Vuijsje were testing an automated method to detect light fluctuations of some 15,700 stars in the OGLE database, compiled from about four years worth of data. One of the stars – OGLE-TR-L9 – was found to exhibit variations in brightness that could be explained by a transit of a planet in front of it, causing the star’s brightness to briefly dip. In this case its brightness was found to decrease by one percent for two hours every 2.5 days. The team used the GROND instrument on the 2.2 metre telescope at ESO's La Silla Observatory to follow up the observations, confirming the presence of a planet.

The discovery came as a complete surprise to project superviser Ignas Snellen. "The project was actually meant to teach the students how to develop search algorithms. But they did so well that there was time to test their algorithm on a so far unexplored database. At some point they came into my office and showed me this light curve. I was completely taken aback!"

In order to confirm the object’s status as a planet and not a brown dwarf or small star, the team made spectroscopic measurements using ESO’s Very Large Telescope,  which also revealed the star to be a hot 7,000 degrees, 1,200 degrees hotter than the Sun. It is also the hottest star with a planet ever discovered, and it is rotating very fast. The radial velocity method – that is used to discover most of the extrasolar planets already known – is less efficient on stars with these characteristics.

Undergraduate students Francis Vuijsje, Meta de Hoon, and Remco van der Burg (left to right) from Leiden University in the Netherlands proudly show off their discovery. Image: Leiden Observatory.

"This makes this discovery even more interesting," comments Snellen. The planet is five times as massive as Jupiter and circles its host star in about 2.5 days. It lies at only three percent of the Earth-Sun distance from its star, making it very hot and much larger than normal planets.

"It is exciting not just to find a planet, but to find one as unusual as this one; it turns out to be the first planet discovered around a fast rotating star, and it's also the hottest star found with a planet," says Meta. Although the planet has officially been assigned the tag OGLE2-TR-L9b, it has been nicknamed ReMeFra-1 by the students, after themselves.