Monday, November 29

Astronomers May Have Discovered a Planet in Another Galaxy

Astronomers May Have Discovered a Planet in Another Galaxy

Extragalactic Exoplanet System Illustration

Astronomers detected the temporary dimming of X-rays from a system where a massive star is in orbit around a neutron star or black hole (shown in the artist’s illustration). This dimming is interpreted as being a planet that passed in front of an X-ray source around the neutron star or black hole. Credit: NASA/CXC/M. Weiss

Using ESA’s XMM-Newton and XMM-Newton M51

This image shows the Whirlpool Galaxy (M51) as seen at X-ray wavelengths by ESA’s XMM-Newton X-ray Observatory. Credit: ESA

This is different when studying X-rays, instead of visible light, in a galaxy. Because there are less objects that shine bright in X-ray light, an X-ray telescope like ESA’s XMM can more easily distinguish between objects when observing a galaxy. Those objects are therefore easier to identify and study, and it might be possible to find a planet around them.

Some of the brightest objects that can be studied in external galaxies are the so-called X-ray binaries. They consist of a very compact object – a

“X-ray binaries may be ideal places to search for planets, because, although they are a million times brighter than our Sun, the X-rays come from a very small region. In fact, the source that we studied is smaller than Composite M51 Labeled

A composite image of M51 with X-rays from Chandra and optical light from NASA’s Hubble Space Telescope contains a box that marks the location of the possible planet candidate. Credit: X-ray: NASA/CXC/SAO/R. DiStefano, et al.; Optical: NASA/ESA/STScI/Grendler

Rosanne and colleagues searched in Chandra and XMM-Newton data of three galaxies for such X-ray transits, dips in the light that could be explained by planets. And they found a very special signal in the Whirlpool Galaxy (M51) that they decided to study in more detail. The dip occurred in X-ray binary M51-ULS-1 and completely blocked the signal for a few hours, before it came back again.

Now the game of carefully crossing off possible explanations began, before the researchers could even consider the option of an extragalactic planet. “We first had to make sure that the signal was not caused by anything else,” says Rosanne, whose team argues against a number of possibilities in their new publication. “We did this by an in-depth analysis of the X-ray dip in the Chandra data, analyzing other dips and signals in the XMM data, and also modeling dips caused by other possible events, including a planet.”

A Planet in Another Galaxy Infographic

Infographic: A planet in another galaxy. Credit: ESA

Could the X-ray dip be caused by small stars like a brown or red dwarf? No, they argue, the system is too young for that, and the transiting object too large.

Could it be a cloud of gas and dust? Not likely, the team says, because the dip indicates a transiting object with a well-defined surface, which would not be the same for a passing cloud. Even if the planet had an atmosphere, it would still have a more well-defined surface than a cloud.

Could the dip be explained by variations in brightness of the source itself? The paper authors are confident that this is not the case, because although the light from the source completely disappeared for a few hours before it came back, the temperature and light colors stayed the same.

Lastly, the team also compared the dip to another blockage of the light caused by the ‘donor’ star passing in front of the compact star. This was partly observed by XMM-Newton and caused a much longer black-out, which was different from the dip caused by a possible planet.

“We did computer simulations to see whether the dip has the characteristics of a planet transiting, and we find that it fits perfectly. We are pretty confident that this is not anything else and that we have found our first planet candidate outside of the Milky Way,” adds Rosanne.

The team also speculates about the characteristics of the planet based on their observations: it would be the size of