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Is this the first extra-galactic planet?

The first confirmed observations of an exoplanet were announced in 1992 and since then over 4,000 have been catalogued with many additional candidates still under investigation and pending confirmation. Despite having known about their existence for a substantial period of time now, exoplanets have continued to make the news on a regular basis in recent years. The discovery of seven exoplanets in the Trappist system and the 2019 Nobel prize in physics awarded for the first detection of an exoplanet around a sun-like star are two notable examples, but one recent observation has caused a lot of additional interest in this busy field.

In the discovery of exoplanets, space missions such as Kepler and TESS working in unison with ground based observations have been crucial in beginning to unravel the details of the exoplanet population. However, these missions have most often worked in the optical wavelengths of the electromagnetic spectrum (the wavelengths of light that human eyes can see). They are primarily looking for dips in a star’s emission as a planet passes in front of it and blocks some of the emitted light (a transit) or a slight change in the wavelength of the star’s emission due to the gravitational pull of a planet (a radial velocity change or Doppler wobble).

The definition of an exoplanet is simply a planet that orbits a star other than our own Sun but, despite the size of the existing catalogue, never before has a planet been observed beyond the boundaries of our local galaxy, the Milky Way. With past methods, observing exoplanets at such great distances has been incredibly challenging but this particular star system, located in the M51 Whirlpool Galaxy, is a near perfect example of a system where such a discovery is possible.

The definition of an exoplanet is simply a planet that orbits a star other than our own Sun

One of NASA’s latest missions is the Chandra X-ray Observatory and, as the name suggests, it has been surveying the universe in the higher frequency X-ray wavelengths. With recent data analysis, there is now a hint of a transiting exoplanet around an X-ray emitting star system but what’s most intriguing about this potential find is the fact that, if confirmed, it would be the first extra-galactic exoplanet in our catalogue. 

Unlike our own, this system is a binary. It has a high mass object at its centre (either a black hole or neutron star) and this object is in the process of accreting material from its companion star. In doing so, material flows across from the ‘donor’ star to the accreting object and the highly energetic infall of material causes emission in the X-ray part of the electromagnetic spectrum.

If confirmed, it would be the first extra-galactic exoplanet in our catalogue 

The central object, whether it is a black hole or neutron star, is incredibly compact and as such, only emits across a very small area of the sky. This means that a planet transiting in front of it could cause the observed X-ray flux to drop temporarily to zero as the planet is large enough to block the entirety of the emission from our perspective. It is the fact that these systems show such a great change in the observed flux that makes it possible to observe them at much greater distances than previous methods and this is exactly what has now been observed.

Further investigation is needed to confirm this exoplanet detection but it has started an important conversation in the field. Only the most exotic systems host X-ray emitting sources but if this method of detection proves successful, there is an opportunity to probe many new and bizarre star systems where conditions are wildly different to our local environment.

When stars move from the main sequence (the stage the Sun is currently in) to later stages of stellar evolution, high mass stars will often exhibit explosive behaviour such as causing supernova. Therefore, exoplanets which have survived in these systems must have undergone drastic changes as the conditions in their host stars have changed. Given existing data in X-ray wavelengths within and outside of the Milky Way, there is potential to find more of these sources, perhaps even within our galaxy, and to use them to better understand the conditions on surviving planets.

The main issue with the observation in M51 is that the orbital period of this exoplanet candidate is approximately seventy years and, as such, there will be a long wait before further observations can be made. That doesn’t mean that work will halt in the meantime though. As already stated, this potential discovery opens up a whole new branch of exoplanetary observations and the continuing observations of the Chandra satellite could yet yield more discoveries of a similar nature.

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