Extragalactic Planet
If you have seen the light curves that indicate transiting exoplanets before, this one might not look all that different. The amount of light coming from the object suddenly drops, and in a symmetrical way, comes right back up to where it was beforehand – suggesting that something briefly moved in the way to block the light. Well, there’s something unusual about this light curve. This one is in another galaxy.
This is a light curve of a small X-Ray Source in galaxy M-51, the same galaxy where the supermassive black hole at the center was imaged by humans a couple years ago. X-Ray sources like this one are thought to come from binary systems, where either a neutron star or a small black hole is accreting mass from its companion star. Because it’s not being swamped out by light from surrounding stars, the discrete X-Ray source could be imaged by the Chandra X-Ray observatory as a single object, despite the distance. Furthermore, the objects are small, so if there happen to be any transiting planets, they would move across quickly and block nearly all the X-Ray light, creating a strong signal that could be observed by the telescope.
After realizing these objects were good candidates for extragalactic planet hunting, a team from Harvard and a few other institutions hunted through observations of X-Ray sources in previous Chandra data and found this one, imaged in 2012. They verified that the light was blocked at every wavelength, as would happen during a transit, and the light curve is symmetric as happens during a planet transit. Although the data are not precise enough to indicate an exact size, the best fit for a size estimate is somewhere around the size of Jupiter or Saturn, again suggesting a planet.
There have been a few other proposed extragalactic planets, but thus far none of them have been fully confirmed by repeat observations. Hopefully that will be plausible with this object.
-JBB
Reference:
https://arxiv.org/pdf/2009.08987.pdf
