Massive Object Found In Milky Way Center, Failed Star Or Giant Planet?

There are a lot of things in the universe that routinely stump scientists trying to understand how everything in it works. And joining the long list is OGLE-2016-BLG-1190Lb, a planetary object so massive the team behind its discovery isn’t even sure if it is a gigantic planet or a failed star.

A large international team of researchers found the planet when looking at data acquired by NASA’s Spitzer Space Telescope in June-July 2016, when the telescope was looking at a recently discovered microlensing event called OGLE-2016-BLG-1190. The event itself was spotted by the Optical Gravitational Lensing Experiment (OGLE) collaboration, a Polish astronomical project based at the University of Warsaw which utilizes the 1.3 meter Warsaw telescope mounted at the Las Campanas observatory in Chile.

Microlensing is a kind of gravitational lensing technique, in which a large object — like a star or a galaxy — in the foreground bends the light coming from an object in the background, so that the background object, otherwise invisible, can be seen by an observer in front. And if a dark object, like a planet, passes across the bent light from the background object, the dark object can be observed too. The planetary object (to be referred to as a planet in the remainder of this article) researchers found using this method is located in the center of the Milky Way.

Milky Way1 The Milky Way seen arching across the Chilean night sky above the European Southern Observatory’s Paranal Observatory. Photo: ESO/P. Horálek

“We report the discovery of OGLE-2016-BLG-1190Lb, which is likely to be the first Spitzer microlensing planet in the Galactic bulge/bar, an assignation that can be confirmed by two epochs of high-resolution imaging of the combined source-lens baseline object,” the researchers, led by Yoon-Hyun Ryu of the Korea Astronomy and Space Science Institute in Daejon, South Korea, wrote in the paper’s abstract.

The planet itself has a mass equivalent to about 13.4 times that of Jupiter, and it orbits its host star approximately every 3 years at a distance of 2 AU (one astronomical unit is the distance between the sun and Earth, roughly 150 million kilometers or 92 million miles). The star, OGLE-2016-BLG-1190L, is a G dwarf with only about 0.89 solar masses, or less than a tenth of the sun.

The mass of the planet places it almost exactly at the conventional boundary that separates planets from brown dwarfs, and this is why scientists cannot determine whether it is, in fact, a planet that was born out of the disk around its host star or if it is a low-mass failed star. Further observations may answer the question in the future, the researchers said.

Milky Way’s bulge refers to the region in the center of the spiral galaxy, where it is much wider than it is along its numerous arms. This region is very densely packed with stars which make it difficult to study using conventional methods of observation.

The paper, titled “OGLE-2016-BLG-1190Lb: First Spitzer Bulge Planet Lies Near the Planet/Brown-Dwarf Boundary,” is currently available on the preprint server arXiv and has been submitted to the Astronomical Journal for publication.

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