An article published in “The Astrophysical Journal” reports multi-epoch observations conducted with the Hubble Space Telescope of the star HBC 672 and the movement of the shadow projected by its protoplanetary disk onto an interstellar cloud. A team of researchers led by Klaus Pontoppidan of the Space Telescope Science Institute (STScI) compared the positions of the shadow over 13 months, noting its movement, which has a visual effect similar to a flapping of wings as its shape reminds of that to the point that the star and its protoplanetary disk have been nicknamed Bat Shadow. It could be a planet that is forming and warping the disk.
About 1,300 light-years from Earth, the star HBC 672, also known by other catalog designations, became famous in 2018, when the Hubble Space Telescope captured the image of the Serpens Nebula in which HBC 672 was born with a large shadow cast onto an interstellar cloud. This is evidence of the existence of a protoplanetary disk that is too far away from Earth to be seen directly by Hubble.
For stars in a growth phase before the main sequence in which they will spend most of their life it’s normal to illuminate dust in the cosmic neighborhood. In the case of HBC 672, the result is the curious shadow that we see from Earth with a shape that looks like black wings, hence the nickname Bat Shadow. That shape reveals details of the disk’s shape and nature, and it’s possible to detect differences in colors at its edges, useful to understand the size and composition of the dust grains in the disk.
After its discovery, more observations of the Bat Shadow have been conducted with the Hubble Space Telescope, and the comparison among images captured at different times over 404 days showed a movement of the shadow, a sort of flapping of wings. The most likely explanation is that a planet is forming in the protoplanetary disk surrounding the star HBC 672 and that it warps the disk making the shadow move.
Klaus Pontoppidan’s team calculated that a planet that warps the protoplanetary disk giving it a saddle-like shape should orbit the star HBC 672 in no less than 180 days. Its distance from HBC 672 should be close to that of the Earth from the Sun.
An alternative hypothesis is that the star HBC 672 has a low-mass companion outside the plane of the protoplanetary disk. This would cause a wobble in HBC 672 which would affect its shadow. The available data indicate a low probability that it’s the correct hypothesis, also because there’s no evidence of the existence of a companion.
Indirect observations of the protoplanetary disk surrounding the star HBC 672 make it difficult to obtain reliable data on the ongoing planetary formation processes. For now we can study star systems in the formation phase only in the cosmic neighborhood, and the shroud of gas and dust makes observations difficult. However, this is an interesting case for the effects created with its shadow.
This video illustrates the “flapping of wings” generated by the star HBC 672 and its protoplanetary disk.