One possible explanation is that an unseen planet's gravity pulls dust and gas into its inclined orbit. Recently, a second shadow has been spotted within a few years between observations stored in the Hubble Space Telescope's MAST archive, possibly indicating another disc within the system. These two discs could be signs of two planets in the process of formation.
TW Hydrae, located about 200 light-years away, is less than 10 million years old. Our Solar System may have resembled the TW Hydrae system when it was in its infancy, around 4.6 billion years ago. As the TW Hydrae system is nearly face-on from Earth's perspective, it provides an excellent opportunity to observe the process of planetary formation.
The second shadow was identified in observations taken on June 6, 2021, as part of a multi-year program aimed at tracking shadows in circumstellar discs. John Debes, from AURA/STScI for the European Space Agency at the Space Telescope Science Institute in Baltimore, Maryland, compared these recent observations to previous Hubble observations.
The team concluded that there could be two misaligned discs casting shadows. In earlier observations, they were too close to distinguish, but over time, they have separated and formed two distinct shadows. "We've never really seen this before on a protoplanetary disc. It makes the system much more complex than we originally thought," said Debes.
The most straightforward explanation is that the misaligned discs are likely caused by two planets' gravitational pull in slightly different orbital planes. Hubble is working to create a comprehensive view of the system's structure.
The discs may represent planets that are orbiting the star at slightly different speeds, like spinning two vinyl records at varying rates. Debes likens it to two racing cars that are close to each other, with one slowly overtaking and lapping the other.
The suspected planets are located approximately the same distance from their star as Jupiter is from our Sun, and their shadows complete one rotation around the star every 15 years, the expected orbital period at that distance.
Furthermore, the two inner discs are inclined by about five to seven degrees relative to the outer disc's plane, similar to the range of orbital inclinations within our Solar System. "This is right in line with typical Solar System-style architecture," said Debes.
The outer disc on which the shadows fall may extend several times beyond the radius of our Solar System's Kuiper belt. Interestingly, there is a gap in this larger disc at twice the distance of Pluto from the Sun, which could indicate the presence of a third planet in the system.
Detecting any inner planets would be challenging due to the star's glare and the dimming effect of dust in the system. ESA's Gaia space observatory may be able to measure a wobble in the star if Jupiter-sized planets are exerting a gravitational pull, but this would take years to confirm due to the long orbital periods.
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