Rendering of a planet taking shape similar to the HD 100546 system (Image Credit: David Cabezas Jimeno (SEA)

The earliest years of a star's life tend to be rather tumultuous. After a previous-generation star reaches the end of its life, it seeds space with the raw materials for star-formation. Once enough material collects, all that's needed is a trigger—like a supernova shockwave—for a nebular cloud to morph into a full-fledged star-making factory. The formation of a planet is a little bit different.

Current models predict that planets slowly take shape with the gas and dust-filled disks—called protoplanetary disks—surrounding young stars. Astronomers even believe that our own solar system formed under the same set of circumstances. As did most of the planets in our galaxy, which are believed to greatly outnumber stars (perhaps by a mindbogglingly large margin). However, we have very little in the way of observational evidence.

Using the ESO's Very Large Telescope Interferometer (VLTI), astronomers have now pierced the amniotic sac of a stellar nursery (its circumstellar disk), producing a "sonogram" of a yet-to-emerge stellar fetus. Ultimately, the star—called HD 100546 (otherwise known as KR Muscae)—is much younger than our Sun: having celebrated just 10 million birthdays (For comparison, the Sun is around 5 billion years old: meaning it is half-way through its 10-billion-year lifespan). So young, it hasn't yet exceeded the upper age threshold for a Herbig Ae/Be star. As such, it is still technically classified as a pre-main-sequence star.

By examining the surrounding disk, and looking for places where gas and dust have been carved out (a telltale sign of planet-formation), they have also discovered clear evidence that HD 100546 has already begun forging at least one, possibly two, planets (one is known as HD 100546 b): a suggestion that was first made a few years ago. Now, a second planet may be in play, only it is more distant.

Of course, this new research poses many questions.

Interestingly, the spectral signature of the innermost part of the disk doesn't match that of similar planet-forming disks. Instead, it more closely resembles strictly barren stars, lacking in evidence of planet formation.

Secondly, the gap in the disk is unusually far away from the star itself. If we were to replace HD 100546 with the Sun, the outermost part of the disk—where one protoplanet appears to be forming—would exceed the orbit of Pluto 10 times.

The gap between the inner disk and the outer also happens to be extremely large. By our estimates, it spans around 10 AU (1 AU is the distance between Earth and the Sun), or 930 million miles (1.5 billion km). Only once before have scientists found something of this magnitude: a protoplanetary disk surrounding a young star with a gap ten times farther out than this example.

a look at HD 100546 and its disk (Image Credit: NASA/ESA)

Everything else aside, since the system is located roughly 325 light-years from Earth (one light-year is 6 trillion miles, or 9 trillion km), it is naturally difficult to resolve. One of the co-authors, Professor Rene Oudmaijer (University's School of Physics and Astronomy), compares the challenge to "trying to observe something the size of a pinhead from 62 miles (100 km) away."

Thankfully, the Very Large Telescope Interferometer (VLTI) is among a small number of observatories up to the job; It utilizes the full power of four different telescopes, each 27 feet (8.2 m) wide—equaling one 427-foot (123-meter) telescope.

Followup observations will surely follow. Until then,"'With our observations of the inner disk of gas in the HD 100546 system, we are beginning to understand the earliest life of planet-hosting stars on a scale that is comparable to our solar system," Professor Oudmaijer pointed out.

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