Astronomers have observed up to three newborn planets evolving from a disk of gas and dust particles circling a distant Sun-like star.

While 1,900 planets have been discovered outside our solar system, these are the first to be seen that are still forming.

The discovery, reported in the journal Nature, has provided scientists with direct evidence of how gas and dust particles coalesce to create planets.

"We have found a clear case where we can join all of the dots showing how planets are forming by accreting the gas and dust left over from the formation of their star," said one of the study's authors, Professor Peter Tuthill of the University of Sydney.

The team of astronomers made the discovery while examining a distant star called Lick-Calcium 15 (LkCa 15), which is located about 450 light-years away in the constellation of Taurus.

The young star is only two million years old and is still surrounded by the circumstellar disk of gas and dust from which it formed.

Over a period of five years the astronomers focused on a large gap in the circumstellar disk that was being cleared out by the newly forming planets as they swept up material that would have otherwise fallen onto the star.

The astronomers identified light emissions caused by very hot gas falling onto the newly forming planet which they named LkCa 15 b.

The process, which is called accretion, occurs as small particles such as gas, dust grains, and tiny meteoroids, come together to form progressively larger and larger bodies.

LkCa 15 b orbits its star at a distance of about 2.4 billion kilometres.

"This is a gas giant planet like Jupiter but a few times more massive," said Professor Tuthill.

A second planet named LkCa 15 c was also detected in the cleared out area, and there are indications of a possible third planet, which would be named LkCa 15 d.

"This provides us with a nice story; we see the star surrounded by a disk of material, we see a gap in the disk where the material's missing, we see the planets that are in the gap, and we see material falling onto the planets."

While astronomers have seen each of these individual events occurring separately before, this is the first time they were able to watch the complete story unfold.

The authors detected the planets by combining different new technologies on the Large Binocular Telescope in Arizona and Chile's Magellan Telescope.

"This was a perfect storm of an observational problem," said Professor Tuthill.

"We're trying to see very faint planets buried in the glare of the star."

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