December 6, 2022


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We were watching a failed star turn into a giant planet

We were watching a failed star turn into a giant planet

Zoom / Image of the AB Aurigae system, with object details shown at right.

At some levels, the formation of stars and planets is simple: they form where there is more stuff. Therefore, while the raw material for a star may be a diffuse cloud of gas, the distribution of this gas is not completely equal. Over time, the gravitational pull of regions with somewhat more material will pull in more material, eventually resulting in enough material to form a star. or two – in many cases, more than one concentration of the substance will form; In other cases, one focus will split into two. Planets also form where matter is, as they are formed by the disk of matter that fuels the forming star.

While this may be true in general, there are two problems with it. First, there’s no clear dividing line between small stars like brown dwarfs and massive planets that we’ve put into a category called super-Jupiter. And the handful of planets we’ve been able to image directly appear to be orbiting away from their host star, where there shouldn’t be a lot of material to trigger their formation.

Astronomers announced this week that they’ve photographed the super-planet Jupiter in the making, far from the star it appears to be orbiting. This suggests that the planet likely formed through a process that typically produces stars rather than through the process that produces gas giants like Jupiter.

We were watching you

The star in question is called AB Aurigae, and it is a very young star located 500 light-years from the Sun. It is embedded in a cloud of gas, some of which is still likely to fall into the star. Beyond them is a cloud of dust. This cloud is thought to be a good candidate for planet formation for several reasons. First, dust has been removed from the region closest to the star. Second, the gas in the inner disk was shaped into spiral arms by the effects of gravity.

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A team of researchers used the time of the telescope to search for planets in AB Aurigae. And the researchers apparently found one, now called AB Aurigae b, at about 100 astronomical units of AB Aurigae (each astronomical unit is the typical distance between the Earth and the Sun). This is more than twice the distance between the Sun and Pluto. This position places AB Aurigae b inside the dust ring and in a position where it should be able to create the kind of spiral arms visible in the gas between the dust and the star. It must also be outside the region where the density of matter is high enough to host the planet’s natural formation.

A look through image archives indicates that we have indications that the planet has been around for quite some time. The images clearly indicate that AB Aurigae b is in orbit.

The researchers used modeling to determine the size of the planet that could produce the light we saw coming from AB Aurigae b. Models suggest that although the planet is still likely to grow, it is already four times the mass of Jupiter. An alternative modeling method suggests that Jupiter’s mass is likely nine times greater. Either way, the planet definitely fits into the super-Jupiter category.

The imaging also shows some faint objects resembling AB Aurigae b, but further afield (430 and 580 AUs). These may be additional planets, but we need additional observations to confirm this.

what is happening here?

So what is going on here? Closer to the host star, gas giants are thought to form by accretion of large rocky cores that then begin to pull gas. This adds to the planet’s growing mass and promotes its further growth. This runaway growth is cut off because the gas feeding it is eventually pushed out by the young star’s radiation.

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However, this process is unlikely to succeed at the distances we see here. While more gas must remain for longer, there is no substance with a high enough density to build a large core. Unbridled growth will never begin.

The alternative is a process similar to that which creates a binary star system. Random fluctuations in the amount of matter lead to a concentration of matter that performs a function similar to a rock core. And since the formation site is far from the star, there is a chance that the growth process will continue for much longer, resulting in a super-Jupiter.

Nature Astronomy, 2022. DOI: 10.1038 / s41550-022-01634-x (About DOIs).