Giant planet discovered orbiting tiny star | Hacker News Reader

Giant planet discovered orbiting tiny star

ucl.ac.uk

31 pointsgnabgib14 comments

The planet (TOI-6894b) is a low-density gas giant with a radius a little larger than Saturn’s but with only ~50% of Saturn’s mass.

IIRC Saturn has very low density, was it lower than water? (IOW it would float) so this would be even lighter.

I understand how they measure mass, but how do they measure an exoplanet's radius, especially to that precision?

I did not read this paper, but typically the diameter can be inferred by the transit time.

Not sure in this specific case, but usually by measuring the brightness of the star as the planet passes in front of it, like a partial eclipse. That’s why most of the planets we have discovered are giants.

Sounds less like a planet, and more like a nebula.

Did a quick calculation that star to planet mass ratio in this system is about 1400x. Does not seem that far from Sun to Jupiter (1047x) but probably crosses some supposed threshold.

It's not really about the ratio.

To get a gas giant, you first need the formation of a "regular" planet through accretion of material in orbit. Once that regular planet is big enough, by capturing enough material, its' strong gravity allows it to start pulling in more and more gas, creating a gas giant.

It was believed that small stars can't possibly host those kind of gas giant, because small stars don't have enough material orbiting around them to create a planet big enough start the runaway process of gas accumulation needed to form a gas giant, because if there was enough material, the star would not have been small in the first place.

> To get a gas giant, you first need the formation of a "regular" planet through accretion of material in orbit. Once that regular planet is big enough, by capturing enough material, its' strong gravity allows it to start pulling in more and more gas, creating a gas giant.

That's just one theory, right? There are competing hypotheses where they form much like stars do, simply by enough gas coalescing to do the same job that the "regular" planet "seed" would.

If we can get a binary star why it's so hard to imagine that it might be so asymmetrical that the other star is not a star but just a gas giant planet instead?

A gas giant is not just a failed star (i.e. an object with the same composition as a star, but that can't gather enough material to achieve fusion), it's formed differently from a star. A star forms from a diffuse molecular cloud collapsing on itself. You can have a binary star system when that molecular cloud is fragmented enough, and so you end up with two stars forming from the two main fragments, where both stars are born roughly at the same time and following the same process.

There could be a scenario where the second fragment is just not massive enough to achieve fusion, which is what you are alluding to, in that case the second fragment could give birth to a sub-brown dwarf [1].

A gas giant has a different composition from a sub-brown dwarf though, because they form differently. A gas giant is formed after the star, not at the same time. First the star forms, and the leftover material around the star starts clumping together in orbit around it, this allows the formation of a rocky core and at some point if the core gets big enough it starts the runaway process of pulling more and more gases, creating a gas giant. So a gas giant would have a higher density than a star of the same radius, because the massive rocky core is there. This was thought not to be a possible scenario around a small star, as we didn't expect a small star to have enough material left in it's orbit to allow the creation of a core big enough to start the runaway gas accumulation necessary for a gas giant.

Here, based on the mass/radius/temperature of the object, they were able to infer that it must have a core of roughly 12±2 Earth masses of dense material. Hence it's a gas giant (formed around a core, and thus after the star formation) and not a sub-brown dwarf in a binary system (formed at the same time as the star).

Obligatory disclaimer that I'm not an astronomer, just a hobbyist in the field.

[1]: https://en.wikipedia.org/wiki/Sub-brown_dwarf

Thank you for this explanation. I didn't know that we can recognize a rocky core at that distance nor that gas giants have one.

"which should not exist under leading planet formation theories"

Every time I read that something should not exist based on current understanding or theories, especially in the field of astronomy, I cry a bit.

Falsifying a theory is a nice thing and assures us that empirical science is working as intended, yes? Or sometimes laypeople were overeager to label as “theory” some things which enjoyed status as mere hypothesis, or conjecture, or proposal, and so update/reform the models and take baby steps again.

I am skeptical that “planet formation” is a type of subject where we could entertain any theories at all, on the human timescales we can handle.

real science, prof who is looking at edge cases finds that the edge is nowhere near it's generaly accepted location and finds a candidate for other interesting direct exo planetary observations, all very low key, but offers a whole new data set in planatary formation, especialy for cold gas giants. thr unfortunate part is that, this adds to the ever growing list of direct observations that contradict the "models" bieng used and taught to students.... MPAPA

Reply on news.ycombinator.com