Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Mariana on 12/02/2019 13:29:44

Title: Are all the planets in the solar system the same age?
Post by: Mariana on 12/02/2019 13:29:44
Hans asks:

For the sun and all the planets to be in orbit, does this mean they are all more or less equal in age?

What do you think?
Title: Re: Are all the planets in the solar system the same age?
Post by: Janus on 12/02/2019 16:03:15
More or less the same age.  Present estimates have Jupiter forming ~90 million years before the Earth, but this is a small difference when compared to the 4.5 billion year age of the Earth.  The planets and and Sun were all formed by the collapse of the same large cloud of gas and dust.
Title: Re: Are all the planets in the solar system the same age?
Post by: Hans on 13/02/2019 04:47:54
Hi Janus

Thanks for the reply. To my mind the Sun should have been formed first which would then attract and keep planets in their orbit. Are you then suggesting that the Sun was there already to keep Jupiter in its orbit around the Sun? 
Title: Re: Are all the planets in the solar system the same age?
Post by: evan_au on 13/02/2019 08:33:03
The Solar System formed by a cloud of gas and dust contracting under its own gravitational field.
- It is a positive feedback system, in that the more the gas contracts, the more intense its gravitational field, so the more it contracts...
- This contraction is opposed by conservation of momentum, which tends to keep the gas in an orbit around the center of mass, rather than converging on the center of mass
- This orbiting gas is opposed by the many random directions of patches of the orbiting gas, which collide with each other, heating up. The increased temperature also opposes contraction.
- This excess temperature is radiated more easily if there are heavier elements mixed in (from carbon to iron), as a result of previous supernovas seeding the dust cloud with heavier elements
- The collisions cancel out different angular momentum, dumping gas closer to the center (which eventually forms the star)
- While some gas remains on the outskirts of a flattish disk, forming the planets
- This protoplanetary disk has been observed by radiotelescopes

So the whole gravitational collapse thing happens together - you don't need the Sun to form first.
- In fact, the orbit of a Jupiter-sized object is determined by the mass within its orbit
- It doesn't matter whether that mass is compact or fuzzy
- or whether it is fusing hydrogen or not

See: https://www.almaobservatory.org/en/press-release/almas-best-image-of-a-protoplanetary-disk/
https://en.wikipedia.org/wiki/Protoplanetary_disk
Title: Re: Are all the planets in the solar system the same age?
Post by: Hans on 13/02/2019 11:53:19
Thanks for an in depth answer. I always understood that e Sun should have, would have been there first.

Appreciate the great answer!
Title: Re: Are all the planets in the solar system the same age?
Post by: acsinuk on 20/02/2019 03:29:09
Evan,  I would love to know whether the ALMA observatory can assess the spectrum from the centre star matter and compare it with the outer gas/dust ring.  I would predict the inner star would have an absorption spectrum and outer gas/dust an emission spectrum
Title: Re: Are all the planets in the solar system the same age?
Post by: evan_au on 20/02/2019 10:23:09
Quote from: acsinuk
I would predict the inner star would have an absorption spectrum and outer gas/dust an emission spectrum

Quote from: ALMA website
For the new TW Hydrae observations, astronomers imaged the faint radio emission from millimeter-size dust grains in the disk
Clear line spectra (emission or absorption lines) come from widely-separated atoms/ions.

Dust grains don't have a clear line spectrum, because the Pauli exclusion principle says that all the electrons must have different energies, and this prevents formation of a line spectrum.

I imagine that ALMA was monitoring the thermal emissions from the dust grains, and this black-body radiation has a broadband nature.
See: https://www.almaobservatory.org/en/press-release/almas-best-image-of-a-protoplanetary-disk/