I'm unable to find an answer to my questions via my searches. This questions pertains to the timeline of a star system creation.

My question:

During the creation of a star system, can satellite planets form before the star has ignited and begun to burn?

In more detail, I understand the basic creation of a star, with the gravitational collapse of a large cloud, leading eventually to fusion. Is this transition from protostar to maturity (or to where "ignition" occurs) long enough that planets have already started to form, even in a primitive state (just a big rock)? Or has the star long since been burning when planets finally start to form?


3 Answers 3


This should be physically possible, it even might happen in the universe right now.

The creation of a stellar system starts with a cloud of matter. This cloud will collapse and form some kind of thing we later might call a star. You might also call this a planet - if the mass is not enough to create a pressure which is high enough to make an initial fusion, you will have a brown dwarf. Also Jupiter is a planet which is a "too light" star.

In my opinion, nothing speaks against the possibility, that maybe two "clusters" forming at the same time. Or a smaller cluster forms first and a bigger - which becomes the star later - forms second.

The point here is more, what you call a planet? Is the sun a planet? Would be Jupiter a planet, if it shines like a star? Maybe you can call the sun a planet, if it rotates around some bigger mass? There might be some sort of definition to be done for this, I think.

  • 2
    $\begingroup$ Whatever definition you might have for a planet, it will not include an object undergoing fusion at its core. An object that does that would be a star and definitely not a planet. $\endgroup$
    – Kyle Kanos
    Nov 5, 2013 at 3:12
  • $\begingroup$ Okay, so we can assume, that a Jupiter-like planet forms before the sun forms - and voila, you have a planet before the central star ignites. I think, it is extremely unlikely, this happens, because within the gravity-field of a central object it's easier to form planets because there will exists the formation of a dust-disk instead of a dust-ball. $\endgroup$
    – Fips
    Nov 5, 2013 at 21:37
  • $\begingroup$ But the star can very well have been compacted enough to form a disc around it but not yet have ignited hydrogen fusion. The star could still shine quite brightly from the thermal radiation alone, though. $\endgroup$
    – Thriveth
    Dec 15, 2013 at 0:14
  • $\begingroup$ @Fips: There will always be a central object with a gravity field, whether it is a dense gas cloud, or a proto star. Once the star ignites, the solar wind will push a lot of the smaller particles out of the planetary zone. There will need to be large particles by this time. $\endgroup$
    – LDC3
    Mar 30, 2014 at 21:10
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    $\begingroup$ How does this answer the question? $\endgroup$
    – ProfRob
    Aug 30, 2014 at 20:09

The answer is yes; according to our current theoretical ideas and observations of circumstellar disk lifetimes, planets must be able to form prior to the ignition of the main phase of hydrogen burning on the main sequence.

The Details:

I take your question to mean planet formation prior to the fusion of hydrogen into helium, not the very brief phase during which the star's primordial deuterium is burned, which for a star like the Sun happens within the first million years and certainly will take place before planets can fully form.

Planets form in a disk of circumstellar material around their parent protostars. The "core-accretion" model of giant planet formation suggests that it takes only 5-10 million years to form a giant planet in this disk. The main competing model (thermal instability in the disk) suggests an even more rapid formation timescale. Stars of a solar mass or less take significantly longer than this to contract sufficiently that their cores reach the necessary temperatures for hydrogen ignition.

To put some numbers on this: according to the pre-main sequence models of Siess, Dufour & Forestini (2000), it takes about 25 million years for a solar mass star to produce an appreciable fraction of its luminosity from hydrogen burning. This timescale gets longer for lower mass stars.

Statistically speaking, we know that most stars lose their disks within about 10 million years (see for example Hillenbrand 2008). They are essentially gone by an age of 25 million years, so all giant planet formation must have already occurred by then. However, we also now know thanks to extensive surveys for exoplanets that a large fraction of stars have gas giant planets. So most stars must form planets within about 5-10 million years and this is definitely not enough time for H-burning to start in a star of $\leq M_{\odot}$. The same argument probably does not apply to small rocky planets, which probably take a bit longer to reach their final configuration (maybe as long as 100 million years in our Solar System), though large planetesimals should be present after only a million years.

  • $\begingroup$ Your first paragraph confuses me. The $p-p$ chain is the conversion to deuterium and is the majority of a star's life. Converting that to helium is most of the remaining life, but still only a fraction of the $p-p$ dominant stage. Where am I mistaken here? $\endgroup$ Aug 30, 2014 at 17:01
  • $\begingroup$ When we say deuterium burning in this context, we are usually referring to the brief phase of time when the primordial deuterium in the star is burned. This occurs within the first million years for a star like the Sun. The p-p chain is the conversion of hydrogen into helium (indeed via deuterium as the first, and rate-limiting step). $\endgroup$
    – ProfRob
    Aug 30, 2014 at 20:06

To the best of my knowledge, the current thinking is that some large chunk of matter starts sweeping up matter in the circumstellar disk during/just after the protostellar phase of the star (e.g., T-Tauri stars) and forms gas-giant planets. The origin of smaller, rocky planets are still debated, but I think most astronomers believe them to be created after the circumstellar disk has been cleared.


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