Wikipedia says the definition of interplanetary space is

the region dominated by the interplanetary medium, which extends out to the heliopause where the influence of the galactic environment starts to dominate over the magnetic field and particle flux from the Sun

While it defines the solar system as the region in which the sun is the primary gravitational influence

The Sun's gravitational field is estimated to dominate the gravitational forces of surrounding stars out to about two light years (125,000 AU).

So, if the region outside the heliopause but inside the solar system isn't interplanetary space, then what is it?

  • $\begingroup$ The way I see it, those are different definitions. The former is based on how far the Sun's plasma is sent out, while the latter is the sphere of influence of the Sun. The space within the Sun's gravitational influence but is outside of the Sun's heliosphere would then be considered interstellar space. That's just as far as I know. $\endgroup$ Commented Jun 27, 2016 at 0:07
  • $\begingroup$ @SirCumference yes, but as came up in The Pod Bay, it is quite possible to get into a conversation where you want to specifically refer to the space outside the heliopause but inside the solar system, and if you can't refer to it, then confusion ensues. Unless you say 'interstellar space within the solar system but outside the heliopause'. Oort cloud doesn't seem to cover it... $\endgroup$
    – kim holder
    Commented Jun 27, 2016 at 0:38
  • $\begingroup$ IIHO the entirety of those objects bound to the Solar System barycenter as their primary is interplanetary space, and, yes that means interstellar space starts a lot farther out than the edge of the heliosphere, especially on the Vega side. $\endgroup$ Commented Jun 27, 2016 at 13:29
  • $\begingroup$ I am pretty sure the heliosphere is considered to be the 'range' of the solar system. $\endgroup$
    – HotSaucey
    Commented Jun 27, 2016 at 20:06
  • $\begingroup$ If the heliosphere has a bow shock with the interstellar medium, then the region to which you refer should be called the heliosheath. Unfortunately, they gave that name to the region between the termination shock and the heliopause... Maybe "interstellar sheath"? $\endgroup$ Commented Aug 2, 2016 at 19:07

2 Answers 2


There doesn't seem to be such a word. Interstellar space within the Solar System is still just interstellar space.

There doesn't seem to be a demand for the word you're looking for either. We distinguish space by its contents; the space within the heliosphere is called the interplanetary medium (it contains solar plasma, dust, etc.), while the interstellar medium comprises sparse gaseous molecules from the Local Fluff and (if you go far enough) the rest of the Local Bubble.

Whereas there are about 10 oxygen molecules per cubic centimeter in the interplanetary medium, there is only 1 oxygen molecule per cubic centimeter in the interstellar medium. Seeing that there is no clear boundary for the Milky Way (nor is there any single definition for a boundary for galaxies), the intergalactic medium just vaguely describes the medium within galaxy filaments; here, the space is so rarified that there is only 1 oxygen molecule per cubic meter.


The Sun's Hill Sphere may be the term you're looking for. Or perhaps "within the Oort Cloud diameter" would do it for you. According to Wikipedia, the outer gravitational zero velocity distance (Hill Sphere) isn't known - it's thought to lie between 50,000 AU and 200,000 AU - or up to over 3 light years.

  • 3
    $\begingroup$ If you are the same Li Zhi we have had in the past, your answer is likely reliable, but since you seem to have chosen not to use a permanent account there is no way to verify that you are the same user. So, since your answer is brief and without reference, it is a target for downvotes. $\endgroup$
    – called2voyage
    Commented Jan 3, 2017 at 15:59

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