Institute of Astronomy


Ask an Astronomer - Planets

Habitable planets

Published on 08/11/2013 

Two questions, which are linked.

Occasionally there is some coverage in the media about the continuing discovery of planets around other star systems in the Milky Way, through the work of the Kepler telescope, and other initiatives. Is there a place on the web I can go to, to check what the latest thinking on all of this work is? I'm thinking of an up to date receod of things like numbers of candidate habitable planets discovered, where they are, stuff like that.

Second question, is there any work being done to try to identify such planets around either Alpha Centauri A or B? Is there somewhere I can go to on the web to see what the current thinking and evidence is from any work on these stars and their surrounding planets, and whether any might be in the 'goldilocks' zone for these stars?

There are several websites that maintain catalogues of exoplanet discoveries, with the two most well known being the Extrasolar Planets Encyclopedia ( run by Jean Schneider at the Paris Observatory, and the Exoplanet Data explorer ( run by NASA, both of which provide facilities for plotting some of the basic parameters of the planets.  Both of those catalogues also have links to other pages about exoplanets, including things like discussion of potential habitable zone planets (though on that particular point be warned that there is no definitive agreement about the location of the habitable zone so you will likely find significant differences between different sources).

Alpha Centauri, as you can imagine being the nearest star system to our own Sun, has been extensively studied for many years to determine whether it has planets.  Last year a roughly Earth-mass planet was reported around Alpha Cen. B orbiting at 0.04AU (about a tenth of the orbital distance of Mercury), however there is considerable controversy surrounding this discovery and it has yet to be confirmed.  The existence of any planets larger than around Neptune has already been ruled out in the Alpha Centauri system, however finding, or ruling out, small, Earth-size, planets at Earth-like orbital distances is exceptionally difficult.  The Wikipedia article on Alpha Centauri Bb actually does a very good job of summarising the current situation.

Blowing up Jupiter

Published on 06/08/2013 

In a hypothetical scenario, if you could bear that in mind I am interested in the effects that would happen to Earth if Jupiter in a hypothetical situation exploded. What kind of magnitude would it be referenced under? And how would the astronomical community report that/those findings back to natons around the world? Would the asteroid belt be perturbed in any way? Shooting out asteroids from it and rocky debris in closer orbit with the gas planet? Your help in this matter to keep your hypothetical hat on just to humour and bear with me would be appreciated, thank you...

It depends in part what you mean by 'exploded'.  The solar system planets are quite widely separated, so at least initially, the effect would be rather small.  The orbits of the other planets would remain close enough to their present orbits that only astronomers would be able to tell the difference, and this will be true whatever type of 'explosion' you want.  As for any immediate, direct effects of the 'explosion', remember that space is a vacuum and so shock waves (sound, earthquakes, etc.) can't propagate.

If we go with a minimal 'explosion' and just magically cause Jupiter to vanish then the only other thing we would have to worry about is what happens to its moons and the asteroids.  The large moons that orbit close to Jupiter would probably all simply be ejected from the solar system and not pose any threat to anyone.  Some of the smaller ones further out might go onto elliptical orbits around the Sun, but there are not that many of them so the chance of a collision would be quite small.  The asteroid belt would likely remain much as it is, interactions with Jupiter actually generally serve to destabilise asteroids rather than the other way around.  The only possible cause for concern would be the Trojan asteroids, which share Jupiter's orbit and lead or follow it by 60 degrees.  These occupy stable regions created by the gravity of Jupiter, so if Jupiter were to disappear some of them might become unstable and come into the inner solar system.  This would likely be a long process rather than a fast one so we would probably have a reasonable length of time to deal with any new asteroid that appeared on a collision course with Earth and not be much worse off than our current situation.  In short, other than the disappearance of Jupiter from the sky, most people probably wouldn't notice the difference.

Now, on the other hand, if you want a 'Death Star' style explosion rather than just a planetary vanishing act things would be a little different.  Jupiter is extremely massive, and all of that mass suddenly flying around the solar system is going to have some rather interesting effects.  Now most of the mass of Jupiter is gas (primarily hydrogen and helium), which the Sun would eventually blow away through X-ray radiation and the solar wind (though it might take some time to do so).  What effect the gas has depends partly on how far the explosion spreads it and so how dense it is, it could potentially exert drag on some asteroids, particularly the Trojans, and cause them to spiral in to the inner solar system.  If Jupiter has a rocky core (we aren't actually entirely certain if it does), then that would cause additional problems.  The core of Jupiter might be as more then 10 times the mass of Earth, by comparison the whole asteroid belt is less than 0.1% the mass of Earth.  Turning all of that into rubble and throwing it across the solar system would cause utter havoc, the conversation between a whole leader and an astronomer would probably go something like this:

Astronomer: Jupiter has exploded, the core, which incidentally we know now was 10 Earth masses, has been pulverized and thrown out across the solar system.
World Leader: That doesn't sound good, is any of it going to hit us?
Astronomer: We haven't determined the exact numbers yet, but we expect the impacts to start within a year.
World Leader: Within a year!?  What kind of damage are we expecting?
Astronomer: It will probably start fairly localised, but the surface will reach saturation quite rapidly.
World Leader: Saturation?
Astronomer: Imagine carpet bombing the entire planet with the largest nuclear weapons you have.
World Leader: Oh... Can we do anything about it?
Astronomer: Nope.
World Leader: So we're doomed then?
Astronomer: Pretty much, yes.

If Jupiter doesn't have a core, or if it is pulverized into small enough pieces in the explosion (think dust), then it might not be so bad.  The gas and dust would probably partially block out the Sun and cause and ice-age, though material raining down into the atmosphere might also cause Earth to heat up, so it might get hotter even with the Sun being dimmed.  Either way the environment would be pretty screwed up but some people might survive.

Eclipses on other planets

Published on 24/06/2013 

Hi, I asked my Dad if any other planets in our solar system have eclipses?  He doesn't know but thinks it is a great question so we would be grateful for your help.

Other places in our solar system can have eclipses, though nowhere has eclipses quite like those on we get on Earth.  The moons of the gas giants, Jupiter and Saturn, regularly experience eclipses because Jupiter and Saturn are so large, their eclipses last a lot longer than ones on Earth do though and cover the whole surface, rather like when the Moon is eclipsed by Earth.  In fact NASA's Cassini space probe took an image of Earth while the Sun was being eclipsed by Saturn a few years ago (, and is planning to take another one on 19th July (

Earth-like moons

Published on 18/02/2013 

I wonder what would happen if neptune somehow ended up in earth's orbit and turned the earth into one of its moons? what would happen to the earth assuming neptune kept its new orbit in the habital zone of the sun and what would happen to neptune and its moons. color changes and melting for ex. and would neptune stay blue? thanks

Assuming that we could arrange a bit of magic such that one day we woke up in orbit around Neptune with Neptune occupying Earth's current orbit there would initially not be much change, aside from the rather obvious one of a large deep blue orb taking up a substantial fraction of the sky.  We'll also assume that we arranged our magic transition such that Earth is not too close to Neptune's other moons, so that there are no immediate major impacts, let's put ourselves at the nice healthy distance of a million km from Neptune, giving us an orbital period of about 30 days.  Triton is the only moon we really need to worry about in that sense as it is far larger than all of the rest of Neptune's moons combined (99.5% of all the mass in Neptune's moons is in Triton).  Some of the others are large enough that we wouldn't want to hit them from the point of view of human civilisation, but it would do any long term damage to Earth itself if we did.

The first thing we would notice after the sudden appearance of Neptune in the sky would be the dramatic increase in the height of the tides.  Although our nominal orbit is around 3 times further from Neptune than the Moon presently is from us, Neptune is much more massive, and so the tidal field strength would be around 50 times higher.  This wouldn't directly translate into a tidal range that is 50 times larger, since the much stronger tidal forces would be more effective at deforming Earth's crust as well as the oceans, so the sea floor itself would rise and fall along with the oceans.  It is difficult to say exactly how much larger the tidal range of the water would be than at present, but the mid-ocean tidal range would almost certainly be in the range of a few metres rather than half a metre as it is now.  This would mean that large swathes of low-lying coast around the world, including many of the worlds major cities, would become tidal plains flooded twice a day.  The much greater tidal flexing of the crust would also lead to a significant, and permanent, increase in earthquakes and volcanic activity.  In the long term the extra energy pumped into the Earth through tidal heating would also become a contributor to global warming, though given the other problems I don't think we would notice.  Life on Earth would take a while to get used to these changes, but it certainly would in time, the new tidal plains would become major new habitats, and although modern civilisation would take some heavy knocks I expect that humans would get used to it too.  We could potentially decrease the effect by placing ourselves in a wider orbit, but the tidal effects are always going to be rather larger than at present.

Assuming we manage to pull ourselves away from our new problems long enough to take a look around the next thing that we would notice would be the lack of our old Moon, which, since Neptune's gravity is much stronger, would become another separate moon of Neptune.

Another thing we would notice would be the monthly total eclipses that would occur every time we passed behind Neptune.  We are used to the usual solar eclipses produced by our Moon, which are only fleeting and cover only a small fraction of Earth's surface, these new eclipses however would last for hours and cover the whole world.

On Neptune it would be the dramatic increase in temperature that would be noticed, at present Neptune has a surface temperature of around -200C and emits two and a half times as much energy from internal sources as it receives from the Sun.  The huge change in the energy balance would certainly affect the Neptunian weather, which would probably become more violent.  It would also almost certainly affect the chemical balance in the atmosphere, though exactly how the colouration would change is uncertain, the methane that gives it its present blue colouration would still be present, but other compounds would likely change.

The changes for Neptune's moons would be dramatic, most of them have large amounts of water and other frozen volatile compounds and elements like ammonia and nitrogen, which would melt, but because they are rather small they would not be able to retain the liquids and gases, and probably would mostly disintegrate due to the outgasing.  Triton would be a different case, almost as large as our moon it may not have hugely strong gravity, but it is enough that it would not immediately loose it's new atmosphere to space.  The new atmosphere would probably be quite thick, composed of nitrogen, carbon dioxide and probably ammonia, and would overly a massive, deep global ocean, since water makes up about 30-40% of Triton's mass, much more than Earth.  In fact if the atmosphere is not too thick, and we could get rid of the carbon dioxide (the ammonia would be destroyed by the Sun fairly quickly) it could become quite a nice second home for us.

Rogue Planets

Published on 22/01/2013 

I understand that astronomers believe so-called rogue planets were likely ejected from their solar systems early in their planetary histories, but it's never clear what event(s) could trigger such a thing. My question: What kind of a catastrophic event in our solar system could cause the Earth to become a rogue planet today?  It's ok to be speculative. I'd love to know. Thank you.

Planetary systems become unstable when the orbits of two planets cross.  By this I don't mean that the planets collide, rather, that their crossing orbits cause them to have a close gravitational encounter.  The close gravitational encounter can transfer a tremendous amount of orbital energy from one planet to the other, potentially shooting it out of the planetary system.
Surprisingly, our own system is barely stable!  A close encounter between the asteroids Vesta and Ceres in about 60 million years ago makes it very difficult to trace Solar system dynamics before the encounter ( and also limits our ability to forward-predict Solar System dynamics on timescales longer than about 10 million years.  For more information about the stability of the Solar system, see this Wikipedia article and its sources:
In summary, to make the Earth a "rogue planet," the Solar system would have to evolve such that another large body (a large asteroid, or Venus or Mars, or eventually Jupiter) crossed orbits with the Earth.  The asteroids and Mars probably don't have enough energy to eject Earth.  A more massive planet like Jupiter would be more effective at ejecting Earth, but Jupiter is quite far away and is less likely to cross orbits with Earth.  But the Solar system is definitely stable for the durations of our lives, and this kind of ejection couldn't happen for at least a few tens of millions of years, if not billions.