Strange New Worlds
Not so much a Blog as a catalogue of the known exoplanets that are:
- Nearest to Earth
- Farthest from Earth
- Closest to its Star
- Farthest from its Star
- Orbiting the Smallest Star
- Orbiting the Biggest Star
- Having Most Companion Planets
- Having Most Suns
- Most Earth-like
Data is largely taken from the Exoplanet Orbit Database, the Extrasolar Planet Encyclopedia and from the Open Exoplanet Catalogue although I also use published papers, NASA websites and other sources. Before reading further, you may find it helpful to read about planet-naming conventions and about the various methods used to detect exoplanets.
Kepler-37 b is probably similar to the planet Mercury in our own Solar System although it is even smaller and hotter. This planet was discovered in 2013 and has a radius of just 1900 km. This compares to 6371 km for the Earth, 2440 km for Mercury and 1737 km for our Moon. This tiny world orbits just 15 million km from a star that’s a little smaller than the Sun and she shares her planetary system with at least two other worlds.
Alien worlds come in all possible sizes. Planets just grade into stars and it’s hard to say where, exactly, the cut-off is between a large planet and a small star. To make matters even more complicated, planet masses have significant uncertainties and so there is much room for debate about whether any given large world should actually be classed instead as a brown dwarf (i.e. a small star). A good example is CT-Chamaeleontis b which is estimated to be 17 times more massive than Jupiter (i.e. 5400 times more massive than the Earth) and which orbits a young star 530 light years from Earth. This planet is so large and so far from its star (440 AU) that it is one of the few exoplanets that can be seen by direct imaging. It only appears as a point of light but, nevertheless, it can actually be seen.
CT-Chamaeleontis b and other “nearly-stellar” planets may be the most massive worlds but they don’t necessarily have the largest diameters. Hot Jupiters, planets about as large as Jupiter but orbiting very close to their host-stars, are inflated by the high temperatures so that they become significantly larger than they would otherwise be. The widest known such planet is HAT-P-32 b a planet with almost the same mass as Jupiter but twice its diameter. This world orbits a star, which is a little bigger and hotter than our own Sun, at an average distance of only 4 million km above the star’s surface. That’s less than a tenth of the distance between Mercury and the Sun and so this is a very hot, and heavily inflated, planet.
Interestingly, the measured diameter of HAT-P-32 b is very similar to the estimated size of massive planets like CT-Chamaeleontis b and so we can’t yet say whether, if you want to be the biggest of the planets, its better to be near your star or nearly-stellar.
Nearest to Earth
Just over 4 light years away lies Alpha-Centauri, the nearest star to Earth, and one member of this triple-star system may have an Earth-sized planet. Alpha-Centauri Bb is 13% more massive than our planet and orbits a star which is only just a little smaller than our own Sun. However, don’t pack your bags yet for a journey to this near neighbour. It orbits just 6 million km from Alpha-Centauri B and so would be an uncomfortably hot place to visit. There’s also significant doubt about whether this world really exists at all since, at this relatively small size, planets are very hard to unambiguously detect by the RV technique used for its discovery. A more secure example of a nearby world is Epsilon-Eridani b, a Jupiter-like world orbiting a Sun-like star at almost a Jupiter-like distance. Epsilon-Eridani is just over ten light years from Earth.
Furthest from Earth
In 2006, the Hubble Space Telescope was pointed at the Sagittarius Window for a week to look for transiting planets orbiting the 180 000 stars it could see there. These stars lie between us and the centre of our galaxy. This SWEEPS survey spotted several such transits and two of the planets orbited stars bright enough to allow their distance to be measured. One of these, SWEEPS-04, orbits a star that is 27 719 light years from Earth.
Closest to its Star
When it comes to weirdness, it would be hard to beat PSR-J1719-1438 b. This body is slightly heavier than Jupiter and orbits a neutron star every two and a quarter hours at a distance of less than 700 000 km. This may sound like a large distance but, if PSR-J1719-1438 was as large as our Sun, this planet would be orbiting just below the star’s surface! However, neutron stars are collapsed objects with the mass of a star compressed into a sphere just a few kilometres across. Collapse occurs during a supernova explosion and so this planet either survived that cataclysm or formed afterwards from the debris. The preferred explanation is that the planet was originally a star too but that its gaseous envelope was sucked onto the companion (probably causing the supernova). The remaining outer layers were either blown away by the supernova explosion or continued to trickle onto the neutron star afterwards. This left a stellar-core behind that is largely composed of carbon (an end product of stellar nucleosynthesis in smaller stars) which, given the strong gravity of any stellar mass object, is compressed to diamond. In fact, this planet has to be very dense since, otherwise, it would have been broken up by immense tidal forces generated by the neutron star. So a diamond-planet that used to be a star is the most plausible description we have of PSR-J1719-1438 b.
However, if we restrict ourselves to planets orbiting normal stars then the closest-in world found so far is Kepler-42 c, a planet a little smaller than Earth which orbits it’s sun twice a day at a distance of less than 900 thousand kilometres. That is still far enough to keep it well away from the surface of its rather small, red-dwarf, star whose radius is less than a fifth that of our Sun.
Farthest from its Star
WD-0806-661 Bb is so far from her parent star that she takes 160 000 years to orbit it just once. To put it another way, this planet lies 64 times further away from its star than Pluto does from our Sun. It’s quite a big planet, too, being eight times heavier than Jupiter and so, if there happened to be a similar world orbiting in our own solar system, we would certainly have spotted it by now. Indeed WD-0806-661 Bb was discovered by direct imaging and so we can see it even though it lies 63 light years from Earth.
Orbiting the Smallest Star
OGLE-2009-BLG-151 MOA-2009-232 b was discovered by gravitational microlensing and is a 7.5 Jupiter-mass planet orbiting a star which is less than 2% of the mass of our Sun. Although OGLE-2009-BLG-151 MOA-2009-232 b orbits it’s star three times closer than we do, our Sun still gives gives us 4000 times more light than this feeble star provides for the only planet discovered, so far, in this distant planetary system.
Orbiting the Biggest Star
Imagine looking up to see a bloated, red-giant appearing 24 times bigger than our Sun looks to us. That would be the view from HD-208527 b, a planet ten times bigger than Jupiter orbiting a star 60% heavier than ours. This star is only 2 billion years old but, because of its high mass, it has already entered the red-giant stage of old-age. A billion years ago this would have been a normal star and its world would have been close to the habitable zone (it would actually have received about 40% more heat than Earth does). We can speculate that HD-208527 b may have had a Moon upon which life began but, even if it did, life has now become impossible under the fierce gaze of the engorged star.
Having Most Companion Planets
One hundred and thirty light years away lies the star HD-10180 which may have nine planets. That’s more than our own solar system (which has 8 planets now that Pluto has been demoted). This star is very like the Sun but its planetary system is certainly not solar-system-like. Seven of these worlds are roughly the size of Neptune whilst the remaining two are slightly larger than the Earth. The system is also much more compact than our own with the most distant planets lying roughly where the asteroid belt does in our own solar system. Sadly, both of the Earth-mass worlds orbit very close to the star and are far too hot to be habitable. However, as is often the case with exoplanets, the detailed structure of this planetary system is hotly debated and not all experts agree that the evidence supports the existence of nine planets. For a less contentious example of a multi-planet system the best example is Kepler-11, a system of 6 planets circling a very Sun-like star. However, once again, the planetary architecture is quite unlike that of our solar system. The orbits of all six worlds would easily fit inside the orbit of Venus and, despite the fact that these planets are mostly just a little larger than the Earth, their densities suggest that they are not rocky worlds.
Having Most Suns
There are 17 known planetary systems which orbit binary stars. The best known of these, Kepler-16 b, was announced in 2011 and was widely compared to Tatooine, the planet from the movie Star Wars which also had two suns. However, PH-1 b (also known as Planet Hunters 1 b and Kepler-64 b) is the current champion since there are 4 stars in this system comprised of two pairs of binary stars with this planet orbiting one of these binary systems. PH-1 b was discovered by amateur astronomers looking at Kepler light curves provided by NASA as part of the Planet Hunters project. This was the first planet discovered by that project; what a way to start!
It’s important to distinguish circumbinary planets, which actually orbit both stars, from planets in binary (or multiple) systems which orbit just one of the stars. If non-circumbinary worlds are allowed in our search for the planet with most suns, then Gliese-667 C is also worth mentioning since this red dwarf star, which has seven planets, itself orbits around the more massive stars Gliese-667 A and Gliese-667 B. Planets in multi-star systems seem to attract comparison to science fiction since Gliese-667 Cc has been nick-named Vulcan after the planet from Star Trek which also orbited a member of a triple-star system. Gliese-667 Cc is at about the right distance from its star to be habitable but it’s also a bit on the heavy side since it weighs in at nearly 4 times the Earth’s mass. The strength of gravity on the surface of this world would be about 50% greater than on Earth (assuming it’s also a rocky planet) and, if Mr Spock came from Gliese-667 Cc, he’d have to be very strong but also probably rather squat. Think “Grumpy” from Snow White and the Seven Dwarfs rather than Leonard Nimoy (or Zachary Quinto for the younger folks out there).
The youngest star with a planet, that I’ve been able to find an age for, is 2M-044144. This star is just one million years old and it’s unlikely that this record will be substantially beaten since, if our planetary formation theories are correct, it usually takes a few million years for planets to form. The planet itself is seven or eight times bigger than Jupiter and orbits almost as far away from its star as Uranus does from ours. This large size and separation has allowed 2M-044144 b’s discovery by direct imaging.
WASP-29 b orbits a star whose age has been estimated as fifteen billion years. That’s older than the Universe! However, the uncertainty in this age estimate is large (eight billion years) and so this isn’t really a problem. In any event, WASP-29 b is at least twice as old as our own planet and perhaps nearly as old as the Universe itself. This world is a classic Hot-Jupiter with a 3-day orbit around its star and a mass about that of Saturn. The surface temperature has been estimated at over 600 °C, as a result of proximity to the star, but perhaps there are other, more habitable, planets in this system that we haven’t yet detected. The idea, that there could be worlds in the Universe that have been inhabited far longer than our own, is a fascinating one.
We’re still waiting to find an Earth-mass planet in the habitable zone of a Sun-like star but, until we do, Gliese-581 g is widely regarded as the most Earth-like world. This planet orbits in the centre of its star’s habitable zone and is almost certainly a rocky planet as it is only about three-times more massive than the Earth. However, this is not yet a confirmed planet and may turn out to be an artefact in the data rather than a real world. Of the confirmed planets, Kepler-62 e and f are perhaps the most Earth-like as they too sit in the habitable zone and, with radiuses just 60% and 40% larger than Earth, are likely to be rocky planets.
Kepler-78 b orbits its Sun-like star every 8.5 hours at a distance of 1.5 million kilometres. The estimated temperature of this Earth-sized world is 2330 °C. However, this planet is almost certainly tidally-locked to its host star so that one side of the world permanently faces the sun. It’s quite possible that the other hemisphere is extremely cold if there is no atmosphere to transport heat.
You’d expect planets that orbit far from their parent-star to be the coolest ones. However, most of these more distal worlds are large planets discovered by direct imaging (see Biggest) and these are worlds that are close to becoming stars. Even Jupiter, in our own Solar System, radiates more heat than it receives from the Sun. So, for a cool planet, we’re looking for a planet which is not too big and which is also fairly distant from it’s sun. One possible candidate for the coldest known planet is Formalhaut Ab, a planet “only” three times heavier than Jupiter orbiting 177 AU from it’s star. Formalhout-A itself is, however, a very bright star which pumps out more than ten-times as much heat as our Sun does. Nevertheless, when you get 177 AU away, temperatures will still be very low even with such a bright host-star. Estimated temperatures depends upon many assumptions (in particular upon the albedo, whether there is any greenhouse effect and where we take the “surface” as being on a gaseous world) but a back-of-an-envelope calculation gives a temperature of -230 °C for Formalhaut Ab.
Probably this one…