… the Universe is not only queerer than we suppose, but queerer than we can suppose.
J.B.S. Haldane
At present we know about 973 confirmed exoplanets, and some of them are very queer indeed. Thanks to the Exoplanet app, written and maintained by a young astronomer, Hanno Rein, it is easy to explore the stunning variety of planets out there. Join me below the orange nebula for some of the queerer members of our little neighborhood.
Orbital period
The slowest planet, WD 0806 661B b, takes 160,000 years to circle its star. In other words, this gas giant is 2,500 times farther from its star than the earth is from the sun. As a gas giant, it was probably sent flying into the outer darkness during a crazy episode of planetary ping-pong.
The fastest planet, PSR J1719-1438 b, only takes 2.17 hours to go around its star, a pulsar. This Jovian (not to say jovial) planet has to be very dense to hang together so close to its star; the consensus is that it is a crystal of carbon. In other words, a planet made of diamond.
Size - Mass
The largest exoplanet by mass is DH Tau b, with a mass of 37.5 Jupiter masses. How so big, and not a brown dwarf star? Well, it has to do with the processes by which a mass forms. Apparently based on its appearance it's considered a planet (a ginormous planet), not a brown dwarf. It's a long way away from its star - its "year" is nearly 10,000 years long.
The smallest by mass is PSR 1257 12b, which is less than half the mass of our Mercury. Just a wee little thing! Speedy, too - it scoots around its star in just 25 days.
Size - Diameter
Because planets can differ in density, the most massive planet is not necessarily the largest across and indeed this is the case. CT Cha b has a larger diameter than DH Tau b even though it is half as massive. At a mass of 17 Jupiter masses and about 1.6 times as dense, CT Cha b is 2.2 times as big across. This is what that looks like:
CT Cha b in scale against the Sun and our planets
The smaller blue and teal planets are Neptune and Uranus, and the ittybitties further to the left are the rocky planets like Earth and Venus.
The exoplanet with the smallest known diameter is Kepler 37 b, a tad bigger than our Moon and a tad smaller than Mercury, like this:
Eccentricity
Eccentricity measures how from circular a planet's orbit is. Although almost certainly all planets start out in circular orbits, many planetary systems go through a time of trouble when the giants play ping pong and fling some or many of the other planets away into the outer darkness. Sometimes one or more of the giants end up in orbits closer to the star, sometimes very close to the star; these are the "hot Jupiters" that took us all by surprise.
When that kind of ping pong is over, the new orbits aren't always circular. Sometimes they are quite eccentric. Just how eccentric? Take a gander:
HD 20782 b is the most eccentric planet known. As the diagram shows, this giant (twice the mass of Jupiter) goes racing far inside where the orbit of Mercury would lie and then swings out and at its furthest extent is farther out than the orbit of Mars. This strange orbit gives a sense of the rough and tumble that some planetary systems pass through in their evolution. The green band is a calculated "habitable zone" for this star, which will come up again later.
At the other end of the scale, there are many planets with perfectly circular orbits, too many to list.
Other oddities
The smallest star known to have a planet, 2M 004144, is pretty dang small - just 20 Jupiter masses, so it is a brown dwarf. Its planet is about a third as massive as it is! So a dinky small star with a Super-Jupiter circling it - almost like a twin star system that just didn't have enough mass.
The largest star known to have a planet is NGC 4349 No 127. It's about four times the mass of our own Sun, and its known planet is a Super-Jupiter way inside the habitable zone. Stars do come much bigger than our Sun, of course. Just what it means that we don't find any planets around bigger stars isn't clear to me. For one thing, bigger stars are rarer. For another, they would be bad targets for detection based on perturbations. It might also be a matter of too short a lifetime as a star, or too big a gravity well for planets to be stable. Since so many systems go through the vicious ping pong stage, maybe the bigger stars just eat all their children.
How many planets can one star have? Well, at least one has seven planets:
KOI-351: A Seven Planet System.
Two independent groups have found a seven planet system in the Kepler dataset. Citizen Scientists at planethunters.org also helped to find the system. The Open Exoplanet Catalogue lists the planets as confirmed because they were reported by two independent groups, there are transit timing variations and because a seven planet system is unlikely to be a false positive. However, note that no planet masses have been measured yet.
This beautiful sapphire planet has rain of glass shards. It is
HD 189733b, a hot Jupiter that is tidally locked to its star and traveling at six times the speed of sound (not a problem in the vacuum of space, of course0.
Anywhere to call home?
While diamond planets are unquestionably intriguing, and everyone was stunned by the discovery of the hot Jupiters, the question that burns in all our minds is Are we alone?
For a variety of reasons, almost all exobiologists believe that life evolving anywhere in the cosmos will use water as its solvent. Based on that estimation, it is possible to mark off a domain around a star called the "habitable zone," meaning one that could have liquid water depending on a planet's size, atmosphere and so on.. The algorithm used in the Exoplanet app is pretty generous; it puts our own habitable zone as stretching from the orbit of Venus to outside the orbit of Mars! It also doesn't take into account the changing temperature of stars, so it is a static image, not a continuously habitable zone. With this wide zone, the app lists about 192 planets as passing at least part of the time in the habitable zone. By this criterion, that wildly eccentric planet is included in the planets in the habitable zone. It also includes a large number of Jupiter sized planets - these might have moons that are habitable but the gas giants themselves are not habitable.
If we limit this list by size, the number of candidate is much smaller. There are four super-Earths (planets that are five to ten times the Earth in mass) that lie in the habitable zone. None of the planets smaller than Earth are there. Even these four are not looking very habitable; they either edge into and out of the zone, or lie at the equivalent of the orbit of Venus. The nine planets that are about the size of Earth are all repositioned into the hotter interior orbits.
There may well be a habitable planet out there. The ones we've seen so far are not wildly promising. Red dwarfs, the most common kind of star, present particular challenges.
10:03 AM PT: A few more collections of strange planets:
Popular Mechanics, including a pink planet and one darker than coal.
Space.com's strangest aliens - each picture comes with a story about that planet.
Discover's gallery of the weird and wonderful.