The day after my birthday the Kepler team announced (I like to think perhaps as a belated present) the discovery of 715 new exoplanets. This is a huge haul. It brings the tally of known planets to almost 1700.
The team was able to confirm the existence of such a large number of planets by making use of a new statistical approach to their analysis. Kepler worked by observing 160,000 stars and looking for periodic dips in brightness. The idea was that these periodic dips could be a sign of a transiting planet. The trouble is, these dips could also be caused by orbiting binary stars eclipsing each other. With the new technique, the Kepler team looked for multiple dips in brightness: this phenomenon must be caused by transiting planets rather than multiple eclipsing stars.
The technique works beautifully: those 715 planets just announced orbit only 305 stars. The Kepler data contains information on planetary systems, not just single planets.
Kepler tells us that planetary systems are common. It tells us small planets are common. And it tells us that some planets will orbit in the habitable zone. Deep-down we knew all those things anyway; but because of this announcement we can be sure.
That astronomers can find exoplanets at all is still a source of wonder to me. That they can find Earth-sized planets is astonishing. But a paper published in today’s issue of Nature is almost miraculous: A sub-Mercury-sized exoplanet, by Thomas Barclay and many others, describes the discovery of an exoplanet that has a radius that’s just 0.3 times that of Earth. It’s smaller than Mercury, in other words.
Kepler 37b, as it’s name implies, was found from data taken by the Kepler mission. The parent star, Kepler-37, is interesting because it’s the densest star in which solar-like oscillations have been detected. Just as a measurement of the frequencies of a musical instrument allows you to determine some of the properties of that instrument, the characteristic “ringing” of a star allows astronomers to determine some of the star’s properties with great accuracy. In this case astronomers were able to determine the radius of Kepler-37 with great precision, and this in turn allowed them to determine the radius of its planets with precision. Transit signals suggest that Kepler-37 has three planets. Kepler 37d has a radius about 1.99 times that of Earth’s; Kepler 37c has a radius about 0.74 times that of Earth’s; and Kepler 37b has a radius just 0.3 times that of Earth’s. It’s not much bigger than our Moon – and Kepler detected it!
An artist’s impression of Kepler 37b (Credit: NASA)
For every 200 stars that Kepler studies you’d expect to see the transit signal in the data of perhaps one star. So the fact that the astronomers were able to identify this sub-Mercury-sized object does rather tend to suggest that small planets are extremely common.
The Kepler mission team announced exciting new results on 7 January 2013, at the 221st meeting of the American Astronomical Society. As I’ve explained in other posts (such as this one here), the team aims to detect exoplanets by staring at more than 150,000 stars in a fixed part of the sky. The Kepler telescope looks for regular dips in the brightness of these stars, variations that might be caused by the presence of a transiting planet.
The technique used by the Kepler team has been tremendously successful. On 7 January Christopher Burke, a Kepler scientist at the SETI Institute, announced the discovery of 461 new planetary candidates. So, as of the time of writing, Kepler has found 2740 potential planets orbiting 2036 stars. This is a hugely impressive number, when you consider that it wasn’t such a long time ago that people were debating whether exoplanets existed at all and, if they did, whether it would be possible to detect them.
The sizes of the planetary candidates discovered by Kepler
The AAS presentation that really caught the attention of the world’s press, however, was that given by Francois Fressin of the Harvard-Smithsonian Center for Astrophysics. Fressin has tried to estimate the fraction of stars that possess of Earth-sized planets, based on the Kepler data. Kepler will inevitably see only a small fraction of exoplanets because the transit technique only picks up those planetary systems in which the orbital plane is more or less side on to our view. If the orbital plane is slanted by more than a few degrees to our line of sight, the planets we won’t see the planets transit and there’ll be no drop in brightness. On the other hand, Kepler is seeing regular brightness fluctuations that are not due to transits; for example, a non-variable star might be extremely close in our line of sight to a regular variable, which would give a similar signal to a transit. After Fressin had corrected for both these effects he arrived at the following estimate: one in six stars host an Earth-sized planet. If that is indeed the case, there are about 17 billion Earth-sized planets in our Galaxy and it’s certain that some of these will be in the habitable zone.
If we’re looking to explain the Fermi paradox, we now know that we can’t invoke a paucity of Earths!
I’m old enough to remember a time when some people thought we’d never discover planets beyond the solar system. It’s not that long ago that astronomers first managed to confirm the existence of a few exoplanets. Techniques improved, and the number of known exoplanets started to increase rather rapidly. Then the Kepler mission was launched, and the number of candidate exoplanets simply rocketed (at the time of writing, Kepler has identified 2321 exoplanetary candidates). That’s impressive progress.
Now that astronomers have identified so many exoplanets, it becomes possible to design a mission that can study those bodies in more detail. That’s precisely what the CHEOPS mission will do. (Yes, CHEOPS is yet another acronym. This one stands for CHaracterising ExOPlanets Satellite.) ESA have selected CHEOPS for study as the first “small” or S-class mission.
If all goes well, CHEOPS will launch in 2017 and study stars that we know have planets around them. The satellite will monitor a star’s brightness, looking for the characteristic dip in brightness as a planet transits. This measurement will allow astronomers to determine the radius of the transiting planet; if the planet’s mass is already known from other measurements then astronomers will be able to calculate the planet’s density. This in turn will provide clues about the planet’s internal structure. CHEOPS will tell us a lot about the formation and evolution of planets with a similar mass to Earth. And, just as Kepler has provided targets for CHEOPS to study, CHEOPS in turn will provide targets for follow-up study by the next generation of powerful telescopes such as E-ELT.
An artist’s impression of CHEOPS. The satellite will be placed in a Low Earth Sun-Synchronous orbit at an altitude of 800km. Its 33cm telescope will observe in the range 400-1100nm.
(Credit: University of Bern)
Solution 41 in Where is Everybody? is entitled “Earth’s system of plate tectonics is unique”. Why should volcanism and plate tectonics have anything to do with life? Well, I don’t plan to go into detail here – you can always read the book – but there are various ways in which plate tectonics might play a role in the emergence of life and the long-term viability of a planet. In Earth’s case, for example, volcanoes vomit lots of CO2: this is a greenhouse gas, which could have kept the surface of our young planet warm enough to allow water to remain in the liquid phase. As Earth aged, volcanism and plate tectonics played a key role in the carbon cycle: plate tectonics captures CO2 and takes it into the planet’s interior while volcanism releases CO2 back into the atmosphere. This carbon cycle, so it’s believed, plays a role in stabilising Earth’s climate – and a stable climate in turn may have been necessary for the development of complex life. And Earth’s magnetic field, which is driven by a rotating liquid metallic core, protects the atmosphere from high-energy cosmic radiation.
I doubt that in a universe as large as ours there is only one planet, Earth, that possesses plate tectonics. Recent work, however, suggests that the phenomenon may not be common – at least not on “super Earths” (rocky planets with a mass between 2-10 times that of Earth).
A team led by Vlada Stamenkovic has studied how temperatures within a super Earth are likely to change over time. There are some uncertainties in the work, but in the scenarios that Stamenkovic’s team studied it turned out that super Earths cool slowly: plate tectonics is unlikely to occur and a slow planetary-core formation (or even no core formation at all) means that magnetic fields are likely to be absent.
I’ve heard lots of people say recently that rocky super Earths, orbiting in the habitable zone, might be suitable places for life to originate and prosper. I think it much more likely that those planets are lifeless.
For full details of the work, see: Stamenkovic, V., Noak, L., Breuer, D. and Spohn, T. (2012) The influence of pressure-dependent viscosity on the thermal evolution of super-Earths. Astrophysical Journal, 748: 41.
A recent paper by Guillen Anglada-Escudé (A planetary system around the nearby M dwarf GJ 667C with at least one super-Earth in its habitable zone) announces the discovery of yet another planet in the habitable zone of its star. The planet, GJ 667Cc, is a super-Earth (with a mass in excess of 4.5 Earth masses). It orbits its star with a period of 28.15 days.
This is a particularly interesting finding because the planet is orbiting an M-class dwarf. Thing is, more than three-quarters of the stars in our neighbourhood are M-class stars (mainly dwarfs, though there are a some red giants too). If rocky planets in the habitable zone are common around M-class dwarfs then there are lots of potentially habitable planets out there!
I doubt that M-class dwarfs are ideal locations for advanced life forms, however.
M-class dwarfs pump out much less energy than our Sun. Thus a planet in orbit around such a star must be close to the star if it is to have a surface temperature that’s similar to Earth’s. That in turn means that the planet is much more likely to be tidally locked. The problem is that tidal locking leads to extremes of climate: the star-facing side of a tidally locked planet would be in permanent light, the other side would be in neverending night. And that in turn means that surface temperatures actually wouldn’t be like Earth’s. One side would be extremely hot, the other side frigid. Furthermore, the temperature on the frigid side would be so low that any atmospheric gases would be frozen out; the day side would be left dry. (If a large planet possessed a moon, however, then conditions on the moon might be more hospitable: a moon that was tidally locked to its planet would have a day-night cycle as it orbited the planet.)
Another problem with M-class dwarfs is that they can be quite variable. Starspots are common, and they reduce the star’s energy output by up to 40% for significant periods; flares are less common, but when they occur they can double the star’s brightness in a matter of minutes.
The discovery of GJ 667Cc suggests that the Galaxy might contain billions of rocky planets where liquid water can exist. But whether those planets can host life … well, that’s a different question. Soon the search for exoplanets needs to become a search for biosignatures.