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Miller: Harold Jeffreys Lecture 2010 Do extrasolar planets go bang? In the 2010 RAS Harold Jeffreys Lecture, Steve Miller discusses how a simple hydrogen molecule controls the fate of the giants. 1: Artist’s impression of the Jupiter-size extrasolar planet HD 189733b being eclipsed by its parent star. (ESA, NASA, M Kornmesser [ESA/Hubble] and STScI) I t has existed for almost as long as molecules have existed. It is the product of the most abundant species in the universe. And it may control the fate of giant planets, whether they stay far away, like our solar system’s Jupiter, or spiral in close to their central star, like many of the extrasolar planets that have been discovered in recent years. “It” is H3+ and this year marks the centenary of its discovery – at least as far as humankind is concerned. The story of our coming to understand this simple molecular ion has already been well told in the pages of A&G by Helge Kragh (2010). Briefly, in the years leading up to the first world war J J Thomson (figure 2) was studying “rays of positive ions”. In 1911 he noticed that he had an ion with a mass-to-charge ratio of three produced alongside H+ and H 2+ in hydrogen discharge tubes. Candidates included a new atomic species, that did not really fit into the periodic table, carbon 4-plus, or a triatomic version of hydrogen, H3. By 1912 Thomson had pretty much ruled out the first two options and had decided on the ion being ionized H3 or H3+. The downside of this choice was the difficulty in conceptualizing a stable molecule of monovalent H in which one or more of the atoms had somehow to become multivalent. Niels Bohr attempted to calculate how this might happen in 1918, using the “old”, semi-classical version of quantum mechanics. His linear model of H3+ was unstable, however, and it was not until the 1930s that Joseph Hirschfelder and co-workers A&G • October 2011 • Vol. 52 Harold Jeffreys Lectures The Lectures commemorate Sir Harold Jeffreys’ long career in mathematics and the physics of the Earth. The Harold Jeffreys Lecture is given annually on a topic from solid-Earth geophysics, solar physics, solar–terrestrial physics, or planetary sciences. – when “modern” quantum mechanics were available – were able to produce a stable triangular H3+ molecule in which each of the three hydrogen nuclei shared the two available electrons in a “delocalized” molecular orbital. In the 1960s it was shown that the ground electronic state has “D3h symmetry” – i.e. an equilibrium geometry that is an equilateral triangle. Later calculations showed that there is an unstable excited electronic state of H3+ that is linear, but that transitions to this state are “forbidden”. The best way to identify and study a molecule is by its spectrum. The upshot of these calculations, however, is, firstly, that H3+ does not have a measurable electronic, visible spectrum. Secondly, a centri-symmetric molecule composed of three atoms of the same species does not have a permanent dipole, and cannot have a pure rotational, microwave spectrum. In the absence of a visible or microwave spectrum the hunt was on for the infrared, rotation-vibration spectrum of H3+. The development of infrared diode lasers and discharge technology all played their part in this hunt, and – in 1980 – Takeshi Oka (then at the Herzberg Institute, Ottawa, now at the University of Chicago) found the very first lines (figures 3 and 4). And what a spectrum it was. Ro-vibrational spectra Normally, ro-vibrational spectral lines are clumped into three groups around the central frequency: P branch lines, in which the molecule hops from a rotational state J to state J–1 as it jumps up the vibrational ladder, are at lower frequencies; a tightly clustered Q branch, with J not changing, are found around the “vibrational band origin”; and R branch lines are at higher frequencies, with both the vibrational quantum number, v, and the rotational quantum number, J, increasing by 1. H3+ had lines all over the place, and it was hard to sort out the pattern. Normally, spectroscopists make use of a variant on quantum mechanics called perturbation theory to explain ro-vibrational spectra. The basic model is that the molecule vibrates as a harmonic oscillator, regular as a watch-spring, and rotates as a rigid rod of fixed length. Perturbations from this ideal model can be accommodated by letting the vibrations become a bit anharmonic, allowing for the molecule to stretch as it rotates faster and faster, and letting the vibrations couple weakly to the rotations. Perturbation theory works if the perturbations represent a small deviation from the ideal harmonic-oscillator/rigid-rotor model. But not in the 5.11 Miller: Harold Jeffreys Lecture 2010 case of H3+ since the perturbations soon became the determining feature, not the basic model. Better understanding of what was happening came from an accurate calculation of the electronic structure of the molecule, by Wilfred Meyer, Peter Botschwina and Peter Burton, and a calculation from first principles, not perturbation theory, of the highly coupled rotations and vibrations that would result from this structure, using a technique developed by Jonathan Tennyson and Brian Sutcliffe. These showed H3+ to be a very “floppy” creature, liable to go from triangular to linear if given enough vibrational energy, and to spin itself to destruction as the rotations increased in ferocity. And as a result, H3+ broke most of the rules of normal spectroscopy. For a start, spun up enough, the molecule would develop a small dipole and – theoretically – develop a microwave spectrum. Secondly, given just a quantum or two of vibrational energy, it would give rise to “forbidden” lines in which the normally infrared “dark” symmetric stretch or breathing mode, which maintains the equilateral triangle symmetry, lit up. Moreover overtone bands, for which the vibrational quantum v number changes by two or more, turned out to be (almost) as intense as the fundamental bands, for which ∆v is just one. That means that H 3+ can radiate from the microwave through the infrared to the visible, although it is brightest in the near-infrared around 3–5 µm. In turn, that means that H 3+ can cool any gas cloud or atmosphere in which it is found with temperatures ranging from a few tens (albeit very slowly) to a few thousands of degrees (very rapidly). This property is one of the key features of H3+ in determining the role it plays in astrophysical environments. Hydrogen atoms make up 9 out of every 10 atoms in the universe, and molecular hydrogen – H 2 – is the most abundant species. H3+ can be formed whenever H 2 gets ionized, provided the gas density is high enough (a few million mol ecules per cubic metre is more than enough). Two Californian chemists, Thorfin Hogness and E G Lunn, first came up with the required sequence of reactions: H 2 + 15.2 eV ionizing particle/radiation → H 2+ + e – H 2+ + H 2 → H3+ + H This ubiquitous reaction set can be found at work in laboratory hydrogen discharges, such as those used by J J Thomson and Takeshi Oka and in space plasmas throughout the universe. In the early universe, around 300 000 to 400 000 years after the Big Bang, temperatures had dropped enough for electrons and atomic nuclei to join up to form neutral atoms – the recombination era. Matter and radiation became decoupled, and the newly formed atoms could take part in chemistry to make molecules. Since helium nuclei combine with electrons at higher temperatures than do protons, probably the first molecule to form was HeH+, a molecular ion 5.12 2 3 4 2: J J Thomson discovered H3+ in 1911. 3: Takeshi Oka found the first lines of the infrared, rotation-vibration spectrum of H3+ in 1980. 4: The plasma tube used in Oka’s ‘Ion Factory’. that was not very stable or productive. As the temperature cooled further, H atoms formed, and it became possible to form H 2 molecules. This step was crucial to forming the earliest stars. Atoms are not very efficient at cooling the proto-stellar gas cloud below about 8000 K, since they only have electronic transitions, with energy gaps that are only accessed at high temperatures. But the ro-vibrational transitions of molecules can get the gas temperature down to 1000 K or less, at which point gravitational attraction can overcome the kinetic forces and a stellar nucleus can form. In a universe consisting of predominantly H and He atoms, plus a smidgeon of deuterium and lithium, the main molecule to form up is H 2 . H 2 is a very inefficient coolant. Nor was it very abundant – some 100 million years after the Big Bang only one out of every 200 000 of the H atoms in the universe had combined to form H 2. So cloud collapse to form the first stars took something like 15 million years. It was at this time that H 3+ made its first appearance and showed some of the properties that make it important today (Glover and Slavin 2009). Although it was never more than a billion times less abundant than H 2 , this molecular ion could, under certain conditions, contribute as much as 1% of the total gas cooling. Molecule for molecule, H 3+ is at least 10 million times as effective as a coolant as H 2 , a property that is crucial to understanding the atmospheres of giant planets like Jupiter, where it was first identified in 1988. Models predict that the stars of the early universe were either very large or very small, depending on just how the gas clouds clumped up. Very small, metal-poor stars, formed before the many enrichments of the interstellar medium by their larger cousins going supernova or otherwise shedding their outer layers, would have had H3+ in their stellar atmospheres. For the very smallest of these, H3+ slows their evolution to the point that some of them may still be around today. In the interstellar medium (ISM) itself, H3+ plays a vital role in the initiation of chains of chemical reactions that would otherwise never get going (figure 5). Temperatures in the gas of the Orion and Taurus molecular clouds, for instance, can be as low as 10–20 K. Chemistry between neutral atoms and molecules at those temperatures is a painfully slow affair. There is simply no energy available to get over the barriers to turning one chemical species into another. But ion-neutral reactions are fast: ions attract neutral atoms and molecules by polarizing their electronic cloud structure; the closer they get, the more polarization occurs and the faster they want to get to know one another. Although the A&G • October 2011 • Vol. 52 Miller: Harold Jeffreys Lecture 2010 5: H3+ in a web of chemical reactions in the interstellar medium. existence of H3+ in the ISM had been proposed in the 1960s, it took another 30 years before Oka and Tom Geballe (1996) first detected key fingerprint lines in gas clouds towards the sources GL 2136 and W33A. As the child of the most abundant of all molecules, H 2 , H3+ is formed whenever ionizing rays and particles can penetrate a molecular cloud. Once formed, it is a very reactive species: almost everything has a higher affinity for protons than H 2 , so reactions that involve H3+ + X → XH+ + H 2 where X might be a carbon, oxygen or nitrogenbearing species, can initiate sequences that end up with some very complex chemistry occurring. This may get as far as the formation of pre-biotic molecules, the building blocks of life itself. Detection on Jupiter Although its astrophysical importance was first considered in terms of its chemical role in the ISM, it is the physical role played by H3+ in planetary atmospheres that is perhaps even more important. And in its very first detection outside of the laboratory, in the aurorae of the giant planet Jupiter (figure 6), its rather particular physical properties manifested themselves. The detection of H3+ in Jupiter’s upper atmo sphere was, for a start, completely fortuitous. In September 1988, a team of astronomers led by Pierre Drossart of the Observatoire de Paris, Meudon, had the idea of studying auroral emission from Jupiter – a massively supercharged version of Earth’s own northern and southern lights – making use of the Canada–France–Hawaii telescope at the Mauna Kea Observatory in Hawaii. The team intended to look for infrared auroral lines of molecular hydrogen: ultraviolet emission had already been detected by the International Ultraviolet Explorer satellite (Clarke et al. 1980) and the Voyager spacecraft (Hamilton et al. 1980) some years earlier. A&G • October 2011 • Vol. 52 6: H3+ emission from the auroral regions of Jupiter, mapped by the NASA Infrared Telescope Facility. (Jack Connerney and Takehiko Satoh) Drossart’s team was looking for very weak H 2 lines that are normally forbidden in the infrared, but that can occur as a result of a small electrical property known as the quadrupole moment. These quadrupole-induced changes in the vibrational and rotational state of H 2 result in transitions that emit infrared photons about once every 100 000 seconds for an individual molecule, or even slower. The team particularly wanted to detect a line known as the v = 1 → 0 S(1) line that has a wavelength of 2.122 µm. Indeed, this line was found. But it was surrounded by a series of sharp lines, some of which were even stronger than the H 2 lines. The lower regions of Jupiter’s atmosphere either absorb radiation or give rise to broad emission features; sharp lines can only come from high in the atmosphere where the gas density is low, and hence the collision rate that broadens lines much lower. The upper atmosphere is almost pure hydrogen – H atoms and H 2 molecules – plus some helium. So the smart money had to be on this series of lines being some form of hydrogen molecule, and H3+ would seem a natural candidate. Except, the wavelengths were roughly half the wavelengths that Oka had measured back in 1980. By a roundabout route, news of the Jupiter spectrum got to Jonathan Tennyson’s group at University College London. The UCL group had just been successfully calculating the H3+ spectrum to high accuracy, and their calculations included not only wavelengths but line strengths, which determine how fast the molecule can make transitions that give rise to rotation-vibration transitions and infrared spectra. Normally, spectroscopists consider that only transitions that involve the molecule changing its vibrational state by one quantum (∆v = ±1) should be allowed to occur. What the UCL calculations showed was that the line strengths of the “forbidden” overtone spectrum, ∆v = ±2, were as large as the allowed fundamental, ∆v = ±1, spectrum. And the wavelengths were roughly half those of the fundamental spectrum that Oka had measured in the laboratory. That meant that, once excited into its v = 2 second vibrational level, an H3+ molecule could radiate back down to the ground state roughly once every 0.01 seconds, at least 10 million times faster than its excited H 2 neighbour. So even though there was only roughly one H3+ for every million H 2 molecules, the ion lines were equally strong, if not stronger. And there were far more of them, because infrared spectra become richer as the number of atoms in a molecule increases. Pretty soon it was realized that as much as the overtone spectrum was very strong, the fundamental spectrum would be even stronger, as it is easier to excite molecules into the first, v = 1, vibrationally excited state than the v = 2 state. What was soon discovered was that the emission spectrum of Jupiter at wavelengths between about 3 and 4 µm was dominated by H3+ lines. Methane in the jovian middle atmo sphere absorbed both the incoming sunlight and the upwelling infrared radiation from the lower reaches of Jupiter, so all that could be seen were sharp emission lines from high in the upper atmosphere. The background was sufficiently dark and the lines sufficiently bright that even at relatively low resolution, images of Jupiter could be obtained that showed nothing but H3+ emission. These images showed a complex structure around the high latitudes of the planet. Arcs of bright aurorae – some hundreds of times more intense that Earth’s northern and southern lights – ringed the poles. Poleward of these “auroral ovals” were more diffuse emissions: some – on the eastern, dusk side of the planet – were sometimes as bright as the main auroral ovals themselves; to the west, dawnward, the polar regions looked relatively dark, although many bright features could still be made out. At latitudes just below the main oval, bright spots and faint trails could be made out. All of these features told stories about how Jupiter’s giant magnetosphere was interacting with the planet’s electrically charged upper atmosphere through a series of multi-million-amp current systems. Earth’s magnetic field carves out a region of interplanetary space which it dominates. As particles from the Sun – the solar wind – flow past our home planet they crash into magnetic field lines that flow from south to north to form a “bowshock”, akin to that formed by an ocean liner through the sea. The pressure of the solar wind creates this stand-off about 60 000 km or so from Earth in the direction of the Sun. In the anti-sunward direction, Earth’s magnetic field is swept downstream to form a wake – the magnetotail – that stretches roughly a million kilometres. This is the region known as the terrestrial magnetosphere. 5.13 Miller: Harold Jeffreys Lecture 2010 7: Simulations of the bottom (left panels) and top (right panels) of the upper atmosphere of “Jupiter” brought into 0.16 AU from the Sun (top panels) and 0.14 AU from the Sun (bottom panels). The temperature scales are on the right of each panel. Note that the scale on the bottom right panel runs from 19 000 K to 23 000 K. 1470 1468 1466 1464 1462 1460 distance = 0.16 AU pressure = 121.62 nbar distance = 0.16 AU, pressure = 5.52 pbar 1540 1530 1520 1510 1500 1490 distance = 0.14 AU pressure = 121.62 nbar Jupiter’s magnetism is approximately 1000 times greater than Earth’s. The jovian magnetosphere is consequently much larger: the stand-off point is anything up to 7 million km from the planet in the sunward direction, and the magnetotail has been observed to stretch anti-sunward as far as the orbit of Saturn, some 750 million km downstream. No wonder the jovian aurorae, powered by its magnetosphere, are so much brighter than their feebly flickering terrestrial counterparts. There are other differences, too. On Earth, the main auroral oval – roughly speaking – maps to a magnetospheric region where the magnetic field lines switch from being closed, as around a schoolchild’s bar magnet, to being open, swept away by the solar wind. On Jupiter, however, there is a more complex system at work. Jupiter’s closest large moon, Io, orbits just 350 000 km above the surface of the giant planet. As a result, Io has its innards continually stirred by the giant planet’s gravitational field such that it is the most volcanic body in the solar system. Gas and dust pour at the rate of one tonne per second into the jovian magnetosphere. Charged up by solar ultraviolet radiation and collisions with other charged particles, this gas and dust is swept up by the planet’s magnetic field so that it rotates not once every 42 hours, like Io, but in just under 10 hours, like the planet itself. Jupiter and Io The additional gas and dust momentum is supplied by Jupiter’s ionosphere – H+ and H3+ ions – at the foot of the magnetic field lines: ionospheric ions drag the magnetic field through the gas and dust torus created by Io; neutral atoms and molecules in the upper atmosphere, rotating at the normal rate of the planet, drag the ions along with them. Gas and dust drift out centrifugally through the magnetosphere to form a disc in the planet’s equatorial plane like a ballet 5.14 3800 3600 3400 3200 3000 2800 2600 2400 x10 4 2.3 2.25 2.2 2.15 2.1 2.05 2 1.95 1.9 distance = 0.14 AU, pressure = 5.52 pbar dancer’s tutu. But there comes a point where the momentum required to keep this skirt in corotation with the planet is too much for the coupled ionosphere–thermosphere to maintain. The skirt lags behind, magnetic fields lines become “bent”, ionospheric ions slip behind their neutral counterparts, and mega-amp currents flow from plasmasheet to planet, firing high-energy electrons into the upper atmosphere. In a ring around the pole, and mapping magnetically to the regions in the magnetosphere where co-rotation has broken down, bright aurorae glow. This is Jupiter’s main auroral oval. Poleward of this oval, on the dark, dawn side, there is a region that is roughly equivalent to the terrestrial oval. The brighter duskward emissions come from regions on the flanks and down the tail of the magnetosphere. The bright spots at lower latitudes maps to the Galilean moons, especially Io and Ganymede, and the trails they leave in the magnetosphere as it sweeps past them. Enormous currents, driven by megavolt electric fields, flow through the upper atmosphere, generating temperatures hundreds of degrees greater than sunlight alone as a result of Joule heating and the friction of ionospheric ions dragged backwards through an otherwise intransigent neutral thermosphere. Absolutely key in all these processes is our H3+ molecular ion. It lights up the atmosphere where these processes are occurring. It provides the ions (and – by electrical balance – the electrons) that allow currents to flow. Dragged against the natural rotation of the planet by straining magnetospheric field lines, it crashes into the neutral atmosphere to create more energy and heating. H3+ is not only the major component of the ionosphere of Jupiter, but also of Saturn and Uranus, although it has yet to be found on Neptune (Melin et al. 2011). Its heating effect may help to explain why the upper atmospheres of these giant planets are so hot. But its effect as a strong radiator has another effect, too. Although insufficient to balance the Joule heating and ion drag inputs generated in Jupiter’s atmosphere, and probably that of Saturn and Uranus, too, it is enough to balance the energy that comes into these planets as a result of electrons being fired in from the magnetosphere. This “H3+ thermostat effect” plays an important role in some extrasolar planets, particularly those that orbit close to their central star. Nowadays, exoplanets are everywhere you look; at the end of July 2011, the Exoplanet Encyclopedia lists 564 (http://exoplanet.eu), and NASA’s Kepler satellite is claiming more than 1200 potential exoplanets, although only 1% of them have so far been confirmed. Some exoplanets are around stars like our Sun, some around stars that are a bit hotter or cooler, some are even reported to be free-floating, detached from any stellar-planetary system at all. But the first detection of such a planet only dates back to the 1980s (Campbell et al. 1988, and this was not really accepted at the time), the first credible detection back to 1992 (Wolsczan and Frail 1992), and the first detection that looked anything like what people were expecting exoplanets to look like to 1995 (Mayor and Queloz 1995). This is a field that has exploded in less than 20 years. So, do (some of) the planets themselves explode – “go bang”? Some might have every reason to do so. Planets such as HD 209458b are Jupitersized affairs that orbit very close to their central star – in this particular case, less than 1/100th of the distance of Jupiter to the Sun. Since the radiation field falls off as the square of the distance, this means that HD 209458b gets some 10 000 as much heating and ionization as does Jupiter. As a result, this planet, which has about 70% of the mass of Jupiter, has a radius about 1.4 jovian radii. Even more importantly, it has been shown to have a very extended atmosphere, going out to three planetary radii and more, well beyond the point where the planet’s own gravity can stop gas being sucked into the star itself. Given our observation of Jupiter, with its tightly held atmosphere extending just a few thousand kilometres, no more than 10% of its notional planetary radius (for a gas giant, without a solid surface, this is usually defined at that level in the atmosphere where the pressure is 1 bar), given our observation of HD 209458b, with its bloated, escaping atmosphere that extends 300 000 km into space, it is an interesting question to ask just where and when the change might take place? And how? This is where the H 3+ thermostat plays a crucial role. Consider this thought experiment: let us move Jupiter in from its chilly 5.2 AU from the Sun so that it now resides on the same orbit as Earth (by definition, 1 AU). The radiation flux from the Sun will have increased by some 27 times. The heating of the planet’s upper atmosphere will have increased by this amount. A&G • October 2011 • Vol. 52 Miller: Harold Jeffreys Lecture 2010 Importantly, there will also be 27 times the ionization as a result of extreme ultraviolet (EUV) radiation. In an upper atmosphere composed almost entirely of H 2 , the result will be a much greater concentration of H 3+. And this H 3+ will radiate out into space (most of) the increased heating. Bring Jupiter another 2.5 times closer to the Sun, inside of the orbit of Mercury, and the increased production of H3+ and its ability to radiate heat away still keeps the planet Jupiter-like rather than HD 209458b-like. This mechanism keeps working even if the planet gets another 2.5 times closer, in to just 0.16 AU (or 24 million km from the Sun). But, by this point, the average temperature of the upper atmosphere has reached a toasty 3000 K or more, and molecular hydrogen is beginning to break down to its atomic constituents. If Jupiter gets just another 3 million km closer to the Sun, then a vicious spiral occurs: H 2 breaks down; H3+ forms less easily; cooling becomes less efficient; the temperature increases; even more H 2 breaks down, etc. At 0.14 AU from the Sun, Jupiter would be transformed rapidly from being jovian to being HD 209458b-like, a bloated version of its former self, with every chance of its atmosphere escaping off into space (Koskinen et al. 2007). But only if its orbit is circular. HD 17156b is a remarkable planet. Its central star is a bit older than the Sun, at 5.7 gigayears, and about 2.5 times brighter. The planet is about the same size as Jupiter, but three times more massive, making it a very dense gas giant indeed. On average, HD 17156b orbits at a distance of 0.16 AU, just inside the stability limit for a circular orbit. But its orbit is not circular. HD 17156b has one of the most eccentric orbits of any planet known, with e = 0.67. This means that at its furthest from its star, HD 17156b is a little over 5% of the distance that Jupiter is from the Sun (0.27 AU). But at its closest, the planet is a mere 1% of the Jupiter–Sun distance (0.052 AU). The effect is that the amount of starlight HD 17156b receives during the course of its 21-day orbit varies by a factor of 27; at its closest approach the planet gets over 25 000 times as much radiation as does Jupiter. Surprise stability Amazingly, HD 17156b is still stable throughout the course of the orbit (Koskinen et al. 2009). If, as is quite probable, the planet is Jupiterlike in terms of the composition of the upper atmosphere, then the H3+ keeps the temperature at the top to a comfortable 2000–3000 K, cool enough for molecular hydrogen to remain stable. But even if there is much more atomic hydrogen in the mix, so less H3+ cooling, and the upper boundary temperature is in the 20 000– 30 000 K range, HD 17156b does not spend enough time during its orbit inside of 0.1 AU for its atmosphere to escape. A&G • October 2011 • Vol. 52 Table 1: The ‘Sun in Time’ targets and thermospheric stability limits name of star EK Dra 1 p UMa 1 HD type age (Gyr) FXUV (erg s–1 cm–2) limit (AU) 129 333 G1.5 V 0.1 513.5 1.68 72 905 G1.5 V 0.3 129.3 0.84 k Cet 20 630 G5 V 0.65 51.1 0.53 b Com 114 710 G0 V 1.6 16.0 0.30 Sun … G2 V 4.58 4.64 0.16 b Hyi 2151 G2 IV 6.7 2.9 0.13 Characteristics of some of the “Sun in Time” stars and the stability of Jupiter-type planets. What, then, of HD 209458b, with its extended atmosphere? It turns out that, although H3+ cooling cannot play a role in the extended atmo sphere, which is mainly atomic hydrogen and ions, there is always a molecular-hydrogen layer where it does form and it does cool down the atmosphere. And, coupled with other effects, this is enough to stop the planet evaporating too rapidly: HD 209458b-type planets are generally stable at least for the lifetime of a stellar/solar system (Yelle 2004). Extrasolar planets do not go “bang”, or do they? Which brings us back to our own Jupiter. Earth today is bathed in a gentle sunlight that creates a Goldilocks world, perfect for the development and maintenance of life. But things were not always so benign. In its early years, our Sun was a much fiercer beast, delivering searing amounts of radiation, particularly in the ultraviolet and X-ray regions of the spectrum. Those are key regions of the spectrum since they are responsible for ionizing planetary atmospheres, with the potential to create H3+ in hydrogen-rich gas. The “Sun in Time” project has attempted to quantify the likely evolution of our home star by looking at analogues of the Sun of known ages (Ribas et al. 2005). When the Sun is about half as old again as its current 4.58 billion years, its output in the X-ray to EUV range (F XUV) will have dropped to less than 60% of its current level. At a third of its current age, the Sun was 3.5 times brighter than now. At a juvenile 100 million years old, the Sun’s XUV output was 110 times fiercer. During its lifetime, then, the “stability limit” at which H3+ cooling could keep a giant planet from going from Jupiter-like to HD 209458blike would gradually have shrunk from just outside of the orbit of Mars, at a solar age of 100 million years, to the 24 million km that is the “safe” distance now (see table 1). But you only have to go back a bit further in time from the 100 megayear-Sun represented by EK Draconis, and the stability limit would have been even further out into the solar system (see table 1 for “Sun in Time” stars and their stability limits, for Jupiter-type planets). The results from the “Sun in Time” project actually fit quite nicely to the formula log10 [F EUV(t) / F EUV(4.58)] ~ 1.23 × log10 [4.58 / t] where t is the age of the Sun in gigayears. So, some time between a solar age of 10 million years and 100 million years, at a time when Jupiter would have been scrambling to grab as much of the gas of the solar nebula as was still available and Earth was little more than a coagulating rubble pile, the stability limit would have been outside of the current jovian orbit. At that point, and for quite some considerable time, the very existence of the Jupiter would have been a battle between solar radiation blowing up the atmosphere and H3+ throwing that radiation back out into space. Without the cooling balm of H3+, how different our solar system might have been! ● Steve Miller is Professor of Science Communication and Planetary Science at University College London, UK. His book The Chemical Cosmos: a Guided Tour is published by Springer in its “Astronomers’ Universe” series. This lecture was originally delivered at Burlington House on 12 November 2010. References Campbell B et al. 1988 Astrophys. J. 331 902–911. Clarke J T et al. 1980 Astrophys. J. 240 696–701. Geballe T R and Oka T 1996 Nature 384 334–335. Glover S C O and Slavin D W 2009 MNRAS 393 911–948. Hamilton D C et al. 1980 Geophys. Res. Lett. 7 813–816. Koskinen T T et al. 2007 Nature 450 845–848. Koskinen T T et al. 2009 Astrophys. J. 693 868–885. Mayor M and Queloz D 1995 Nature 378 355–359. Melin H et al. 2011 MNRAS 410 641–644. Ribas I et al. 2005 Astrophys. J. 622 680–694. Wolszczan A and Frail D A 1992 Nature 355 145–147. Yelle R V 2004 Icarus 170 167–179. Further reading Much of the material and many of the key references for this article can be found in these review articles: Kragh H 2010 A&G 51 6.25–6.27. Miller S et al. 2000 Phil. Trans. R. Soc. Lond. A358 2485–2502. Miller S et al. 2006 Phil. Trans. R. Soc. Lond. A364 3121–3128. Oka T 1983 Molecular Ions eds Miller T A and Bondeybev V E (North Holland, Amsterdam) 73–90. Oka T 1992 Rev. Mod. Phys. 64 1141–1149. 5.15