Barrie W. Jones

Infrared spectra of protostars - Composition of the dust shells

Nearly complete 2 to 13 ..mu..m spectra are presented for 13 compact infrared sources associated with molecular clouds, as well as partial spectra of six additional objects. The spectra resemble blackbodies with superposed absorption features from 2.8 to 3.5 ..mu..m, at 6.0 and 6.8 ..mu..m, and in the silicate band centered near 9.7 ..mu..m. Correlations among the features are studied in an attempt to confirm possible identifications. A good correlation between the deepest part of the absorption at 3.1 ..mu..m, its long wavelength wing, and the 6.0 ..mu..m features suggests that all may be due to large amorphous water ice particles. The relatively poor correlation between the 3.4 and 6.8 ..mu..m optical depths adds no evidence to support the suggestion that these bands may be due to CH bonds.

The evolution of habitable zones during stellar lifetimes and its implications on the search for extraterrestrial life

A stellar evolution computer model has been used to determine changes in the luminosity L and effective temperature T e of single stars during their time on the main sequence. The range of stellar masses investigated was from 0.5 to 1.5 times that of the Sun, each with a mass fraction of metals (metallicity, Z ) from 0.008 to 0.05. The extent of each stars habitable zone (HZ) has been determined from its values of L and T e . These stars form a reference framework for other main sequence stars. All of the 104 main sequence stars known to have one or more giant planets have been matched to their nearest stellar counterpart in the framework, in terms of mass and metallicity, hence closely approximating their HZ limits. The limits of HZ, for each of these stars, have been compared to their giant planet(s)s range of strong gravitational influence. This allows a quick assessment as to whether Earth-mass planets could exist in stable orbits within the HZ of such systems, both presently and at any time during the stars main sequence lifetime. A determination can also be made as to the possible existence of life-bearing satellites of giant planets, which orbit within HZs. Results show that about half of the 104 known extrasolar planetary systems could possibly have been housing an Earth-mass planet in HZs during at least the past billion years, and about three-quarters of the 104 could do so for at least a billion years at some time during their main sequence lives. Whether such Earth-mass planets could have formed is an urgent question now being investigated by others, with encouraging results.

Habitability of known exoplanetary systems based on measured stellar properties

Habitable planets are likely to be broadly Earth-like in composition, mass, and size. Masses are likely to be within a factor of a few of the Earths mass. Currently, we do not have sufficiently sensitive techniques to detect Earth-mass planets, except in rare circumstances. It is thus necessary to model the known exoplanetary systems. In particular, we need to establish whether Earth-mass planets could be present in the classical habitable zone (HZ) or whether the giant planets that we know to be present would have gravitationally ejected Earth-mass planets or prevented their formation. We have answered this question by applying computer models to the 152 exoplanetary systems known by 2006 April 18 that are sufficiently well characterized for our analysis. For systems in which there is a giant planet, inside the HZ, which must have arrived there by migration, there are two cases: (1) where the migration of the giant planet across the HZ has not ruled out the existence of Earth-mass planets in the HZ; and (2) where the migration has ruled out existence. For each case, we have determined the proportion of the systems that could contain habitable Earth-mass planets today, and the proportion for which this has been the case for at least the past 1000 Myr (excluding any early heavy bombardment). For case 1 we get 60% and 50%, respectively, and for case 2 we get 7% and 7%, respectively.

Prospects for Habitable “Earths” in Known Exoplanetary Systems

We have examined whether putative Earth-mass planets could remain confined to the habitable zones (HZs) of the 111 exoplanetary systems confirmed by 2004 August. We find that in about half of these systems there could be confinement for at least the past 1000 Myr, though in some cases only in variously restricted regions of the HZ. The HZ migrates outward during the main-sequence lifetime, and we find that in about two-thirds of the systems an Earth-mass planet could be confined to the HZ for at least 1000 Myr sometime during the main-sequence lifetime. Clearly, these systems should be high on the target list for exploration for terrestrial planets. We have reached our conclusions by detailed investigations of seven systems, which has resulted in an estimate of the distance from the giant planet within which orbital stability is unlikely for an Earth-mass planet. This distance is given by nRH, where RH is the Hill radius of the giant planet and n is a multiplier that depends on the giants orbital eccentricity and on whether the Earth-mass planet is interior or exterior to the giant planet. We have estimated n for each of the seven systems by launching Earth-mass planets in various orbits and following their fate with a hybrid orbital integrator. We have then evaluated the habitability of the other exoplanetary systems using nRH derived from the giants orbital eccentricity without carrying out time-consuming orbital integrations. A stellar evolution model has been used to obtain the HZs throughout the main-sequence lifetime.We have shown that Earth-mass planets could survive in variously restricted regions of the habitable zones (HZs) of most of a sample of nine of the 102 main-sequence exoplanetary systems confirmed by 19 November 2003. In a preliminary extrapolation of our results to the other systems, we estimate that roughly a half of these systems could have had an Earth-mass planet confined to the HZ for at least the most recent 1000 Ma. The HZ migrates outwards during the main-sequence lifetime, and so this proportion varies with stellar age. About two thirds of the systems could have such a planet confined to the HZ for at least 1000 Ma at sometime during the main-sequence lifetime. Clearly, these systems should be high on the target list for exploration for terrestrial planets. We have reached this conclusion by launching putative Earth-mass planets in various orbits and following their fate with mixed-variable symplectic and hybrid integrators. Whether the Earth-mass planets could form in the HZs of the exoplanetary systems is an urgent question that needs further study.

The stability of the orbits of terrestrial planets in the habitable zones of known exoplanetary systems

We show that terrestrial planets could survive in variously restricted regions of the habitable zones of 47 Ursae Majoris, Epsilon Eridani, and Rho Coronae Borealis, but nowhere in the habitable zones of Gliese 876 and Upsilon Andromedae. The first three systems between them are representative of a large proportion of the 90 or so extrasolar planetary systems discovered by mid-2002, and thus there are many known systems worth searching for terrestrial planets in habitable zones. We reach our conclusions by launching putative Earth-mass planets in various orbits and following their fate with a mixed-variable symplectic integrator.

Origin and dynamical evolution of Neptune Trojans: I. formation and planetary migration

We present the results of detailed dynamical simulations of the effect of the migration of the four giant planets on both the transport of pre-formed Neptune Trojans and the capture of new Trojans from a trans-Neptunian disc. The cloud of pre-formed Trojans consisted of thousands of massless particles placed on dynamically cold orbits around Neptunes L4 and L5 Lagrange points, while the trans-Neptunian disc contained tens of thousands of such particles spread on dynamically cold orbits between the initial and final locations of Neptune. Through the comparison of the results with the previous work on the known Neptunian Trojans, we find that scenarios involving the slow migration of Neptune over a large distance (50 Myr to migrate from 18.1 au to its current location, using an exponential-folding time of τ= 10 Myr ) provide the best match to the properties of the known Trojans. Scenarios with faster migration (5 Myr, with τ= 1 Myr ), and those in which Neptune migrates from 23.1 au to its current location, fail to adequately reproduce the current-day Trojan population. Scenarios which avoid disruptive perturbation events between Uranus and Neptune fail to yield any significant excitation of pre-formed Trojans (transported with efficiencies between 30 and 98 per cent whilst maintaining the dynamically cold nature of these objects – e < 0.1, i < 5° ). Conversely, scenarios with periods of strong Uranus–Neptune perturbation lead to the almost complete loss of such pre-formed objects. In these cases, a small fraction (∼0.15 per cent) of these escaped objects are later recaptured as Trojans prior to the end of migration, with a wide range of eccentricities (<0.35) and inclinations (<40°) . In all scenarios (including those with such disruptive interaction between Uranus and Neptune), the capture of objects from the trans-Neptunian disc (through which Neptune migrates) is achieved with efficiencies between ∼0.1 and ∼1 per cent. The captured Trojans display a wide range of inclinations ( <40° for slow migration, and <20° for rapid migration) and eccentricities (<0.35), and we conclude that, given the vast amount of material which undoubtedly formed beyond the orbit of Neptune, such captured objects may be sufficient to explain the entire Neptune Trojan population.

Jupiter - friend or foe? I: The asteroids

The asteroids are a major source of potential impactors on the Earth today. It has long been assumed that the giant planet Jupiter acts as a shield, significantly lowering the impact rate on the Earth from both cometary and asteroidal bodies. Such shielding, it is claimed, enabled the development and evolution of life in a collisional environment, which is not overly hostile. The reduced frequency of impacts, and of related mass extinctions, would have allowed life the time to thrive, where it would otherwise have been suppressed. However, in the past, little work has been carried out to examine the validity of this idea. In the first of several papers, we examine the degree to which the impact risk resulting from a population representative of the asteroids is enhanced or reduced by the presence of a giant planet, in an attempt to understand fully the impact regime under which life on Earth developed. Our results show that the situation is far less clear cut that has previously been assumed, that is, the presence of a giant planet can act to enhance the impact rate of asteroids on the Earth significantly.

Determining habitability: which exoEarths should we search for life?

Within the next few years, the first Earth-mass planets will be discovered around other stars. Some of those worlds will certainly lie within the classical ‘habitable zone’ of their parent stars, and we will quickly move from knowing of no exoEarths to knowing many. For the first time, we will be in a position to carry out a detailed search for the first evidence of life beyond our Solar System. However, such observations will be hugely taxing and time consuming to perform, and it is almost certain that far more potentially habitable worlds will be known than it is possible to study. It is therefore important to catalogue and consider the various effects that make a promising planet more or less suitable for the development of life. In this work, we review the various planetary, dynamical and stellar influences that could influence the habitability of exoEarths. The various influences must be taken in concert when we attempt to decide where to focus our first detailed search for life. While there is no guarantee that any given planet will be inhabited, it is vitally important to ensure that we focus our time and effort on those planets most likely to yield a positive result.

Jupiter: friend or foe? II: The centaurs

It has long been assumed that the planet Jupiter acts as a giant shield, significantly lowering the impact rate of minor bodies upon the Earth, and thus enabling the development and evolution of life in a collisional environment which is not overly hostile. In other words, it is thought that, thanks to Jupiter, mass extinctions have been sufficiently infrequent that the biosphere has been able to diversify and prosper. However, in the past, little work has been carried out to examine the validity of this idea. In the second of a series of papers, we examine the degree to which the impact risk resulting from objects on Centaur-like orbits is affected by the presence of a giant planet, in an attempt to fully understand the impact regime under which life on Earth has developed. The Centaurs are a population of ice-rich bodies which move on dynamically unstable orbits in the outer Solar system. The largest Centaurs known are several hundred kilometres in diameter, and it is certain that a great number of kilometre or sub-kilometre sized Centaurs still await discovery. These objects move on orbits which bring them closer to the Sun than Neptune, although they remain beyond the orbit of Jupiter at all times, and have their origins in the vast reservoir of debris known as the Edgeworth-Kuiper belt that extends beyond Neptune. Over time, the giant planets perturb the Centaurs, sending a significant fraction into the inner Solar System where they become visible as short-period comets. In this work, we obtain results which show that the presence of a giant planet can act to significantly change the impact rate of short-period comets on the Earth, and that such planets often actually increase the impact flux greatly over that which would be expected were a giant planet not present.

Habitability of exoplanetary systems with planets observed in transit

We have used the measured properties of the stars in the 79 exoplanetary systems with one or more planets that have been observed in transit, to estimate each systems present habitability. Such systems have the advantage that the inclination of the planetary orbits is known, and therefore the actual mass of the planet can be obtained, rather than the minimum mass in the many systems that have been observed only with the radial velocity technique. The measured stellar properties have been used to determine the present location of the classical habitable zone (HZ). To establish habitability we use the estimated distances from the giant planet(s) within which an Earth-like planet would be inside the gravitational reach of the giant. These distances are given by nR H , where R H is the Hill radius of the giant planet and n is a multiplier that depends on the giants orbital eccentricity e G and on whether the orbit of the Earth-like planet is interior or exterior to the giant planet. We obtained n int (eG) and n ext (e G ) in earlier work and summarize those results here. We then evaluate the present habitability of each exoplanetary system by examining the penetration of the giant planet(s) gravitational reach into the HZ. Of the 79 transiting systems known in 2010 April, only two do not offer safe havens to Earth-like planets in the HZ, and thus could not support life today. We have also estimated whether habitability is possible for 1.7 Gyr into the past, i.e. 0.7 Gyr for a heavy bombardment, plus 1.0 Gyr for life to emerge and thus be present today. We find that, for the best estimate of each stellar age, an additional 28 systems do not offer such sustained habitability. If we reduce 1.7 Gyr to 1.0 Gyr, this number falls to 22. However, if giant planets orbiting closer to the star than the inner boundary of the HZ have got there by migration through the HZ, and if this ruled out the subsequent formation of Earth-like planets, then, of course, none of the presently known transiting exoplanetary systems offers habitability. Fortunately, this bleak conclusion could well be wrong. As well as obtaining results on the 79 transiting systems, this paper demonstrates a method for determining the habitability of the cornucopia of such systems that will surely be discovered over the next few years.