The changing view of planets orbiting low mass stars, M stars, as potentially hospitable worlds for life and its remote detection was motivated by several factors, including the demonstration of viable atmospheres and oceans on tidally locked planets, normal incidence of dust disks, including debris disks, detection of planets with masses in the 5–20 M⊕ range, and predictions of unusually strong spectral biosignatures We present a critical discussion of M star properties that are relevant for the longand short-term thermal, dynamical, geological, and environmental stability of conventional liquid water habitable zone (HZ) M star planets, and the advantages and disadvantages of M stars as targets in searches for terrestrial HZ planets using various detection techniques Biological viability seems supported by unmatched very long-term stability conferred by tidal locking, small HZ size, an apparent short-fall of gas giant planet perturbers, immunity to large astrosphere compressions, and several other fact

M stars as targets for terrestrial exoplanet searches and biosignature detection.

Formation and destruction of dust grains

The region beyond Neptune's orbit is populated by numerous bodies with semimajor axes from 31 to 48 AU. This region, known as the Kuiper belt, should contain primitive bodies, perhaps among the most primitive objects in the solar system. These bodies could be remnants of the solar system formation. They seem to be dark, volatile-rich objects showing a strong relationship with comets: the Kuiper belt is probably the source of most short-period comets and Centaurs. The Kuiper belt objects (KBOs) could still contain ices and organic compounds in unaltered proportions with respect to those of their formation. Thermal models of bodies moving in Kuiper belt orbits have been developed to follow their evolution and differentiation and to better understand the relations between them and the short-period comets and Centaurs. In these models, we assume that KBOs are porous bodies composed of ices and dust. The solar energy is very low, between 30 and 50 AU, and radiogenic heating becomes a nonnegligible source of energy for differentiation. The radioactive elements, if they exist in sufficient quantity, may modify the original composition of cometary nuclei. In the models reported here, we have assumed that the radiogenic elements stored in the refractory component are 40K, 232Th,235U, and 238U, in meteoritic proportions. In some models, we have also included the short-lived radio nuclide 26Al. The aim of this work is to see how an undifferentiated Kuiper belt body can change its internal structure under the combined effect of radiogenic heating and solar irradiation. Moderate heating can permit the sublimation of the most volatile ices both from the interior and from the surface, depending on the dominant heat source. The main result is that Kuiper belt objects can be strongly volatile depleted. From the surface down to several hundred meters below the surface, the most volatile ices (like CO) can be completely absent.

Thermal Evolution and Differentiation of Edgeworth-Kuiper Belt Objects

Transition Elements between Comets and Asteroids

We discuss the results obtained by simulating the thermal evolution of 2060 Chiron, the largest of the Centaur objects. The study of Chiron is extremely important because of the presence of activity in spite of its large distance from the Sun. In this paper we study the thermal evolution of a Chiron-like body under diUerent assumptions concerning the initial composition and taking into account its evolutionary history. The composition of Centaurs and Kuiper belt objects is almost unknown: we can guess that Chirons ii cometary behavior ˇˇ is an indication of a composition dominated by diUerent ices. We have therefore computed the thermal evolution of Chiron using a numerical code developed to study the thermal evolution of cometary nuclei. As in the case of comets, we assume that the ice is mixed with dust and that the body is highly porous. CO is present in the mixture both as ice and as gas trapped in the amorphous ice. Our conclusion is that Chirons activity can be explained only by assuming a composi- tion in which CO is present not far from the objects surface as an ice or as a gas trapped in the amor- phous ice.

Chiron Activity and Thermal Evolution

Some mechanisms which are expected to produce the growth of dust grains in the protosolar nebula are studied during the isothermal and the adiabatic phase of the gravitational collapse.

Grains accretion processes in a proto-planetary nebula: II. Accretion time and mass limit

In a previous work the authors estimated cross-sections for constructive and destructive collisions (‘constructive’ and ‘destructive’ cross-sections) related to silica grains embedded in a protoplanetary nebula. The outstanding point of the problem has induced the authors to examine more closely the physical conditions in which the accretion processes occur, and their dependence on the materials, by which the particles are probably made. In this paper the interaction processes among conducting grains (iron, graphite) and among the insulating (silica) ones have been considered. The following results have been obtained: (1) insulating grains have smaller constructive crossections than conducting; and (2) conducting grains show very large cross-sections for masses up to about 10−9 g.

G. Magni

Papers

Grains accretion processes in a proto-planetary nebula: II. Accretion time and mass limit

Grains accretion processes in a protoplanetary nebula: Conducting and insulating materials