Stars, planets and origins

At Imperial, our research in this area straddles both observations and theory (analytic and simulations) with a large degree of interplay between these techniques. On the formation side, our current understanding of the processes involved is conditioned on explaining the Solar System; however, we now know planetary systems similar to our Solar System are rare. Thus, one of our major focuses is reconciling our theories of planet formation with the properties of exoplanets. One of the major stumbling blocks is that planet formation is a relatively short process, only lasting a few 10s of Myrs compared to the ages of most observed exoplanets at 1 Gyr or older. Therefore, have been trying to understand how planetary systems evolve over their billion year lifetimes into the population we see today, including planetary evaporation and dynamical evolution. For the planet formation processes itself, planets are born out of the star-girdling disks of gas and dust that surround young stars. The ALMA telescope has taken unprecedented images of these planetary nurseries, revealing a wealth of structure that is telling us something about the planet formation process, it's just unclear what these images are telling us at this stage. At Imperial, we are focused on understanding, modelling and observing these discs in-order to build the next-generation of planet formation theories. One current avenue, involves connecting the atmospheres of exoplanets, which are being observed by telescopes such as JWST, with the properties of the discs in which they form. No picture of planet formation is complete, without an understanding of how these disks are born. They are a natural outcome of the star formation process, and thus now picture of planetary birth is complete without an understanding of star formation.

The origin and properties of both stars and planets are closely linked to those of brown dwarfs (BDs). By definition, these are substellar objects: with masses < 75MJUP, BDs cannot sustain stable H-fusion. Instead, like planets, they simply cool down and grow fainter with time (after an initial period of Deuterium fusion); unlike planets, however, most BDs do not orbit stars, but are free-floating bodies. Their importance to stars and planets is threefold. First, the genesis of BDs is deeply puzzling, and key to unraveling the physics of low-mass star formation in general. Second, the ultra-low, planetary temperatures attained by these objects, combined with the ease with which free-floating BDs can be studied directly (in contrast to planets, which circle overwhelmingly brighter stars), makes BDs invaluable guides to planetary atmospheres. Third, accretion disks are ubiquitous around young BDs, raising the novel possibility of forming planets around these ultra-low-mass objects, just as in stars.

Finally, and much closer to home, we visit the question of habitability and the possible detection of life on Venus. This planet has long been regarded as totally hostile to life, but recent observations with the JCMT and ALMA telescopes have found a molecule, phosphine, which might be the first hint that something unusual is happening in the clouds of Venus. While the surface of Venus is undeniably hostile to life, with pressures in excess of 90 atmospheres and temperatures high enough to melt lead, the clouds, at an altitude of about 55km, are rather more hospitable, with pressure matching Earth at sea level and temperatures of about 40 to 50 C. This seems to be where phosphine, PH3, has been detected. Such a molecule has no right to be present in the oxidised atmosphere of Venus - normal chemical processes, and even such exotic sources as volcanism, lightening or cometary impactors, predict levels of phosphine many orders of magnitude below what we have found. There thus seems to be some unknown chemical route leading to its production or, just possibly, a biological route. Imperial is thus leading a long term observing project with JCMT to monitor phosphine and other molecular species, as well as conducting other observations, to better understand the processes that produce this unexpected gas and to determine if there really is any evidence of life in the clouds of Venus.

Our research at Imperial focuses on this nexus of low-mass stars, BDs and planets, with the ultimate goal of painting a unified picture of the intertwined origins and properties of these three classes of objects.