This article will explore the potential for life to develop in the outer planetary systems of red giant stars. It will then discuss the death-throes of red giant stars, and whether the subsequent outward thrust of stellar material might provide another mechanism for free-floating planets in interstellar space.
Exoplanets have already been found orbiting extremely old stars, one some 11 billion years old (1). This star, named Kepler-444, makes our own Sun, at a mere 4.6 billion years old, seem like an infant in comparison. The implication of this is that life could readily have got going early on in the history of the universe, long before the birth of our Sun. Furthermore, if these exoplanets were to benefit from a relatively stable stellar environment during that long timescale, then the chances of life evolving into higher forms are statistically more probable. Scale this up across trillions of stars, and the possibilities become clear.
Our own Sun has a shorter lifespan than this. Its main sequence life is expected to last another 5 billion years, by which point it will have burned up all of its hydrogen fuel. Then it will swell into a red giant star, before collapsing down into a white dwarf. For Earth, this post-main sequence (post-MS) phase of the Sun’s life will be pretty disastrous. The Sun’s expansion to a red giant will swallow the Earth up. However, a less catastrophic outcome might be expected for planets in the outer solar system, beyond, say, Jupiter. In fact, their climates might significantly improve – for a while, at least. The habitable zone of the solar system will expand outwards, along with the expanding star. Saturn’s largest moon Titan, for instance, might benefit greatly from a far milder climate – as long as it can hang onto its balmy atmosphere in the red heat of the dying Sun.
The expansion of habitable zones, as late main sequence stars become hydrogen-starved, offers the potential for life to make a new start in previously frigid environments. The burning question here is how long these outer planets have to get life going before the red giant then withdraws into its cold white shell. A study published last year by scientists at the Cornell University’s Carl Sagan Institute attempted to answer this question (2), choosing to examine yellow dwarf stars whose sizes range from half that of the Sun, to approximately twice its mass. They argue that the larger stars along this sequence could well have larger rocky terrestrial planets in their outer planetary systems than our Sun does (at least, insofar as we know it does!) This is because the density of materials in their initial proto-planetary disks should be that much greater for larger stars (3). Larger Earth-like planets in outer regions mean more potential for stable atmospheric conditions during the post-MS period under consideration. In other words, the growing red giant (which is shedding its mass pretty wildly at this point) would not necessarily blast away an outer planet’s atmosphere if that rocky planet had sufficient gravity to hold onto it.
A new theory about planet formation has posited that stars, placed under inordinate stress, could break apart catastrophically, flinging their smouldering remains out into the void at tumultuous speeds. It would take quite a force to render stars apart in this way. The supermassive black hole which lies at the centre of the galaxy creates just such an impression. Wayward stars drifting inexorably into the depths of its immense gravitational well would not fare well, during what are termed Tidal Disruption Events (1,2).
Researchers from Harvard University (namely, undergraduate Eden Girma and James Guillochon, an Einstein fellow at the Harvard-Smithsonian Center for Astrophysics), have conducted computer simulations to model what happens to this streaming material, and the results are quite extraordinary:
“Every few thousand years, an unlucky star wanders too close to the black hole at the center of the Milky Way. The black hole’s powerful gravity rips the star apart, sending a long streamer of gas whipping outward. That would seem to be the end of the story, but it’s not. New research shows that not only can the gas gather itself into planet-size objects, but those objects then are flung throughout the galaxy in a game of cosmic “spitball.”” (3)
A couple of brown dwarfs have been discovered in a close binary system some 240 light years away, whose two stars circle each other at a distance of about 19AU, similar to that of Uranus around the Sun. The two new exoplanets orbit close to the primary Sun-like star HD 87646 (1). These two sub-stellar companions are HD 87646b, which is a minimum 12MJupiter sub-brown dwarf (a ‘hot Jupiter’-type exoplanet) orbiting every 13 days just 0.117AU from the star (2); and HD 87646c, which is a 57MJupiter brown dwarf circling the star every 673 days (1). The orbital eccentricity of the brown dwarf is greater than that of the inner sub-brown dwarf, which is in keeping with other observations of brown dwarfs orbiting stars.
Image Credit: Janella Williams, Penn State University
The international team that discovered this remarkable system is perplexed as to how it might have come about:
“Given the fact that HD 87646 is the first known system to have two massive substellar objects orbiting a star in a close binary and the masses of the two objects are close to the minimum masses for burning deuterium and hydrogen, these peculiarities raise questions about the system’s formation and evolution.
“”The large masses of these two substellar objects suggest that they could be formed as stars with their binary hosts: a large molecular cloud collapsed and fragmented into four pieces; the larger two successfully became stars and formed the HD 87646 binary, and the other smaller ones failed to form stars and became the substellar objects in this system. This scenario might be relevant for the binary stars but seems problematic for the two substellar objects on orbits within one AU because it is unclear whether fragmentation on such a small scale can occur,” the paper reads (1)
“Other hypothesis offered by the scientists is that the two newly discovered giant objects were formed like giant planet in a protoplanetary disk around HD 87646A. However, they added that such massive disks are rare in close binaries, and further investigation is needed to confirm this explanation.” (3)