Despite a large number of exoplanets detected to his date none of these objects have been detected orbiting around white dwarf stars. In a paper published this week on arXiv.org, the scientists argue that in this way we are not accounting for (or, better to say, missing) more than 95% of theoretically existing planetary systems. But what real technical possibilities are available for detection of planets near white dwarfs, stars that are much dimmer and smaller compared to the majority of their counterparts?
Well, many astrophysicists would agree that our chances to detect even a giant Jupiter-class planets after including white dwarfs in the ongoing exoplanetary surveys won’t be much higher. But the authors of the current study say, that modern orbital telescopes – namely, Gaia – have a potential to widen the scope for new discoveries.
Undoubtedly, the idea itself is very appealing. During the last decade a multitude of astrophysicists investigated the possibility of white dwarfs containing any planets at all. The reason of such assumption is very simple: prior to becoming a dwarf, a star has to undergo major evolutionary changes. That includes a drastic expansion of outer layers of the star even beyond the orbits of many inner planets; later, the star also loses a significant fraction of its initial mass and these mass variations and related tidal effects inevitably impact the orbits of surviving planets, if any do remain after earlier scorching. Some of the planetary orbits expand (when stellar mass loss dominates) or shrink (when tidal effects dominate).
Several recently developed astrophysical models suggested that planets may survive so called post-main sequence star evolution. “At the end of the evolution, when the star becomes a white dwarf, we expect therefore that some planets have migrated outwards and some inwards, so tat the final distribution of orbital periods shows a gap. Close to the star we expect to find only massive companions in very tight orbits, while all the other planets were presumably pushed out at several AUs from their host star”, the authors of the study say. They also note that inner planets would remain only if their initial mass was large enough to survive the immersion into the outer layers of the host star during its expansion phase.
According to the authors, Radial Velocity (RV) and transit methods are two main techniques for detection of exoplanets. Certainly, these methods are not the only methods available, but currently they yield the most of results. Limited resolution of spectroscopic radial velocity is the main disadvantage of RV technique when considering white dwarfs. In case of transit method, the small radii of white dwarfs result in very deep transit signals. That could allow us to detect objects smaller than Moon, at least theoretically. But small radii is a disadvantage at the same time, because this factor vastly reduces the probability of orbiting body transiting directly in front of the star.
Meanwhile, the astrometric method is the most promising method to detect planets orbiting around white dwarfs in wide orbits, the authors argue. Significant improvements are expected with the recent launch of Gaia. In their study, the team decided to investigate the actual technical possibilities of this novel telescope by simulating what Gaia can find. For this purpose, the scientists selected a sample of white dwarfs located relatively near our Solar system. The sample was limited in the magnitude (R<20.5), distance (d<200 pc) and some other type-related astronomical parameters.
The planetary frequency and distribution parameters, as well as Gaia sensitivity data were selected according to the data provided in recently published scientific works. The team assumed a constant expansion of orbits by a constant factor of 2.5, corresponding to the stellar mass loss during white dwarf transformation for the star with initial mass equal to 1.5 solar masses. The simulation was simplified further as no tidal effects were considered. Still, the team is certain that these simplifications are matched to some degree by specifically selected sample of stars. The astrometric planet detection efficiency was calculated as a result of simulations.
According to the calculations, Gaia should be more sensitive to white dwarf companion planets with orbital periods between 1 and 2 years, the authors say. The best thing is that the results generally indicate that the detection of exoplanets around white dwarfs is not out of the realm of possibility. However, masses of these planets should outmatch our Jupiter at least by a factor of 3. The scientists expect to provide more detailed results after reducing the number of assumptions in their model.
Written by Alius Noreika