I’m in Puerto Rico for the week, for a conference on all things related to planets and discs around stars at late stages in their lives, including giants, white dwarfs, and neutron stars. Both the natural and the technological settings are amazing:
The first day’s sessions focussed on planets around neutron stars and hot subdwarfs. Alex Wolszczan opened with a recounting of the announcement of the discovery of the first reliably-detected extra-Solar planets, around the pulsar PSR B1257+12 almost exactly 20 years ago, after their discovery at Arecibo Observatory. While the original data showed the Doppler shift of the pulsar’s signal, attributed to two planets on almost circular orbits, another 20 years’ worth of timing data have not only thrown up another planet, but have allowed the detection of the perturbations of the two planets on each others’ orbits, thus confirming that the signals are indeed planetary and not from some other source.
Wolszczan proceeded to outline some current problems in the field of pulsar planets. First, how do they form? Any previously existing planets would likely be destroyed by the supernova in which the neutron star was born. And second, where are all the others? With of order 100 more milisecond pulsars now known than in the early 1990s, why have we only found a couple more pulsars with planets? The rarity suggests a formation mechanism which is only rarely successful, such as the kick from the supernova shooting the neutron star through a companion star, whose material the neutron star accretes into a disc that can form planets.
Scott Ransom then asked this same question. For a “good” milisecond pulsar, planets with a mass as low as Mercury’s should be easily visible, yet we don’t see them. He also favoured the kick mechanism for formation of pulsar planets, pointing out that a strong candidate for hosting a planet exists in a globular star cluster, where stellar densities are very high and the neutron star may have been kicked through another star.
Sam Bates discussed the planet orbiting PSR 1719-1438, which has a very high density (>25 g/cm^3; cf Earth’s 5.5 g/cm^3), and may be the left-overs of a white dwarf that was largely consumed by the neutron star. The “planet” is on a very close orbit with a period of about 2 hours.
Bettina Posselt discussed early results from a direct imaging search for planets around neutron stars, with nothing found so far. This emphasises that planets around pulsars are uncommon, although the sensitivity of direct imaging observations in terms of mass is much less than that of the timing method. It can, however, probe much larger semi-major axes.
Roberto Silvotti opened the session on planets around hot subdwarf stars. These are stars that lost their envelope during the Red Giant stage. In many cases this can be attributed to the envelope’s being unbound as it interacts with a stellar companion on the giant branch. However, around half of hot subdwarfs have no known companion. Perhaps planets or brown dwarfs, harder to detect, could be supplying the energy needed to unbind the primary’s envelope? There are now a few known substellar or planetary companions to these stars, and they are of very different types: radial velocity searches have detected companions close to the hydrogen-burning mass limit of 70 Jupiter masses, while Kepler photometry has detected some two comapnions to KOI 55 whose masses are less than that of Earth’s.
These were the subject of the next talk by Stephane Charpinet. In contrast to most planets discovered by Kepler, which transit their host star, blocking its light, these were detected through the changing amount of light they reflect at different points in their orbits, like the phases of the moon. This required especially careful work to rule out non-planetary origins of the signals. No known stellar oscillations have frequencies in the same range as those observed for KOI 55. The star’s very slow rotation period–40 days, compared to the photometric varibility of 5 and 9 hours–makes any origin governed by rotation, such as the motion of spot patterns, impossible. The variations are far too small for a stellar companion to be responsible, and the Kepler team managed to rule out their origin due to background contamination. So a planetary origin seems likely. However, the survival of these planets in the preceeding RGB phase would be very difficult, as they would be deep inside the star’s envelope and vulnerable to drag forces moving them into the core and evaporation due to the high temperatures.
Ronny Lutz then described the discovery of substellar companions to hot subdwarfs by measuring the Doppler shift of stellar pulsation frequencies. Becasue these can change due to a variety of causes, it is important to verify that all pulsation modes are Doppler shifted in the same way by the planet. In these cases, two independent modes displayed the same behaviour, so a planetary origin is not ruled out.
Stephan Greier then discribed the detection of substellar companions through traditional RV measurements. The precision of these is very poor because the hot subdwarfs have few metal lines, but detections of brown dwarf objects have now been made, and planets are just about becoming possible.
Richard Wade gave a very well-received talk cautioning the attribution of timing variations of the eclipses of close binaries to the effects of planets. There are many ways a close binary’s period can change, due to processes intrinsic to one or both stars, such as apsidal precession, or magnetic or structural changes inside the stars. Some debate ensued as to what should be the “default” position regarding planetary explanations. Traditionally they are invoked when all other explanations for an effect have been ruled out, but several audience members pointed out that, since they are so common in many unexpected places, perhaps we should be more ready to accept a planetary origin for signals ahead of other explanations.
To close Monday’s talks, William Danchi discussed changes to the Habitable Zone as a star evolves off the Main Sequence. The Habitable Zone is defined as the region where liquid water can survive on a planet’s surface, and is further from the star as the star’s luminosity increases. He pointed out that, although Earth will be a parched desert by the end of the Sun’s Main Sequence, a Habitable Zone will exist at around a few AU when the Sun is on the Horizontal Branch, following the RGB. Since this phase of evolution lasts for around 1 Gyr, this may be ample time for life to emerge on the currently icy worlds of the outer Solar System, or planets on relatively wide orbits around stars currenty at this stage of evolution.