Stratospheres on Hot-Jupiters

Figure 1: Artist’s impression of HD 209458b - a hot-Jupiter. Credit: NASA, ESA, and G. Bacon (STScI)

Hot-Jupiters are a class of extrasolar planets that have similar characteristics to Jupiter but unlike Jupiter, they orbit very close to their parent stars. As a result, hot-Jupiters exhibit high atmospheric temperatures ranging from hundreds to thousands of Kelvin. Additionally, hot-Jupiters are expected to be tidally-locked with respect to their parent stars and this means that a hot-Jupiter always presents the same hemisphere towards its parent star, resulting in a permanent dayside and nightside hemisphere. This establishes a large temperature contrast between the dayside and nightside. Condensable species such as titanium oxide (TiO) and silicates that are stable in the gas phase on the dayside may condense into particles on the much cooler nightside and gravitationally settle there. In the absence of vertical mixing in the atmosphere, condensation on the nightside can deplete the atmosphere of these condensable species. In this regard, the nightside can serve as a cold trap by depleting the atmosphere of these condensable species.

Observations of some transiting hot-Jupiters have revealed the presence of stratospheres on the daysides of these planets. A stratosphere is a layer in a planet’s atmosphere where the temperature increases as altitude increases (thermal inversion). In the atmosphere of a hot-Jupiter, a stratosphere is thought to result from the absorption of starlight by strong visible/ultraviolet absorbers and gaseous TiO is a good candidate for such an absorber. The presence of the nightside cold trap on a hot-Jupiter brings up the question of whether such a cold trap is effective enough to deplete the atmosphere of TiO and prevent the formation of a stratosphere. This is investigated by using 3D global circulation models of HD 209458b - one of the best studied hot-Jupiter which is also though to have a stratosphere and a large day-night temperature contrast.

Figure 2: Temperature and horizontal winds (arrows) in the global circulation model of the hot-Jupiter HD 209458b. The top three panels show the temperature and flow at three different pressures (0.1 millibar, 1 millibar and 10 millibar). The substellar point is located at longitude 0 degrees and latitude 0 degrees while the dayside is between the longitudes of -90 degrees and +90 degrees.

Based on the global circulation models of HD 209458b, a stratosphere is clearly visible on the dayside of the planet at altitudes above the 10 millibar level due to strong absorption by TiO in the atmosphere. Furthermore, at altitudes above the 10 millibar level, temperatures reach 2200 Kelvin around the substellar point on the planet and the day-night temperature contrast is as large as 1600 Kelvin. The large temperature contrast is due to the short radiative timescales at low atmospheric pressures. Centred along the equator of HD 209458b is a strong eastward superrotating jet. A pair of convergence/divergence shock-like features can be seen along the superrotating jet. The convergence/divergence feature located about 40 degrees west of the substellar point creates a region of strong upward flow while the convergence/divergence feature located about 150 degrees east of the substellar point creates a region of strong downward flow.

As shown from the global circulation models of HD 209458b, large scale circulation in the atmosphere naturally creates vertical flows that are strong enough to keep a condensable species such as TiO aloft if it condenses into particles no larger than a few microns on the planet’s nightside. If TiO is able to condense into particles larger than a few microns, then the nightside cold trap will be efficient enough to deplete TiO from the atmosphere. However, if TiO is unable to condense into particles larger than a few microns, there will be sufficient TiO on the planet’s dayside for a TiO-induced stratosphere to be present. Due to the relatively small abundance of TiO in the atmosphere, condensing TiO into particles larger than a few microns is difficult. However, if TiO can be efficiently incorporated into other more abundant condensable species such as silicates, then it can condense into larger particles. In such a scenario, the nightside cold trap can be effective enough to prevent the formation of a TiO-induced stratosphere on the planet’s dayside.

Reference: Vivien Parmentier, Adam P. Showman and Yuan Lian (2013), “3D mixing in hot Jupiter atmospheres I: application to the day/night cold trap in HD 209458b”, arXiv:1301.4522 [astro-ph.EP]

Characterizing Habitable Earth-like Planets

A major goal in the study of exoplanets is the detection and characterization of Earth-like planets in the Habitable Zones of nearby stars. However, direct imaging of exoplanets is extremely difficult because a star is typically billions of times brighter than the orbiting planet and from a distance of several light-years, an exoplanet will appear just tens of milli-arcseconds from its host star. This makes it almost impossible to directly image an exoplanet since it will be lost in the star’s overwhelming glare. A proposed mission known as the New Worlds Observer (NWO) will allow the direct detection and characterization of Earth-like planets around stars. NWO consists of a 50 meter diameter starshade placed 80,000 kilometres in front of a 4 meter aperture space telescope. The starshade suppresses the starlight by a factor of several billion and allows planets orbiting the star to be imaged by the telescope.

The starshade is attached to a spacecraft whose main purpose is to move the starshade to block different target stars and maintain precise alignment with the telescope during observations. To align with each target star, the starshade is expected to travel thousands of kilometres. For such retargeting manoeuvrers between target stars, the spacecraft uses a solar-powered ion propulsion system to move the starshade. This method of propulsion is selected to allow for the largest number of target stars with a given amount of propellant mass. During retargeting manoeuvrers by the starshade which can account for up to 70 percent of the telescope observing time, the telescope will be dedicated to general astrophysics. Typical starshade travel time between target stars is 5 to 10 days and typical observation durations at each target star ranges from 24 hours for imaging to 14 days for detailed spectroscopic observations. The planned mission duration for NWO is 5 years with the goal for an extended mission of an additional five years. Both the sunshade and telescope will be placed at the Sun-Earth L2 point, which is a low-acceleration environment in space. To prevent sunlight from illuminating the telescope-facing side of the starshade, the starshade will be tilted so that the telescope-facing side will always remain dark.

NWO can observe an entire planetary system around a target star at once. After suppressing light from a target star, NWO can image every planet from the Habitable Zone outward in a span of just several hours. Although the primary goal of NWO is to detect and characterize Earth-like planets in the Habitable Zones of nearby stars, it can also image and characterize planets beyond the Habitable Zone such as Jupiter-like and Neptune-like planets in long-period orbits. As a planet rotates, photometric observations by NWO will show variations in colour and intensity as different surface features rotate in and out of the telescope’s field-of-view. For Earth-like planets, this allows for the detection of surface features such as oceans, continents, polar ice-caps and clouds.

Spectroscopic observations of an Earth-like planet in the Habitable Zone of a target star can quickly reveal information about the planet’s atmospheric composition, surface conditions and even the presence of life. The spectrum of an Earth-analogue exoplanet observed by NWO for just several hours will reveal a brightening at short wavelengths that is indicative of Rayleigh scattering which accounts for the blue sky we see here on Earth. At longer wavelengths, the presence of molecular oxygen will produce two moderate-strength absorption features at the 0.76 and 1.26 micrometer wavelengths. Molecular oxygen is a key biosignature since it is chemically reactive in the Earth’s atmosphere and must be continuously replenished by the biosphere. The presence of water also produces six strong absorption features (0.72, 0.82, 0.94, 1.13, 1.41 and 1.88 micrometers) which get dramatically stronger towards longer wavelengths. NWO will allow astronomers to easily detect Earth-sized planets and image entire solar systems. This mission may prove to be the quickest and most affordable path to the discovery of life on other planets.

Searching for Nearby Earths

A complete census of planetary systems around Sun-like stars (FGK dwarfs) in the neighbourhood of the Sun out to a distance of 50 light years is highly desirable and will be a major milestone in the search for exoplanets. This can be achieved with a proposed mission called the Nearby Earth Astrometric Telescope (NEAT) which is sensitive enough to detect Earth-mass planets within the habitable zones of these nearby stars. NEAT is a space telescope that will carry out an astrometric survey of the 200 closest F, G and K-type stars in the Sun’s neighbourhood. Such an astrometric survey involves precisely measuring the position of a star and observing tiny changes in the star’s position caused by the presence of planets orbiting around the star. To accomplish this, NEAT performs extremely high precision astrometric measurements down to the 0.05 micro-arcsecond accuracy level.

The proposed mission architecture for NEAT consists of a pair of satellites flying in formation with one satellite being the telescope spacecraft and the other being the detector spacecraft. Both satellites are positioned precisely 40 metres apart since the telescope spacecraft consists of a one metre diameter parabolic mirror whose focal plane is located 40 metres away. The detector spacecraft will be at the focal plane where it will have ten 512 × 512 CCDs. Eight of the CCDs can be moved in the X and Y directions on the focal plane to image the reference stars while the central two CCDs are fixed in position - one CCD to image the target star and the other CCD to track the telescope’s axis. The use of 10 small CCDs instead of a billion-pixel focal plane dramatically reduces the mission cost.

Data obtained by NEAT will allow follow-up observations of planetary systems to be scheduled when the configuration of the planetary system is most favourable. In addition to detecting planets down to one Earth-mass around nearby stars, NEAT can also detect Jupiter-sized planets in long orbital periods around those planetary systems already discovered by the Kepler space telescope. Since the F, G and K-type stars in the Sun’s neighbourhood are all visible with unaided eyes or with simple binocular, the discovery of Earth-mass planets around these stars will dramatically change humanity’s view of the night sky.

Capturing an Asteroid

In April 2012, a report was published which investigated the feasibility of sending a robotic spacecraft to capture and haul back an entire Near-Earth Asteroid (NEA) to the vicinity of the Earth by the mid-2020s. The expected size of an asteroid that can be retrieved from such a mission depends on the overlap between the smallest NEAs that can be detected and characterized, and the largest NEAs that can be captured and brought back. This overlap appears to centre on NEAs roughly 7 m in diameter. Depending on its bulk density, a NEA with a diameter of roughly 7 m is expected to have a mass of about 500,000 kg. In comparison, the Apollo program brought back 382 kg of Moon rocks from six surface missions.

Capturing a 500 ton asteroid and placing it in high lunar orbit will provide an interesting and affordable destination for future astronauts since NASA expects a human presence in cislunar space to be well established by the mid-2020s. Having an asteroid in high lunar orbit provides a convenient test bed for new technologies and operational experience that will benefit future manned missions to larger NEAs. Returning samples from the captured asteroid will be easier since the flight time to and from high lunar orbit will be a lot shorter than a full-fledged mission into deep space to retrieve samples from even the most accessible NEAs. Finally, the extraction of water and other resources from the captured asteroid allows the asteroid to serve as a source of raw materials for manned deep space missions.

Figure 1: Illustration of an asteroid retrieval spacecraft in the process of capturing a 7 m, 500 ton asteroid. (Image Credit: Rick Sternbach / KISS)

The spacecraft proposed for this asteroid retrieval mission involves a robotic vehicle that is propelled by a 40-kilowatt solar-electric propulsion (SEP) system. At launch, the initial mass of the spacecraft is 18,000 kg and 12,000 kg of the initial mass is in the form of xenon propellant for the SEP system. The SEP system consists of five 10-kilowatt Hall thrusters that are powered by two solar array wings, each with a collecting area of 71 square meters. The spacecraft’s asteroid capture mechanism consists of a large inflatable bag measuring 10 metres by 15 metres that will deploy and envelope the asteroid.

The mission to retrieve an asteroid will be a rather long duration one. An Atlas V rocket will launch the spacecraft to low-Earth orbit (LEO). Once in LEO, the spacecraft will gradually spiral out to the Moon over a period of 2.2 years. Near the Moon, the spacecraft will undergo a lunar gravity assist followed by a 1.7 years cruise to the target asteroid. After rendezvous with the asteroid, the spacecraft will undergo 90 days of asteroid operations where it will capture and de-spin the asteroid. Finally, another 2 to 6 years is required to transport the captured asteroid to high lunar orbit. Hauling back a more massive asteroid will require a longer flight time. As a result, the total mission duration is expected to be between 6 to 10 years.

Figure 2: Asteroid return mission concept. Return flight time of 2 to 6 years depending on the asteroid mass.

NASA is already considering such an asteroid retrieval mission which is estimated to cost $2.6 billion - slightly more than NASA’s Curiosity Mars rover. The potential benefits of using a robotic spacecraft to snag and haul back a NEA to high lunar orbit for further study and exploration are expected to be huge. The team involved in this feasibility study mentioned: “Placing a NEA in lunar orbit would provide a new capability for human exploration not seen since Apollo. Such an achievement has the potential to inspire a nation. It would be mankind’s first attempt at modifying the heavens to enable the permanent settlement of humans in space.”

Accretion-Induced Collapse

When the mass of an accreting white dwarf grows towards the Chandrasekhar limit, the white dwarf eventually becomes unstable and destroys itself as a Type Ia supernova. In some cases, a white dwarf may not explode as a Type Ia supernova but instead undergoes a process known as an accretion-induced collapse to form a rapidly rotating neutron star. Accretion-induced collapse can occur for accreting oxygen-neon-magnesium white dwarfs or from the merger of carbon-oxygen white dwarfs. The occurrence rate for accretion-induced collapse is expected to be less than one percent of the occurrence rate of Type Ia supernova. No accretion-induced collapse event has yet been directly observed.

During an accretion-induced collapse event, a white dwarf collapses into a rapidly rotating neutron star. The ejected mass from such an event is expected to be small (less than 0.1 solar masses) and travelling at high velocity (up to about 10 percent speed of light). As such, the amount of emitted radiation will be orders of magnitude less than a typical supernova. Furthermore, the expected duration of observable optical radiation being emitted from such an event is expected to last for only a day or so.

As the white dwarf collapses, conservation of angular momentum and amplification of the magnetic field leads to the creation of a rapidly spinning magnetar. A magnetar is a type of neutron star with an exceptionally strong magnetic field. The spin-down of the newly formed magnetar powers a pulsar wind nebula which injects energy in the form of magnetic fields and relativistic particles into the ejecta surrounding it. Because the spin-down energy of the magnetar is so much greater than the initial kinetic energy of the ejecta, it is the spin-down energy that determines the ejecta speed. As the ejecta expand outwards, the low-density pulsar wind nebula pushing up against the high-density ejecta can lead to the development of Rayleigh-Taylor instabilities. The ejecta plough through the interstellar medium and will begin to decelerate after it has swept up a mass comparable to its own.

Reference: Anthony L. Piro and S. R. Kulkarni (2013), “Radio Transients from the Accretion-induced Collapse of White Dwarfs”, The Astrophysical Journal Letters Volume 762 Number 2