Friday, March 25, 2011

The search for habitable Earth-size planets has primarily been focused on stars similar to our Sun. In recent years, the search has also gone on to focus on low mass red dwarf stars as these stars are by far the most common and an Earth-size planet around such a star will be much easier to detect due to the lower mass and luminosity of a red dwarf star. In this article, I will be exploring the possibility of detecting Earth-size planets located in the habitable zone of cool white dwarf stars. White dwarf stars are the final evolutionary state of all stars that are not massive enough to explode as supernovae and this includes stars such as our Sun. Typically, a white dwarf star has a mass that is comparable to our Sun and all its mass is contained within a tiny volume that is comparable to the size of the Earth. Hence, a white dwarf star is a very dense object as each cubic centimetre of its material can weight over a metric ton.

White dwarf stars are as common as Sun-like stars and as they slowly cool, they can provide energy to planets in orbit around them for billions of years. A paper entitled “Transit Surveys for Earths in the Habitable Zones of White Dwarfs” describes the prospect of detecting habitable Earth-size planets around white dwarf stars by searching for transits of such planets in front of white dwarf stars. Compared to a typical Sun-like star, the habitable zone around a white dwarf star will be located much closer in due to the much lower luminosity of a white dwarf star. The most common surface temperature for white dwarf stars is around 5000 degrees Kelvin and white dwarf stars with surface temperatures of over 10000 degrees Kelvin are rare because white dwarf stars spend little time at high temperatures as they cool very rapidly at such high temperatures. Furthermore, the high ultraviolet flux from a hot white dwarf star that has a surface temperature of over 10000 degrees Kelvin will affect the retention of an atmosphere around an Earth-size planet. Therefore, only cool white dwarf stars will surface temperatures that are considerably less than 10000 degrees Kelvin are considered for the detection of habitable Earth-size planets.

A white dwarf star does not have an internal source of energy like a typical star and this means that it will gradually radiate away its energy and cool down over a period of billions to trillions of years. Hence, the term “continuously habitable zone” is defined as the range of orbital distances from a white dwarf star where an Earth-size planet can stay habitable for a specified minimum duration. For an Earth-size planet to remain habitable for at least 3 billion years, the continuously habitable zone will extend from a distance of 0.005 AU to 0.02 AU for white dwarf stars with masses ranging from 0.4 to 0.9 times the mass of our Sun, whereby 1.0 AU is basically the mean distance of the Earth from our Sun.

The orbital period of any planet in the continuously habitable zone of white dwarf stars will range from around 4 to 32 hours and the planets are expected to be tidally-locked whereby the star-facing hemisphere of the planet will experience permanent day, while the other hemisphere will experience permanent night. The night side of such a planet can be warmed by the global circulation of heat from the day side of the planet which can prevent the formation of a cold-trap on the night side. Since the orbital period and spin period of a tidally-locked planet are both the same, an Earth-size planet in the continuously habitable zone of a white dwarf star will experience Coriolis and thermal forces that are similar to those on the Earth.

Earth-size planets in or near the continuously habitable zone of white dwarf stars can be detected via the transit method where the individual photometric output of a large number of white dwarf stars can be continuously monitored to look for any dimming that can be associated with the transit of an Earth-size planet in front of a white dwarf star. Due to the small size of a white dwarf star, the transit of an Earth-size planet will block out a significant fraction of the white dwarf star’s total photometric output or even completely block out the entire star if the star is sufficiently small. The small size of a white dwarf star also favours the detection of transiting objects that are smaller than the size of the Earth. The transit durations of Earth-size planets in the continuously habitable zone of white dwarf stars are estimated to last for a couple of minutes or so, thereby requiring high cadence observations to record the proper light curves that are indicative of such transit events.

Measurements of the distance and spectrum of a white dwarf star will allow its mass, luminosity, atmospheric composition and radius to be determined. Therefore, with the size of the white dwarf star known, the measured transit depth of a transiting planet enables the size of the transiting planet to be directly determined. On the contrary, the mass of the transiting planet cannot be determined from Doppler measurements as the spectra of cool white dwarf stars are generally featureless. However, if the white dwarf star has multiple transiting planets, gravitational interactions among the planets can cause measurable transit timing variations which can be use to estimate the mass for each of the planets.

Friday, March 18, 2011

NASA’s MESSENGER spacecraft was launched into space onboard a Delta II 7925 rocket on 3 August 2004 at 06:15:56 UTC from Space Launch Complex 17B at the Cape Canaveral Air Force Station in Florida. After travelling through space for 6 years, 7 months and 16 days and covering an impressive distance of 7.9 billion kilometres, MESSENGER finally entered orbit around the planet Mercury on 18 March 2011 at 01:00 a.m. UTC after a 15 minutes Mercury orbit insertion (MOI) engine burn. MESSENGER is the second mission to Mercury after a final flyby performed by Mariner 10 in 1975 and it is the first spacecraft to enter orbit around the planet. The primary mission of MESSENGER will be to study the chemical composition, geology and magnetic field of Mercury.

Getting to Mercury from the Earth requires a large velocity change because the closeness of Mercury to the Sun places the planet deep within the Sun’s gravitational potential well. Furthermore, Mercury’s extremely tenuous atmosphere makes it impossible for an aerobraking manoeuvre to be employed to sufficiently slow an incoming spacecraft for capture into orbit around Mercury. To solve this issue, MESSENGER extensively used gravity assist manoeuvres by making flybys of the inner planets to gradually decelerate the spacecraft such that the amount of propellant required to slow the spacecraft into orbit around Mercury is greatly reduced. However, this comes at the cost of prolonging the trip to Mercury by a few years. The trajectory that MESSENGER took through the inner solar system to get to Mercury included one flyby of Earth, two flybys of Venus and three flybys of Mercury itself.

On 18 March 2011 at 12:45 a.m. UTC, the orbital insertion manoeuvre brought MESSENGER into a highly elliptical orbit around Mercury whose lowest point is 200 kilometres above the planet’s surface while the highest point is over 15000 kilometres above the planet’s surface. The three previous flybys of Mercury by MESSENGER have already generated an astonishing amount of interesting science that has changed our understanding of the enigmatic innermost planet of the solar system. However, these flybys are merely a sneak preview of the discoveries that are expected to come as MESSENGER is now the first spacecraft ever to orbit Mercury for long-term observations. Visit http://messenger.jhuapl.edu/index.php to learn more about MESSENGER and its mission around Mercury.

Thursday, March 17, 2011

The Fermi Gamma-ray Space Telescope (FGST) is a space observatory which observes the universe in gamma-rays from its vantage point in low Earth orbit. One interesting discovery by Fermi are two enormous gamma-ray-emitting bubbles that extend about 30 thousand light years above and below the centre of the Milky Way galaxy. The existence of the two gamma-ray-emitting bubbles was first hinted by previous detections of a localized excess of radio signals. In this article, the two gamma-ray-emitting bubbles will be referred to as the Fermi Bubble. I recently read a paper entitled “Origin of the Fermi Bubble” and this paper suggests that the periodic capture of stars by the supermassive black hole at the centre of the Milky Way galaxy can inject the required amounts of high energy plasma into the galactic halo to form the Fermi Bubble.

A supermassive black hole with a mass of approximately 4 million Suns sits in the heart of the Milky Way galaxy. Stars which happen to come too close to the supermassive black hole can be destroyed by tidal disruption. When a star gets tidally disrupted by the supermassive black hole, about half of its mass becomes tightly bound to the black hole while the other half gets violently ejected. The amount of energy carried by the ejected mass can significantly exceed the amount of energy released by a normal supernova explosion. Approximations have shown that the supermassive black hole at the galactic centre destroys a star by tidal disruption at a rate of roughly one star every ten thousand years or so. This means that tens of stars are expected to get tidally disrupted every one million years.

The ejecta from each tidally disrupted star expand as a spherically symmetric wind of high energy plasma and ‘snowploughs’ its way out of the galactic centre to form a pair of bipolar outflows which contribute to the existence of the Fermi Bubble. The high energy outflows from each tidal disruption event expand hydrodynamically out of the galactic centre and into the galactic halo, forming shock fronts which accelerate electrons to near the speed of light. Interaction of the high energy electrons with background photons via synchrotron radiation and inverse Compton scattering produces the observed radio and gamma-ray emissions respectively. Since the mean interval between each tidal disruption event is smaller than the timescale for energy loss, the gamma-ray emissions produced from each individual shock front can be approximated to be uniformly distributed over the entire Fermi Bubble.

Finally, the existence of the Fermi Bubble cannot be explained by a previous episode of starburst activity in the galactic centre because there is no evidence of an excessive amount of supernova explosions in the past 10 million years or so in the galactic centre. Furthermore, supernova remnants can be traced by the radioactive aluminium-26 they produce and the sparse concentration of aluminium-26 in the galactic centre does not support a previous episode of starburst activity.

Wednesday, March 9, 2011

Brown dwarfs are objects that are too low in mass to sustain hydrogen fusion in their cores and they occupy the mass range between gas giant planets and the lowest mass stars. The upper limit for the mass of a brown dwarf is around 80 times the mass of Jupiter while the lower limit for the mass of a brown dwarf is undefined as it overlaps with the masses of gas giant planets. Methane-bearing spectral class T brown dwarfs are the coolest known class of brown dwarfs. Although a large number of brown dwarfs are know, there remains a large gap between the temperature of the coolest known brown dwarfs and the gas giant planets in our solar system. The coolest known brown dwarfs have temperatures of around 500 degrees Kelvin while the gas giant planets in our solar system have temperatures of around 150 degrees Kelvin. Theoretical studies have shown that brown dwarfs in this temperature range exhibit spectroscopic characteristics that are distinct from the spectral class T brown dwarfs, such as ammonia absorption lines and scattering from clouds of water ice. Any brown dwarfs in this temperature range can be categorized into a new and cooler spectral class known as spectral class Y.

A paper by Kevin Luhman, et al. (2011) entitled “Discovery of a Candidate for the Coolest Known Brown Dwarf” describes the discovery of what might be the coolest known brown dwarf and a likely prototype for the spectral class Y. With an estimated temperature of 300 degrees Kelvin, WD 0806-661 B is a candidate for the coolest known brown dwarf and also cool enough for its atmosphere to contain clouds of water ice. WD 0806-661 B is in a wide orbit around a white dwarf star and if a similar age to its host star is assumed, WD 0806-661 B will be around 1.5 billion years old. Furthermore, based on evolutionary models of cooling brown dwarfs, WD 0806-661 B is estimated to have a mass of around 7 times the mass of Jupiter and this falls well within the range of masses for the more massive extrasolar planets. There are two mechanisms in which an object like WD 0806-661 B could have formed. Firstly, WD 0806-661 B could have formed from the coalescence of a fragmented cloud of gas at its current large distance from its host star. Secondly, WD 0806-661 B could be a gas giant planet that had been dynamically scattered into a much more distant orbit around its host star. If subsequent observations confirm WD 0806-661 B to be the coolest known brown dwarf, it will become a valuable target for studying atmospheres in an entirely new temperature regime that will consequently aid searches for the coldest brown dwarfs with facilities such as the Wide-field Infrared Survey Explorer (WISE) and the James Webb Space Telescope (JWST).

Friday, March 4, 2011

The formation of planets around a young star occurs within a disk of gas and dust encircling the young star called a protoplanetary disk. This results in a close alignment between the rotation axis of the star and the orbital motion of the planets after they have formed. However, measurements of the relative spin-orbit alignment of transiting extrasolar planets via the Rossiter-McLaughlin effect have shown that a number of these planets have orbits that are significantly misaligned with the rotation axes of their host stars. Gravitational interactions with other planets or the Kozai mechanism are two proposed means in which planets in initially aligned orbits can get perturb into misaligned orbits. A recently published paper entitled “Misaligned and Alien Planets from Explosive Death of Stars” proposes an alternative mechanism to explain the planets that are observed to be in misaligned orbits around their host stars.

In this paper, planets whose orbits are misaligned with the rotation axes of their host stars are suggested to have formed from high-speed blobs of gas that are produced in supernova remnants and planetary nebulae. Supernova remnants are formed following the death of massive stars in supernova explosions while planetary nebulae are formed from the death throes of Sun-like stars as they shed off their outer layers into space. These blobs of gas have been observed in great numbers around supernova remnants and planetary nebulae. High resolution images of young supernova remnants and planetary nebulae have shown that each of them is surrounded by thousands of blobs of gas that have cometary-like appearance possibly shaped by overtaking winds. As these blobs of gas travel through interstellar space, they sweep up ambient matter along the way, causing them to increase in mass and decelerate. Over time, these blobs of gas gradually cool by emitting radiation. Once the blobs of gas become sufficiently massive and cool, self-gravity takes over and cause the blobs of gas to collapse gravitationally to form gas giant planets.

Regardless of whether these blobs of gas eventually contract gravitationally to form gas giant planets, they can explain the considerable number of planets that are found to be in misaligned orbits around their host stars through a number of different ways. For a blob of gas that has already collapsed gravitationally to form a gas giant planet, it can be captured into a misaligned orbit around a star or perturb the original planets that have previously formed around a star into misaligned orbits. On the other hand, an uncollapsed blob of gas can be captured into a misaligned orbit around a star to form a misaligned disk of gas and dust from which the in situ formation of planets with misaligned orbits can occur. Furthermore, uncollapsed free-floating blobs of gas can also be captured by stars and strongly perturb their planetary systems.

Around young supernova remnants and planetary nebulae, a typical blob of gas has a similar mass as the Earth and a ‘head’ which spans tens of billions of kilometers across. Shaped into cometary-like morphologies by faster overtaking winds, a typical blob of gas travels with an average projected velocity on the order of a few hundred kilometers per second outwards from its parent supernova remnant or planetary nebula. Blobs of gas in young supernova remnants can be classified into two populations distinguished by their velocities where the low velocity population formed in the ejecta of the progenitor star before its supernova explosion while the high velocity population formed following the supernova explosion.

As the blobs of gas travel outwards, they cool by radiation and grow in mass by accreting gas and dust in the interstellar medium. The accretion process causes the blobs of gas to decelerate, making the blobs of gas or the eventual gas giant planet slow enough to be gravitationally captured during sufficiently close encounters with stars. Finally, very high velocity blobs of gas from supernova explosions that travel through very low density regions of space will escape into the low-density intergalactic space where they will expand before they ever reach the required mass and compactness necessary for them to collapse gravitationally into gas giant planets. These very high velocity blobs of gas will enrich the intergalactic medium with heavy elements produced from supernova explosions.