Planets Orbiting White Dwarfs

An extrasolar planet is basically a planet which orbits a star other than our Sun and a transiting extrasolar planet is one which periodically blocks a small fraction of the light from its parent star as its orbit brings it in front of the star’s luminous disk. Current missions such as NASA’s Kepler space telescope are sensitive enough to detect transiting extrasolar planets that are as small as the Earth around Sun-like stars.

Transits only occur when a planet’s orbit around its parent star happens to be orientated nearly edge-on with respect to our line of sight. Stars have randomly orientated planetary orbits and the probability that a planet is observed to transit its parent star is inversely proportional to the distance of the planet from its star. Therefore, a planet orbiting at a smaller distance from its star will have a higher probability of being observed to transit its star as compared to a planet orbiting at a larger distance.

For stars like the Sun, the fraction of light that a transiting planet blocks is small because the size of the star is much larger than the size of the planet. For example, if the Earth were to be observe transiting the Sun, it will cause the brightness of the Sun to dip by approximately 0.01 percent while if Jupiter were to be observe transiting the Sun, it will cause the brightness of the Sun to dip by approximately 1 percent.

White dwarfs are basically the end result of the evolution of main sequence stars such as the Sun and stars with less than 8 times the mass of the Sun eventually end up as white dwarfs. The Sun is 333 thousand times more massive than the Earth and a typical white dwarf can contain as much mass as the Sun gravitationally compactified into a dense sphere that is approximately the size of the Earth. In comparison, the Sun has a diameter that is 109 times larger than the Earth’s and a volume that can fit 1.3 million Earths.

As a result, when it comes to the relative fraction of starlight that can be blocked by a transiting planet, white dwarfs do offer a huge advantage over main sequence stars like the Sun. This is because the size of a white dwarf is much smaller than the size of a main sequence star and this allows a much greater fraction of a white dwarf’s light to be blocked by a transiting planet to generate a proportionally stronger transit signal. In fact, a Jupiter-size transiting planet can completely block out the light from a white dwarf since such a planet will be larger in size than the white dwarf.

The transit of an Earth-size planet across the luminous disk of a white dwarf will block out a significant fraction of the white dwarf’s light. In certain cases, it is even possible for the Earth-size planet to completely block out the white dwarf. Even the transit of an object as small as the Earth’s Moon in front of a white dwarf will occult a few percent of the white dwarf’s light. In comparison, the transit of an object as small as the Moon in front of a star like our Sun will only block out a minuscule 6 parts-per-million of the star’s light and such a weak signal will probably be hardly distinguishable from background noise. Therefore, white dwarfs offer an enormous advantage over main sequence stars as a means to detect small planetary objects.

Nonetheless, the detection of small transiting planetary objects around white dwarfs will require a higher observational cadence as compared to the detection of planets around main sequence stars like our Sun. This is because the small size of a white dwarf means that any transiting planet takes on the order of only a few minutes to transit across the entire luminous disk of the white dwarf. In comparison, the transit of a planet across the luminous disk of a main sequence star like our Sun takes on the order of a few hours which permits a much lower observational cadence.

White dwarfs are the final result of the evolution of main sequence stars like our Sun and there are ways in which planets can exist around white dwarfs. Before a star becomes a white dwarf, it will undergo a red giant phase where it will swell to many times its original size and engulf or vaporize planets that might be orbiting it in close vicinity. However, planets that orbit further out and planets that are sufficiently massive can survive the star’s evolution to a white dwarf and continue to orbit the white dwarf.

As a star evolves to a white dwarf, it can lose a significant fraction of its mass and this can destabilize any system of planets that is in orbit around the star. In such a scenario, planets can gravitationally interact with each other and can either be scattered into a tighter orbit around the white dwarf or get boosted into a more distant orbit around the white dwarf. It is also possible for planets to get completely ejected from the system. Planets that are scattered into a tighter orbit around the white dwarf will improve their probability of being observed to transit the white dwarf because the transit probability of a planet is inversely proportional to the distance of the planet from its star.

Interestingly, there is also an alternate mechanism that can allow planets to exist around white dwarfs. When a closely spaced pair of white dwarfs eventually merges due to the loss of orbital energy via the emission of gravitational radiation, a second generation of planets can form out from the disk of debris from the tidal disruption of the lower mass white dwarf. Therefore, the existence of an entire second generation system of planets, moons and asteroids in close orbit around a white dwarf is quite plausible.

The detection of transiting planetary objects around white dwarfs will help constrain the effectiveness of the mechanisms in which planets can exist around white dwarfs. To detect such transits, a large number of white dwarfs have to be continuously sampled at a high observational cadence. Currently, NASA’s Kepler space telescope is most suited for the detection of transiting planetary objects around white dwarfs. This is because Kepler has an extremely high observational cadence since its CCDs are read out every six seconds and Kepler is theoretically sensitive enough to detect objects as small as asteroids as they transit in front of white dwarfs.

Rogue Worlds

The formation of planets in protoplanetary disks around stars is a messy process and a significant number of protoplanets with masses similar to planets such as the Earth or Mars may get ejected from their solar systems by gravitational interactions with massive gas giant planets. It is even possible that more protoplanets are ejected than retained in protoplanetary disks around stars and a considerable number of such ejected worlds might be wandering in the immense and dark expanses of interstellar space. Interstellar space is basically the vast and enormous spaces between stars.

An ejected planet with the mass of the Earth can retain an atmosphere of hydrogen in the frigid temperatures of interstellar space since the atmosphere of hydrogen will be cold enough to be bounded by the gravity of the planet. In comparison, at the distance where the Earth is from the Sun, a planet has to be over 10 times more massive than the Earth for its own gravity to be sufficiently strong enough to keep an atmosphere of hydrogen from escaping into space. In this article, I shall denote such ejected worlds as interstellar planets. From a distance, an interstellar planet will appear as a dark silhouette against a field of distant background stars.

The effective temperature of an interstellar planet is expected to be just several degrees Kelvin above absolute zero and any form of water at this temperature will be frozen solid. However, if an interstellar planet has a sufficiently thick atmosphere of hydrogen where the pressure at the bottom of such an atmosphere ranges from a hundred to a few thousand bars, the pressure-induced infrared opacity of molecular hydrogen will greatly insulate the planet from dissipating its internal radiogenic heat into space. At high pressures, molecular hydrogen is very effective at trapping heat and this significantly reduces the amount of heat lost into space.

With such an overlying atmosphere of hydrogen, the surface temperature of an interstellar planet can potentially exceed the meting point of water! In an environment like this, it will be possible for oceans of liquid water to exist on the planet’s surface and the ocean can be kept warm by the persistent flux of heat generated by the decay of radioactive isotopes in the planet’s interior.

On the surface of an interstellar planet, the sky will appear totally back and it is highly unlikely that stars will be visible from the planet’s surface due to the thick hydrogen atmosphere. However, areas of the planet’s surface can still be illuminated by occasional flashes of lightning. In the lower and warmer reaches of the atmosphere, it is possible for clouds of water droplets to form and drive a hydrological cycle. The atmospheric temperature decreases higher up in the atmosphere, possibly enabling other gases such as ammonia and nitrogen that have lower condensation temperatures to condense into clouds at these higher altitudes. Therefore, an interstellar planet can have several cloud layers of different condensates in its atmosphere.

To consider the Earth as the only populated world in infinite space is as absurd as to assert that in an entire field sown with millet, only one grain will grow.

- Metrodorus of Chios, 4th century BC

An interstellar planet can provide a stable environment for life for billions of year until the surface temperature declines below the meting point of water as the heat generating radioisotopes in the interior of the planet slowly gets depleted. Interstellar planets will be extremely difficult to detect as the amount of radiation they emit is exceedingly miniscule compared to the large amount of solar radiation the Earth reflects back into space as the Earth basks in the warm vicinity of the Sun. Finally, if interstellar planets do exist in significant numbers in the vast and uncharted expanses between the stars, these dark worlds might serve as common sites for life-supporting environments in the universe.

Triton's Shooting Stars

Triton is by far the largest moon in orbit around the planet Neptune and with a diameter of 2700 kilometers; Triton is the seventh largest moon in the Solar System. Interestingly, Triton is the only large moon in the Solar System with a retrograde orbit, whereby it orbits Neptune in the opposite direction to Neptune’s rotation. Triton has a relatively high rock/ice ratio of approximately 70/30 and this composition is remarkably similar to large Kuiper Belt objects such as Pluto and Eris. Because of Triton’s retrograde orbit and its similar composition to objects such as Pluto and Eris, Triton is believed to be a Kuiper Belt object that was captured into orbit around Neptune.

Triton orbits Neptune at a distance of 354800 kilometers and it takes 5 days and 21 hours for Triton to orbit once around Neptune. Like the other large moons in the Solar System, Triton is tidally locked to Neptune where it keeps the same hemisphere oriented towards Neptune at all times. Triton was discovered by British astronomer William Lassell in 1846, only several days after Neptune itself was discovered. The first and only closed-up images of this elusive and far-flung moon of Neptune came on August 1989, after a close fly-by of Triton by NASA’s Voyager 2 spacecraft.

Apart from Saturn’s moon Titan, Neptune’s moon Triton is the only other moon in the Solar System with an appreciable atmosphere. Similar to both the Earth and Titan, nitrogen is the main constituent of Triton’s atmosphere. The atmospheric pressure on the surface of Triton is over 50000 times less than the atmospheric pressure at sea-level on the Earth. This is actually equivalent to the atmospheric pressure up in the Earth’s mesosphere at well over 50 kilometers above the Earth’s surface. Triton’s extremely frigid and cold environment allows the nitrogen in its atmosphere to be deposited onto the surface as frozen nitrogen. On the surface of Triton, the Sun will appear almost a thousand times dimmer than from the Earth’s surface.

The atmosphere of Triton contains thin clouds of nitrogen ice particles that are located a few kilometers above the surface. Above the clouds of nitrogen ice particles is a haze layer which extends up to 30 kilometers above Triton’s surface. This haze layer is made up of hydrocarbons and nitriles created when ultraviolet light from the Sun breaks down methane in Triton’s atmosphere. The surface of Titan also contains numerous erupting geysers of nitrogen gas. By itself, nitrogen gas is invisible and it is the entrained dust within the nitrogen gas which allows the geysers to be seen as plumes rising from the surface up to a height of 8 kilometers above Triton’s surface. The entrained dust within a plume can be deposited to over a hundred kilometers downwind of the plume and these plumes are responsible for creating the long and dark streaks on the surface of Triton.

Although Triton’s atmosphere is rather tenuous compared to the Earth’s atmosphere, it is still dense enough to ablate micrometeoroids when they pass through. The combination of a micrometeoroid’s orbital velocity around the Sun, the orbital velocity of Triton around Neptune, the orbital velocity of Neptune around the Sun and the gravitational accelerations of both Triton and Neptune, can allow the micrometeoroid to achieve a fast enough impact velocity that will enable the micrometeoroid to be sufficiently heated to visibility by friction with the air molecules in Triton’s atmosphere.

As Triton orbits Neptune, most micrometeoroids will impact the leading hemisphere of Triton and this is where most micrometeoroids can be observed. When a micrometeoroid penetrates an atmosphere, it heats up due to friction with the air molecules in the atmosphere. This causes material to sputter off the surface of the micrometeoroid particle in a process called ablation. Sufficiently strong heating and ablation can enable a micrometeoroid to become a visible meteor.

Due to the thin atmosphere of Triton and the much lower speeds at which micrometeoroids will impact the atmosphere of Triton as compared to the Earth, micrometeoroids can penetrate all the way down to the surface of Triton. This is unlike the Earth where micrometeoroids vaporize entirely high in the atmosphere. On Triton, visible meteor trails can extend all the way down to the surface. Icy micrometeoroids are expected to produce brighter meteor trails than stony micrometeoroids because icy micrometeoroids have a greater rate of ablation and the brightness of a meteor trail is directly related to the rate of ablation of the micrometeoroid.

Additionally, large variation in the brightness of meteors is expected to occur along different phases of Triton’s orbit around Neptune. The brightness of a meteor trail is expected to be the greatest when Triton’s orbital velocity around Neptune adds up most positively with Neptune’s orbital velocity around the Sun. The Neptune-Triton system is unique, because unlike other planet-satellite systems, it features a remarkably large variation in the meteoroid impact velocity onto Triton as a function of Triton’s orbital phase position around Neptune.

In addition to producing visible meteor tails, the ablation of incoming micrometeoroids can deposit metallic atoms and molecules that were once part of the micrometeoroid into the atmosphere of Triton. These metallic atoms and molecules can condense into dust particles in the atmosphere of Triton and these dust particles could serve as nucleation centers for the condensation and formation of cloud and haze particles observed in Triton’s atmosphere. Most of these metallic atoms and molecules will eventually get deposited together with the condensates onto the surface of Triton.

Stellar Behemoth

The Tarantula Nebula is an extremely luminous nebula that is located approximately 165000 light years away in the Large Magellanic Cloud and it is the largest and most active star formation region known in the Local Group of galaxies. The Large Magellanic Cloud is basically a nearby irregular galaxy which is roughly one-tenth as massive as the Milky Way Galaxy.

At the heart of the Tarantula Nebula lies an exceptionally dense cluster of stars called R136 which generates most of the light that illuminates the Tarantula Nebula. 4 exceedingly massive and luminous stars sit in the core of the R136 star cluster and they are designated R136a1, R136a2, R136a3 and R136c respectively. Each star is well over 100 times more massive than the Sun and each star is millions of times more luminous than the Sun.

The most massive and luminous of the 4 central stars in the R136 star cluster is the star called R136a1. This behemoth is currently the most massive and luminous star discovered so far. R136a1 has a current mass that is 265 times the mass of the Sun and an initial mass that is estimated to be 320 times the mass of the Sun! Since its birth, R136a1 has shed over 50 times the mass of the Sun in extremely powerful stellar winds.

R136a1 also shines with an “off-the-charts” luminosity that is approximately 10 million times greater than the Sun’s luminosity! To put this extreme luminosity into perspective, R136a1 emits as much energy in 3 seconds as the Sun emits in an entire Earth year! With an age of just over a million years, R136a1 is already a middle-aged star. In comparison, our Sun is already 5000 million years old and the Sun is only halfway through its lifespan.

Although the R136 star cluster contains approximately 100000 stars, its 4 brightest stars (R136a1, R136a2, R136a3 and R136c) account for approximately half of the wind and radiation power of the entire cluster of stars! Because very massive stars are so exceedingly rare, it is unlikely that there is any other star in the Tarantula Nebula or possibly even in the entire Local Group of galaxies that will be comparable to the brightest components of the R136 star cluster. An ultra-massive star like R136a1 is considered to be a very extreme case for a star and R136a1 might well be one in a trillion.

Source: Paul A Crowther, et al. (2010), “The R136 Star Cluster Hosts Several Stars Whose Individual Masses Greatly Exceed the Accepted 150 M_Sun Stellar Mass Limit”, arXiv:1007.3284v1

Mercurian Ice

Mercury is the closest planet from the Sun and it orbits the Sun once every 88 Earth days. Due to its eccentric orbit, Mercury has a minimum distance of 46.0 million kilometers from the Sun and a maximum distance of 69.8 million kilometers from the Sun. This causes the intensity of sunlight on Mercury to vary by over a factor of two as Mercury orbits the Sun. Mercury is also locked in a spin-orbit resonance where the ratio of orbital period to spin period is precisely 3:2. This means that Mercury completes three rotations about its axis for every two orbits around the Sun.

With a diameter of 4880 kilometers, Mercury is the smallest of the terrestrial planets and despite its size; it is the only other terrestrial planet besides the Earth that has a global magnetic field. Mercury also has a large and dense iron core which makes up well over half the planet’s mass. This makes Mercury the second densest planet in the Solar System with Earth being the densest due to gravitational compression effects. Had it not been for Earth’s gravitational compression effects, Mercury would have been the densest planet in the Solar System.

Mercury has a large surface temperature range which goes from a maximum of 700 degrees Kelvin at the subsolar point when Mercury is closest to the Sun to a minimum of below 100 degrees Kelvin at the bottoms of craters located around the poles. The subsolar point on a planet such as Mercury is where the Sun is directly overhead and hence, the Sun’s rays strike the surface perpendicularly at the subsolar point.

Permanently shadowed craters are known to exist around the north and south poles of Mercury. Since Mercury has an axial tilt that is almost zero, the rims of many of these craters are able to shield the Sun and keep the floors of the craters in permanent darkness. The temperatures within these permanently shadowed craters can go below 100 degrees Kelvin as the Sun never rises above the crater rims to warm the frigid interiors of these craters. Within these permanently shadowed craters, water can exist in the form of ice and remain stable over billions of years. In fact, radar observations have revealed the presence of ice deposits within permanently shadowed craters in Mercury’s polar regions.

These permanently shadowed regions within craters around the poles of Mercury receive only scattered sunlight and thermal emissions from the surrounding topography. The temperatures within these permanently shadowed regions are therefore sensitive to the orientations of the surface and surrounding topography. Finally, burial under a thin regolith layer can enable the ice deposits to remain stable at higher temperatures and can extend the presence of ice deposits to lower latitudes.

Currently, NASA’s MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) spacecraft is on its way to Mercury and MESSENGER is expected to enter orbit around Mercury on 18 March 2011. Since its launch on 3 August 2004, MESSENGER has made one Earth flyby, two Venus flybys and three Mercury flybys. These flybys are a form of gravity assist maneuvers which greatly reduce the amount of fuel required to fly MESSENGER on the right trajectory that will allow it to eventually enter orbit around Mercury. The three flybys of Mercury by MESSENGER have already generated an astonishing amount of interesting science that is poised to greatly change and increase our understanding of the elusive closest planet from the Sun. If you want to find out more, visit the mission homepage at http://messenger.jhuapl.edu/.

Frozen Oasis

Liquid water cannot exist on the surface of the Moon as it will rapidly evaporate on its airless surface or get broken down into hydrogen and oxygen by sunlight. However, water can be present in the form of ice within permanently shadowed craters at the Moon’s poles. In such places, the Sun never gets high enough over the horizon to cast its rays over the rims of these craters and illuminate the floors of the craters.

Without ever being warmed by the Sun, these permanently shadowed regions can maintain incredibly low temperatures which make them ideal for water in its frozen form to exist over billion-year timescales. In fact, an instrument onboard NASA’s Lunar Reconnaissance Orbiter recorded temperatures as low as 25 degrees Kelvin or -248 degrees Centigrade in areas within these permanently shadowed regions, making them amongst the coldest known places in the Solar System!

[29] From whose womb comes the ice? Who gives birth to the frost from the heavens [30] when the waters become hard as stone, when the surface of the deep is frozen?

- Job 38:29-30 (New international Version)

The permanently shadowed craters at the Moon’s poles can serve as cold traps where water brought to the Moon by impacting comets can accumulate in these places. This week, I researched on the retention of water from the impacts of comets onto the surface of the Moon. Comets are small icy objects which orbit the Sun and they range in sizes from a few hundred meters to tens of kilometers across. Comets are known to contain a large amount of volatiles, especially water in the form of ice. When a comet impacts the Moon, a fraction of the water from the comet can eventually end up in these permanently shadowed craters and accumulate there in the form of ice.

In my research, I derived a method which estimates the fraction of the comet’s mass that remains gravitationally bound to the Moon after the impact. I carried out the computations and analysis for various impact velocities and various impact incident angles. From my results, a significant fraction of the comet’s mass remains gravitationally bound to the Moon after the impact as long as the impact velocity of the comet is less than 30 to 40 kilometers per second. In my analysis, the comet’s mass is assumed to be entirely made up of water.

Of the fraction of the comet’s mass in water which remains gravitationally bound to the Moon after the impact, a portion can survive long enough in its migration across the surface of the Moon to eventually accumulate in the permanently shadowed craters at the Moon’s poles. The presence of water on the Moon is an important factor in determining lunar habitability since a large and easily accessible source of water on the Moon will render needless the prohibitively expensive feat of transporting water from the Earth. Water can be separated into hydrogen and oxygen to provide breathable oxygen and to serve as a form of rocket fuel.

I have also extended my method of analysis in estimating the retention of water from the impacts of comets onto the surface of the Moon to other worlds such as the planet Mercury. With a stronger gravitational field, Mercury is able to retain a larger fraction of a comet’s watery mass and like the Moon; Mercury also has permanently shadowed craters at its poles where frozen water can accumulate.

Visiting Europa

Jupiter has over twice the mass of all the other planets in our Solar System combined and it is the archetype of large gas giant planets, especially so for the countless giant planets now known to orbit other stars. Orbiting Jupiter are 4 large moons named Io, Europa, Ganymede and Callisto. Additionally, Jupiter also has a few dozen small irregular satellites and a ring system in orbit around it. The 4 large moons of Jupiter, also known as the Galilean satellites, are particularly fascinating. Io is by far the most volcanically active world in the Solar System while Europa has a huge global ocean of water hidden beneath just a thin layer of ice. Ganymede and Callisto are large moons that are believed to also harbor internal oceans. Ganymede is the largest moon in the Solar System and it is even larger than the planet Mercury.

A paper by K. Clark, et al. (2010) entitled “Return to Europa: Overview of the Jupiter Europa Orbiter Mission” describes a mission to explore Jupiter’s ocean moon Europa. The Europa Jupiter System Mission (EJSM) is a proposed mission by the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) to explore Jupiter and its moons. The Jupiter Europa Orbiter (JEO) will make up the NASA-led portion of the EJSM and it is a satellite that will be placed into orbit around Europa.

Europa is a particularly interesting and intriguing moon of Jupiter because its subsurface ocean is in direct contact with the rocky interior of Europa, enabling the water to be infused with minerals and energy that is necessary for life through features such as hydrothermal vents. Thus, conditions at the bottom of Europa’s ocean could be very similar to the Earth’s ocean floor. Europa’s subsurface ocean is estimated to contain far more water than all the oceans on the Earth combined and the aquatic environment of Europa’s ocean is likely to be within the constraints of known life on Earth.

The Jupiter Europa Orbiter (JEO) is expected to be launched onboard an Atlas V 551 launch vehicle in the first quarter of 2020 and it will use a Venus-Earth-Earth gravity assist interplanetary trajectory to get to Jupiter. It will take approximately 6 years for JEO to get to Jupiter where it is expected to arrive at the end of 2025 or the beginning of 2026. Upon reaching Jupiter, JEO will perform a series of gravity assist with the moons Io, Europa, Ganymede and Callisto over a 30 month period to reduce its orbital energy with respect to Europa. This mission phase provides a unique opportunity to explore the Jovian system as it also includes several flybys of each of the 4 large moons of Jupiter.

In the middle of 2028, the Europa Orbit Insertion (EOI) will occur whereby a main engine burn will decelerate JEO into a low circular orbit around Europa. JEO will then begin its nominal Europa science campaign which is expected to last for 9 months and a mission extension beyond 9 months is very likely because the compounding effect of applying worst-case assumptions at every level in the design of the spacecraft tends to severely underestimate the mission lifetime.

JEO will be powered by Multi-Mission Radioisotope Thermoelectric Generators (MMRTGs) where the radiogenic heat from the decay of Plutonium-238 will be used to power the onboard systems. The huge amount of radiation that JEO will be subjected to throughout its mission poses a unique technical challenge. The 4 main sources of radiation are solar radiation, galactic cosmic rays, high energy particles trapped within the Jovian magnetosphere and neutrons and gamma ray photons from the onboard MMRTG nuclear power source. The original paper briefly addresses the radiation risks and various mitigation methods for the JEO mission.

The JEO mission science objectives, as defined by the international EJSM Science Definition Team are:

1. Europa’s Ocean: Characterize the extent of the ocean and its relation to the deeper interior.

2. Europa’s Ice Shell: Characterize the ice shell and any subsurface water, including their heterogeneity, and the nature of surface-ice-ocean exchange.

3. Europa’s Chemistry: Determine global surface compositions and chemistry, especially as related to habitability.

4. Europa’s Geology: Understand the formation of surface features, including sites of recent or current activity, and identify and characterize candidate sites for future in situ exploration.

5. Jupiter System: Understand Europa in the context of the Jupiter system.

If you want to read more about the Europa Jupiter System Mission (EJSM), you can download the final report from http://opfm.jpl.nasa.gov/library/.

Alien Planets

Kepler is NASA’s first mission that is capable of finding Earth-sized worlds orbiting other stars and Kepler is a space telescope that is named after German astronomer Johannes Kepler. Kepler was launched into space on 7 March 2009 onboard a Delta II rocket. On 12 May 2009, Kepler completed its commissioning phase and started searching for planets around other stars.

Kepler utilizes the transit photometry method to detect extrasolar planets by precisely monitoring the brightness of about 150000 selected stars in its field-of-view. A transit occurs when a planet passes in front of its parent star and causes a slight decrease in the star’s apparent brightness from the blocking of a small fraction of the star’s light by the planet’s opaque disk. Therefore, transits only occur when a planet’s orbit around its parent star happens to be orientated nearly edge-on with respect to an observer’s line of sight. A larger planet will block out a greater fraction of the star’s light compared to a smaller planet and Kepler is sensitive enough to observe the miniscule drop in brightness when an Earth-sized planet transits a Sun-like star.

An Earth-sized planet transiting a Sun-like star will cause an 84 parts-per-million decrease in the star’s apparent brightness while a Jupiter-sized planet transiting a Sun-like star will cause a one percent decrease in the star’s apparent brightness. For the same star, the dip in its apparent brightness from the transit of a Jupiter-sized planet is over 100 times greater than the signal from a transiting Earth-sized planet. Kepler has to observe at least three transits to be sure that the dimming of a star is caused by a planet. Therefore, the discovery of Earth-sized planets in Earth-like orbits around Sun-like stars is expected to take three years or longer. However, Kepler has already turned out a myriad of potential planetary candidates during its first several days of observations.

On 15 June 2010, the Kepler mission team released data on all but 400 of the 706 targets from data collected during the first 43 days of Kepler’s nominal observation phase. This set of data contains viable extrasolar planet candidates with sizes ranging from as small as that of the Earth to larger than that of Jupiter! The paper detailing this is entitled “Characteristics of Kepler Planetary Candidates Based on the First Data Set - The Majority are found to be Neptune-Size and Smaller (2010)” and the appendix of this paper shows a list of 306 viable extrasolar planet candidates.

The publicly released list of 306 targets with viable extrasolar planet candidates shows that most candidate planets are significantly smaller than Jupiter. In fact, most of the candidate planets range in size from being slightly larger than the Earth to Neptune-sized worlds. Interestingly, 5 of the 306 targets are stars with multiple transiting candidate extrasolar planets and a paper entitled “Five Kepler Target Stars that Show Multiple Transiting Exoplanet Candidates (2010)” further describes this.

Data for 400 of the 706 targets with viable planetary candidates were not publicly released because they are bright enough for high-quality Doppler measurements or contain candidate planets with less than 1.5 times the diameter of the Earth, or both. Data for these 400 targets will only be released in February 2011.

Considering that there are approximately 470 known extrasolar planets as of July 2010, follow-up confirmations of the targets in this first set of data from Kepler is expected to dramatically increase the known planet count! Besides the discovery of Earth-sized planets, Kepler is also able to detect interesting objects such as a Saturn-style ring system around a planet or an Earth-sized moon of a gas giant planet. I bet that this initial set of data with 706 targets containing viable extrasolar planet candidates is just a sneak preview of things to come!

Trillion Years

Red dwarf stars are by far the most common stars in the galaxy and they have masses ranging from 0.08 to 0.4 times the mass of the Sun. 0.08 times the mass of the Sun is just about the lowest possible mass a star can have and still be able to sustain hydrogen fusion within its core. The least massive red dwarf stars shine at only 0.01 percent the luminosity of the Sun while the most massive ones do not exceed 10 percent the luminosity of the Sun.

Assuming that the lifespan of a star is the total duration in which it is able to sustain nuclear fusion reactions, the lowest mass red dwarf stars can have lifespans that exceed 10 trillion years. In comparison, the current age of the universe is a mere 13.7 billion years and the estimated lifespan of the Sun is just 12 billion years. At the current age of 13.7 billion years, all the red dwarf stars in the universe have only just begun their seemingly eternal existence.

One reason why red dwarf stars have such incredibly long lifespans when compared to more massive stars is because red dwarf stars have fully convective interiors and this means than almost all of the hydrogen within such stars is available for sustaining nuclear fusion within the cores of these stars. A more massive star such as the Sun has a mostly radiative interior and this means that only the hydrogen within the core of the Sun is available for nuclear fusion due to the absence of any convective mixing between the matter in the core with the matter in the overlying layers. The other reason for the longevity of red dwarf stars is that such stars burn their hydrogen via nuclear fusion at a much smaller rate than more massive stars.

In this article, we shall follow the evolution of a red dwarf star that has 0.1 times the mass of the Sun. It takes an estimated 2 billion years for this red dwarf star to contract from an initial cool cloud of hydrogen and helium to the point where is able to sustain hydrogen fusion within its core. At the onset of stable hydrogen fusion within its core, the newly formed red dwarf star will have a surface temperature of 2200 Kelvin and shine at 0.04 percent the luminosity of the Sun. Since the red dwarf star has a fully convective interior, almost all of its hydrogen is available to sustain the fusion reactions within its core. With 0.1 times the mass of the Sun, this red dwarf star is estimated to have a nuclear burning lifespan of over 6 trillion years. In fact, the current age of the universe is not even a quarter of a percent of the multi-trillion year lifespan of this red dwarf star.

At age zero, the mass of the red dwarf star is three quarters hydrogen and one quarter helium. Over the subsequent trillions of years, the red dwarf star will fuse hydrogen into helium within its core, gradually converting more of its fraction by mass into helium. The steady rise in the helium mass fraction of the red dwarf star increases the rate at which energy is being generated by nuclear fusion in the core of the star, causing the surface temperature and the overall luminosity of the star to also increase.

After 3.1 trillion years, the red dwarf star’s fraction of mass that is helium surpasses the fraction of mass that is hydrogen. At this point, the red dwarfs star will have a surface temperature of 2500 Kelvin and shine at 0.1 percent the luminosity of the Sun. As the red dwarf star crosses the age of 5.7 trillion years, over 85 percent of its mass will now be in the form of helium and this is the point where radiative transport of energy replaces convection in the core of the star. At this stage, the red dwarfs star will have a surface temperature of 3500 Kelvin and shine at 0.3 percent the luminosity of the Sun.

The creation of the radiative core within the red dwarfs star signifies the closing stages of its near eternal lifespan as the evolution of the star begins to accelerate. The core of the red dwarf star increases in mass via the buildup of helium as the remaining hydrogen undergoes fusion into helium in a shell surrounding the core which gradually moves outward through the star. During this process, the surface temperature and luminosity of the red dwarf star continues to increase until it eventually reached a maximum surface temperature of 5800 Kelvin and shines with just under one percent the luminosity of the Sun. In fact, the surface temperature of the red dwarf star is now slightly greater than the surface temperature of the Sun even though its overall luminosity is much lower due to its vastly smaller size compared to the Sun. At this stage, the red dwarf star is a far cry compared to what it initially was.

After the red dwarf star attains its maximum surface temperature, it beings to turn around and evolves towards a lower surface temperature and a lower luminosity. At this point, the red dwarf star is still producing energy by burning hydrogen into helium in a shell surrounding a large and inert helium core. The rate at which energy is being generated by the fusion of hydrogen into helium in the shell gradually diminishes and it is eventually extinguished at 540 billion years after the red dwarf star first develops its radiative core. At this point in time, the red dwarf star has a surface temperature of 1700 Kelvin and shines with 0.0005 percent the luminosity of the Sun, 80 times dimmer than its luminosity at birth. Since the onset of the radiative core occurs 5.7 trillion years into the lifespan of the red dwarf star, the total duration of nuclear burning within the star adds up to just over 6 trillion years.

The red dwarf star now ends its life as a low mass helium white dwarf star with a final mass fraction where 99 percent of it is helium with the remaining 1 percent being hydrogen. This final mass fraction shows the extraordinary efficiency in which the red dwarf star generates energy by burning its hydrogen into helium through nuclear fusion. In comparison, the Sun burns only 10 percent of its hydrogen throughout its entire lifespan.

A 6 trillion year lifespan is not the longest a red dwarf star can possibly have. In fact, a red dwarf star with 0.08 times the mass of the Sun has an estimated lifespan of 12 trillion years, making it twice as long as a red dwarf star with 0.1 times the mass of the Sun. In this incredibly distant future universe, the red dwarf star with 0.1 times the mass of the Sun has finally evolved into a white dwarf star. After many trillions of years of further cooling, this white dwarf star will eventually become a black dwarf where its surface temperature gets ever nearer to absolute zero.

Given that red dwarf stars make up the vast majority of stars in a galaxy and that these stars can live for trillions of years, most of the stellar evolution that will occur has yet to occur. In the far future universe, red dwarf stars will play an increasingly important role in contributing to the total luminosity of a galaxy as the rate of star formation decreases and as the more massive stars in the galaxy age and fade away. This is because the gradual increase in the luminosities of red dwarf stars nearly compensates the loss in luminosity as the rate of star formation declines and as the more massive stars fade away. This ultimately causes the total luminosity of the galaxy to remain fairly constant over trillions of years.

Intergalactic Wanderer

Hypervelocity stars are stars with sufficiently large velocities that they are no longer gravitationally bound to the galaxy. While ordinary stars have velocities on the order of 100 kilometers per second, hypervelocity stars have velocities on the order of 1000 kilometers per second. At such velocities, hypervelocity stars will escape their home galaxy forever and become lone wanderers of intergalactic space. In February 2010, I wrote and posted a short article about hypervelocity stars, including possible mechanisms that can lead to such stars.

For a moment, imagine a Sun-like hypervelocity star with an entire system of planets orbiting it, where one of the planets is an Earth-like world that is not too different from ours. The Sun-like star and its system of planets are traveling in excess of 1000 kilometers per second, on a trajectory that has already taken them far from the home galaxy. How will the night sky from the surface of such an Earth-like world appear as it wanders the dark and immense distances of intergalactic space?

In the vast and starless voids of intergalactic space, the night sky from the surface of this Earth-like planet will be totally devoid of any stars. Assuming that the Earth-like planet and its parent star left their home galaxy a hundred million years ago, an alien observer on the surface of the Earth-like planet will be able to see the entire galaxy as a disk of wispy arms spiraling out from a glowing central bulge. The galaxy will span across a huge swath of the starless night sky and none of the hundreds of billions of stars that make up the galaxy will be individually distinguishable with unaided eyes. Every several years or so, a star in the galaxy will end its life in a supernova explosion and it will appear as a brilliant point of light which will suddenly appear and gradually fade.

It may take a huge leap of imagination and ingenuity for an extraterrestrial civilization living on this planet to realize that each of the hundreds of billions of miniscule points of light that comprise the galaxy are actually stars not too different from their yellow Sun. Imagine the idea of interstellar space travel in such a scenario where instead of a mere 4.37 light years away, the nearest stars are many thousands of light years away! Additionally, this extraterrestrial civilization might even contemplate about the possibilities of other extraterrestrial civilizations living on worlds within the galaxy and how these civilizations might possibly figure out the shape of the galaxy in which they live in!