Orbital Alignments: An Exoplanet Diagnostic Tool

The Rossiter-McLaughlin effect is an evolving tool for exoplanet research, one that has already begun to pay off. We recently looked at a paper studying whether this quirk of radial velocity methods could help in the detection of a terrestrial-class planet. The effect causes a distortion in radial velocity data during a planetary transit, one that seems to indicate a change in the velocity of the star under study. But in reality there is no change — what observers see is the effect of the transiting planet on the starlight, as shown in the diagram below.

Rossiter Effect

It turns out the effect might be useful in finding planets larger than two Earth radii, but perhaps less so with smaller worlds. However, new work by a Japanese/American team using the Subaru Telescope points to a different observational capability. Observing the extrasolar system TrES-1, the team has been able to measure the angle between the parent star’s spin axis and the planet’s orbital axis, only the third time such an alignment has been measured.

Image: The Rossiter-McLaughlin effect is defined as the radial velocity anomaly during a transit from the known Keplerian orbit caused by the partial occultation of the rotating stellar disk. For example, if a planet occults part of the blue-shifted (approaching) half of the stellar disk, then the radial velocity of the star will appear to be slightly red-shifted, and vice-versa. The radial velocity anomaly depends on the trajectory of the planet across the disk of the host star, and in particular on the spin-orbit alignment of the system. Thus by monitoring the Rossiter-McLaughlin effect one can measure the spin-orbit alignment. Credit: National Astronomical Observatory of Japan.

TrES-1, a K0 star with a visual magnitude of 12, is the faintest target ever used in such work; its planet TrES-1b is a ‘hot Jupiter’ with an orbital period of three days. The outcome is welcome considering that ongoing transit surveys often work with faint stars indeed. In fact, we’re getting more and more interested in M-dwarfs in a variety of ways (especially in an astrobiological sense), knowing that the dim red objects may help us snare transits of objects of Earth’s size and even smaller. Now we have a way to measure spin/orbit alignments, allowing the collection of further data by which to study planet formation.

Orbital spin alignment

Ponder this: Given the preponderance of Jupiter-class worlds close to their parent stars that has emerged from early radial-velocity work, we’d like to know more about how they formed. Migration inward through the early solar system seems a reasonable assumption. Planets that form and migrate like this ought to show relatively minor misalignments between the stellar spin axis and the planet’s orbital axis.

Image: An illustration of the concept of the spin-orbit alignment (described by lambda) in an exoplanetary system. Credit: National Astronomical Observatory of Japan.

But planets dispersed by the gravitational effects of giant planets (‘planet-planet scattering’) through a protoplanetary disk should show tilts different from the original orbital axis. If such mechanisms are in play in a given system, measuring the spin/orbit alignment through the Rossiter-McLaughlin effect should reveal the resultant misalignment. That makes this tiny observational twist a genuine diagnostic tool in the continuing development of planet formation theories.

What’s next is surely to tighten up the numbers. The current study can only show an alignment angle of 30 degrees with an error of plus or minus 21 degrees. These are useful contraints (enough to demonstrate the prograde orbital motion of TrES-1b), but they’ll be refined with future work. The paper is Narita et al., “”Measurement of the Rossiter–McLaughlin Effect in the Transiting Exoplanetary System TrES-1,” Publications of Astronomical Society of Japan, Vol 59, No. 4 (August 25, 2007), pp. 763-770, available here.

Watery Birth of a Stellar System

A Class 0 protostar is a star so young that the bulk of its light is emitted at long infrared wavelengths, blocked from Earth-based observatories by our atmosphere. It takes space-borne platforms like the Spitzer Space Telescope to make sense out of these objects, hundreds of which have now been identified, though few studied with the precision of the one designated IRAS 4B. There, signs abound of a region within the protostellar envelope that is warmer and denser than the material around it.


Located about a thousand light years from Earth in the nebula NGC 1333, the infant star presents an interesting signature to Spitzer’s infrared spectrograph. Out of thirty protostars examined by University of Rochester astronomers, IRAS 4B is the only one to show the infrared spectrum of water vapor, a fact understood to mean that material is falling from the protostar’s envelope onto the surrounding, denser disk. As the ice hits the protoplanetary disk, it heats rapidly and emits its characteristic infrared signature. We seem to be looking at the formative mechanism from which planetary systems later emerge.

Image: Artist’s rendition of the forming system at IRAS4B. Credit: JPL/CalTech.

At temperatures in the area of 170 degrees Kelvin (-102 C, or -153 F), the disk is currently an active place, says Dan Watson (University of Rochester):

“Icy material from the envelope is in free-fall, reaching supersonic speeds and crashing into the protoplanetary disk. The ice vaporizes on impact, and the warm water vapor emits a distinctive spectrum of infrared light. That light is what we measured. From the details of the measured spectrum we can tease out the physical details of this brand-new, pre-planetary disk.”

Enough water to fill Earth’s oceans five times forms what the astronomers are calling the ‘puddle’ on the disk’s surface, filling an area whose perimeter seems to extend out about 40 AU. An interesting implication of this work involves the icy bodies of our own outer Solar System, usually thought to carry ices descended directly from the interstellar medium. Not without an intervening step, says Watson:

“On Earth, water arrived in the form of icy asteroids and comets. Water also exists mostly as ice in the dense clouds that form stars. Now we’ve seen that water, falling as ice from a young star system’s envelope to its disk, actually vaporizes on arrival. This water vapor will later freeze again into asteroids and comets.”

The chemistry of the disk is essentially ‘reset’ by this process, making the uses of outer system ice in understanding interstellar conditions problematic. And note the interesting assumption that the reason only one of the 30 protostars under study showed signs of water is that the ‘watery’ phase of system formation is short-lived and therefore tricky to observe. As more and more protostars become available for study, that notion will surely be tested.

The paper is Watson et al., “The development of a protoplanetary disk from its natal envelope,” Nature 448 (30 August 2007), pp. 1026-1028 (30 August 2007). Abstract available.

Mini-Mag Orion: A Near-Term Starship?

Physics breakthroughs aside, are there more conventional ways we can reach the stars? Centauri Dreams often cites (with admiration) Robert Forward’s work on beamed laser propulsion, which offers a key advantage: The spacecraft need carry no bulky propellant. Forward’s missions involved a 7200-GW laser to push a 785 ton unmanned probe on an interstellar mission. A manned attempt would involve a 75,000,000-GW laser and a vast vehicle of some 78,500 tons. The laser systems involved in such missions, while within our understanding of physics, are obviously well beyond our current engineering.

Are there other ways to accomplish such an interstellar mission? One possibility is a hybrid system that combines what is known as Miniature Magnetic Orion technologies with beamed propulsion. The spacecraft would carry a relatively small amount of fission fuel, with the remainder of the propellant — in the form of particles of fissionable material with a deuterium/tritium core — being beamed to the spacecraft. In a recent paper in Acta Astronautica, Dana Andrews (Andrews Space) and Roger Lenard (Sandia National Laboratories) describe these technologies and their own recent studies of the Mini-Mag Orion concept.

Mini-Mag Orion, of course, harkens back to the original Project Orion, an attempt to develop a spacecraft that would be driven by successive detonations of nuclear bombs. Mini-Mag Orion takes the concept in entirely new directions, reducing the size of the vehicle drastically by using magnetic compression technology, which Andrews and Lenard have studied using Sandia National Laboratories’ Z-Pinch Machine, the world’s largest operational pulse power device. Their experimental and analytical progress is outlined in the paper referenced below; they now propose a follow-on program to extend their experimental work.

Mini-Mag Orion

The originally envisioned spacecraft would compress small fuel pellets to high density using a magnetic field, directing plasma from the resultant explosion through a magnetic nozzle to create thrust. This highly efficient form of pulsed nuclear propulsion is here paired for interstellar purposes with beamed propulsion methods, taking advantage of a pellet stream that continuously fuels the departing spacecraft. The interstellar Mini-Mag Orion attains approximately ten percent of light speed using these methods, and as Andrews and Lenard show, the hybrid technologies here studied reduce power requirements from the departing star system and the timeframe over which acceleration and power have to be applied.

Image: The Mini-Mag Orion interstellar concept, a hybrid starship accelerated by beamed pellet propellants, and decelerated with a magnetic sail. Credit: Roger Lenard/Dana Andrews; Andrews Space.

With the bulk of the propellant being supplied externally, deceleration in the target star system is an obvious challenge, one met through the use of a magnetic sail. Here is the authors’ explanation of what is essentially ‘free’ deceleration:

In 2003 both Andrews and Lenard postulated using a large superconducting ring to intercept charge particles in interstellar space to slow the spacecraft down from high speeds. Additionally, the solar wind emanating from a star system provides an additional source of charged particles that can interact with the magnetic ?eld. Deceleration can actually begin a sizable distance from the target star system… [T]he ?rst phase of the deceleration starts at 21600 AU with a two-turn superconducting carbon nano tube reinforced loop. This loop captures the charged interstellar medium and de?ects it to decelerate the spacecraft. This initial hoop size is 500 km in radius and carries 1,000,000 A of current. The spacecraft decelerates from .1 c to 6300 km/s by the time the spacecraft reaches 5000 AU. This will be quite a light show, so if there are any intelligent life forms with an observing system, they should be able to see the arrival.

Quite a light show indeed! But note this: Even in the absence of a paradigm-changing physics breakthrough, Andrews and Lenard, as Forward before them, have demonstrated that there are ways to reach nearby stars with technologies we understand today and may be able to build within the century. Assume methods no more advanced than these coupled with advances in biology and life extension and it is conceivable that long-lived human crews could populate the galaxy in a series of 60 to 90 light year expansions, an interstellar diaspora that, the authors calculate, could occur every four to five thousand years.

Work out the numbers and you get half the galaxy populated within a million years (Fermi’s question again resonates). The paper is Lenard and Andrews, “Use of Mini-Mag Orion and superconducting coils for near-term interstellar transportation,” Acta Astronautica 61 (2007), pp. 450-458.

A Relativistic Probe of Exotic Matter

We’d like to know a lot more about neutron stars. They’re doubtless the home of exotic matter of the sort we’re unable to create in any laboratory, and their extraordinary density leads to conditions in the space around them that are, shall we say, extreme. Gases whipping around three neutron stars at forty percent of the speed of light have now been used to take measurements of their diameter and mass.

A neutron star binary

Figure out the properties of such gases and you’ve nailed down a maximum size for the diameter of the neutron star Serpens X-1, for example, a figure that turns out to be between 18 and 20.5 miles across. A team led by Edward Cackett (University of Michigan) looked at the spectral lines from hot iron atoms around Serpens X-1 and two other neutron star binaries, GX 349+2, and 4U 1820-30. Independent work by Sudip Bhattacharyya and team (NASA GSFC) bolsters Cackett’s results and demonstrates the efficacy of the method.

Image: Many neutron stars are accompanied by a companion star, as portrayed in this illustration. The powerful gravity of the neutron star siphons off gas from the companion, which then settles into a slowly in-spiraling disk around the neutron star. Credit: NASA.

The factor at work here is the extreme velocity of the gas, which smears and distorts the iron line because of the Doppler effect and beaming effects that are in accordance with Einstein’s Special Theory of Relativity. Moreover, the powerful gravity of the neutron star warps spacetime (now we’re in the realm of the General Theory) and shifts the object’s iron lines to longer and longer wavelengths. These skewed iron lines tell the tale and the GSFC work confirms it. Says Cackett’s colleague Jon Miller:

“Now that we’ve seen this relativistic iron line around three neutron stars, we have established a new technique. It’s very difficult to measure the mass and diameter of a neutron star, so we need several techniques to work together to achieve that goal.”

It’s a goal that goes beyond the study of exotic particles or states of matter, although that’s a major plus. Because the behavior of these gases in the extreme environment near a neutron star can be deduced from the General Theory, observations like these offer a useful tool for examining and testing that theory’s parameters. In such ways do space-based platforms like the European Space Agency’s XMM-Newton x-ray observatory and the Japanese/NASA Suzaku x-ray observatory, both used in this work, help us mine observational data from a cosmic laboratory where no condition is too extreme to to be modeled.

The paper by Cackett and Miller is “Relativistic iron emission lines in neutron star low-mass x-ray binaries as probes of neutron star radii,” submitted to Astrophysical Journal Letters (abstract). The GSFC work by Sudip Bhattacharyya and Tod Strohmayer is “Evidence of a Broad Relativistic Iron Line from the Neutron Star Low-Mass X-ray Binary Serpens X-1,” in Astrophysical Journal Letters Volume 664, Number 2, Part 2 (August 1, 2007), pp. L103-L106 (abstract).

Dark Energy Paints the Void

A vast, empty region in Eridanus may be giving us hints about the operation of dark energy in the distant universe. The region shows up on the Wilkinson Microwave Anisotropy Probe’s map of the cosmic microwave background (CMB) radiation. The remnant of the Big Bang, the faint radio waves of the CMB provide the earliest picture we have of the cosmos. What the WMAP displayed to us was a view of its structure at a time just a few hundred thousand years after the Big Bang.

The Eridanus region stands out on the WMAP data because it’s slightly colder, and I do mean ‘slightly’ — we’re talking about temperature differences in the area of millionths of a degree. Two possibilities thus arise: The cold spot could be intrinsic to the CMB itself, a structural anomaly in the early universe. Or it could indicate something through which the CMB radiation had to pass on its way to our detectors. Now a study using data from the National Radio Astronomy Observatory VLA Sky Survey offers a possible confirmation of the latter.

For the Eridanus region shows a marked drop in the number of galaxies that would be expected there. Says Lawrence Rudnick (University of Minnesota):

“Although our surprising results need independent confirmation, the slightly lower temperature of the CMB in this region appears to be caused by a huge hole devoid of nearly all matter roughly 6-10 billion light-years from Earth.”

And that’s abnormal indeed. Yes, the universe is known to feature voids largely empty of matter, but none on this scale, nor does the magnitude of this ‘hole’ gibe with computer simulations of the large-scale structure of the universe. And the observational effect being examined may involve dark energy. The lack of matter creates lower temperatures in the CMB, the team theorizes, because CMB photons that pass through the void before reaching Earth should have less energy than those that pass through space filled with a normal distribution of matter.

Here’s the gist of what the team is arguing. With the paper not yet available online, I’ll have to work solely off the news release:

In a simple expansion of the universe, without dark energy, photons approaching a large mass — such as a supercluster of galaxies — pick up energy from its gravity. As they pull away, the gravity saps their energy, and they wind up with the same energy as when they started.

But photons passing through matter-rich space when dark energy became dominant don’t fall back to their original energy level. Dark energy counteracts the influence of gravity and so the large masses don’t sap as much energy from the photons as they pull away. Thus, these photons arrive at Earth with a slightly higher energy, or temperature, than they would in a dark energy-free Universe.

Conversely, photons passing through a large void experience a loss of energy.

But this work, as Rudnick said above, needs confirmation. As team member Liliya Williams (also at the University of Minnesota) emphasizes, “What we’ve found is not normal, based on either observational studies or on computer simulations of the large-scale evolution of the Universe.” Accounting for the size of this void and its relationship to the rest of the WMAP data will doubtless yield new surprises. Let’s hope it also has more to tell us about dark energy itself. The paper is slated for publication in The Astrophysical Journal; full references when they become available.