Narrowing Down the Hunt for Giant Exoplanets

Despite advances in exoplanet research over the past decade much remains unknown. For example, how do the detection rates of giant planets vary as a function of the host star’s metal content? Are giant planets more frequent around massive stars?  Do giant planets form under different mechanisms depending on the star’s metal content?

To that end a team of astronomers led by Annelies Mortier and Nuno C. Santos explored what mathematical function characterizes the detection rate across a distribution of stars (i.e., from metal-rich to metal-poor objects).  “Finding the exact functional form of the metallicity-planet detection frequency will foster our understanding of both planet formation and the number of planets roaming the galaxy,” Santos told Universe Today.

Giant planets are most often found around metal-rich stars, and a figure from the team’s study (shown below) reaffirms that ~25% of stars featuring twice the Sun’s metal content host a giant planet, while the probability falls to ~5% for stars with a metal content analogous to the Sun.

Establishing that metal-rich stars exhibit an increased probability of hosting a giant planet constrains planet formation models.  Specifically, the observations suggest that larger metallicity promotes the growth of rocky/icy cores, which subsequently accrete gas.  However, the team notes that although the giant planet-metallicity trend is solid for stars exhibiting metallicities greater than (or analogous to) the Sun, the results are less certain for metal-poor stars.  Indeed, there is an active debate in the literature pertaining to what function links the metal-rich and metal-poor regimes. In particular, does an exponential decline extend into the metal-poor regime, or does the function level off?

Depending on the manner in which the frequency trend extends into the metal-poor regime, it may indicate that a separate mechanism is responsible for creating that subsample’s giant planets. Thus continued surveys of metal-poor stars are important, despite the decreased frequency of finding a giant planet.  Moreover, Mortier (Centro de Astrofisica, Universidade do Porto) notes that, “Studying metal-poor stars should be encouraged, since several theoretical models show that Earth-like planets are more common around these stars than around their metal-rich counterparts.”

Frequency of giant planets as a function of metallicity (A. Mortier et al., arXiv:1302.1851).
Frequency of giant planets as a function of metallicity (credit: Mortier et al., arXiv 1302.1851).

The team focused their efforts on trying to discern a difference between the viability of various functional forms in the metal-poor regime (i.e., does the detection rate of giant planets in that domain flatten, rather than decline exponentially?).  In the end no statistical difference was found between the scenarios, and it was likewise unclear whether a mass-dependence exists behind the frequency of giant planet detections.  The team noted that a larger sample was needed to reach definitive conclusions, and added that ongoing surveys to discover planets would ensure the problem may soon be resolved.

“Kepler and Gaia will significantly increase the amount of planet discoveries, not only for giant planets, but also for smaller planets,” said Mortier.

In sum, to answer the questions posed at the outset planet-hunting efforts should be focused on metal-poor and metal-rich stars, despite the former exhibiting a reduced frequency of giant planets.  The team’s findings will appear in Astronomy & Astrophysics, and a preprint is available on arXiv.   The results from the study are tied in part to observations acquired via the HARPS (High Accuracy Radial Velocity Planet Searcher) instrument, which is shown below.

HARPS (High Accuracy Radial Velocity Planet Searcher) instrument (credit: ESO).
HARPS (High Accuracy Radial Velocity Planet Searcher) instrument (credit: ESO).

Exomoons? Kepler‘s On The Hunt

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Recently, I posted an article on the feasibility of detecting moons around extrasolar planets. It was determined that exceptionally large moons (roughly Earth mass moons or more), may well be detectable with current technology. Taking up that challenge, a team of astronomers led by David Kipping from the Harvard-Smithsonian Center for Astrophysics has announced they will search publicly available Kepler data to determine if the planet-finding mission may have detected such objects.

The team has titled the project “The Hunt of Exomoons with Kepler” or HEK for short. This project searches for moons through two main methods: the transits such moons may cause and the subtle tugs they may have on previously detected planets.

Of course, the possibility of finding such a large moon requires that one be present in the first place. Within our own solar system, there are no examples of moons of the necessary size for detection with present equipment. The only objects we could detect of that size exist independently as planets. But should such objects exist as moons?

Astronomers best simulations of how solar systems form and develop don’t rule it out. Earth sized objects may migrate within forming solar systems only to be captured by a gas giant. If that happens, some of the new “moons” would not survive; their orbits would be unstable, crashing them into the planet or would be ejected again after a short time. But estimates suggest that around 50% of captured moons would survive, and their orbits circularized due to tidal forces. Thus, the potential for such large moons does exist.

The transit method is the most direct for detecting the exomoons. Just as Kepler detects planets passing in front of the disc of the parent star, causing a temporary drop in brightness, so too could it spot a transit of a sufficiently large moon.

The trickier method is finding the more subtle effect of the moon tugging the planet, changing when the transit begins and ends. This method is often known as Timing Transit Variation (TTV) and has also been used to infer the presence of other planets in the system creating similar tugs. Additionally, the same tugs exerted while the planet is crossing the disk of the star will change the duration of the transit. This effect is known as Timing Duration Variations (TDV). The combination of these two variations has the potential to give a great deal of information about potential moons including the moon’s mass, the distance from the planet, and potentially the direction the moon orbits.

Currently, the team is working on coming up with a list of planet systems that Kepler has discovered that they wish to search first. Their criteria are that the systems have sufficient data taken, that it be of high quality, and that the planets be sufficiently large to capture such large moons.

As the team notes

As the HEK project progresses, we hope to answer the question as to whether large moons, possibly even Earth-like habitable moons, are common in the Galaxy or not. Enabled by the equisite photometry of Kepler, exomoons may soon move from theoretical musings to objects of empirical investigation.

Kepler Spacecraft Back in Action After Computer Glitch

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NASA’s Kepler spacecraft is back in planet-hunting action after a computer malfunction put it into “safe mode” for 144 hours (six days.) The anomaly occurred on March 14, 2011 immediately after the spacecraft issued a network interface card (NIC) reset command to start a computer program update. During the reset, the NIC sent invalid reaction wheel data to the flight software, which caused the spacecraft to enter the self-protecting safe mode. The NIC is the interfaces between the spacecraft’s flight software, attitude determination, and its control subsystems and sensors. Mission managers said an anomaly response team will continue to evaluate the spacecraft data to determine the cause of the safe mode event.

A safe mode is a measure the spacecraft takes to protect itself when something unexpected occurs. Kepler mission managers described what happens during a safe mode event:

“During safe mode, the spacecraft points the solar panels directly at the sun and begins to slowly rotate along a sun-aligned axis. This safe mode orientation provides the vehicle with the maximum power and limits the buildup of momentum from solar wind. The spacecraft also swapped to its backup subsystem interface box (SIB), an electronics component that provides thermal and power distribution control to all spacecraft subsystems, and powered off the photometer, the instrument used to measure light intensity to detect planets. This is a normal procedure when the spacecraft enters safe mode.”

Kepler spacecraft returned to science data collection at 2:45 p.m. EDT Sunday, March 20, 2011.

Kepler launched in 2009 to look for alien worlds, hoping to find one like Earth in the just-right “Goldilocks Zone” around another star. So far, Kepler has discovered 1,235 possible planets, with 54 of those candidates in that potential habitable zone where liquid water could exist on a planet’s surface. Further study is needed to see if any of these planets have the potential to harbor life.

But given how many potential habitable planets were found in just one area of the sky, astronomers have estimated that our Milky Way galaxy could hold as many as 50 billion alien planets, with 2 billion of those being about the size of Earth.

Stay tuned!

Source: Kepler

Buzz About Gliese 581g: Doubts of Its Existence; Aliens Signals Detected

Goldilocks Zone

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Ever since the announcement of the discovery of exoplanet Gliese 581g, there has been a buzz in the news, on websites, Twitter – pretty much everywhere, about the first potentially habitable extrasolar planet. But the past couple of days there has been a different sort of buzz about this distant world. Two stories have surfaced and they both can’t be true. The first one is fairly off the deep end: an astrophysicist from Australia claims that while doing a SETI search two years ago, he picked up a “suspicious signal” from the vicinity of the Gliese 581 system, and a couple of websites have connected some dots between that signal and a potentially habitable Gliese 581g.

The second one is more sobering. At an International Astronomical Union meeting this week, other astronomers have raised doubts whether Gliese 581g actually exists.

Unless you’ve been under a rock the past two weeks, you likely know that this newest and most promising of potential habitable extra solar planets was described by the scientists who discovered it as a rocky world about 3 times the mass of Earth, and it orbits within the red dwarf star’s habitable zone, the place that is just right for water to remain as a liquid on a planetary surface. And it is fairly close to us, too, at about 20 light years away, located in the constellation Libra.

Also announced was the discovery of planet ‘f’, a 7-Earth mass planet with a 433-day orbit around Gliese 581.

Astronomer Steven Vogt announced the discoveries by his team, which used the HIRES instrument on the Keck I telescope in Hawaii. They also used 119 measurements from the HARPS instrument on the La Silla telescope at the European Southern Observatory in Chile.

On Monday, Steinn Siggurdson broke the news on his Dynamics of Cats blog that an astronomer who works on HARPS data at the Geneva Observatory, said at the IAU meeting this week that his team could not confirm the existence of Gliese 581 g.

In an article on the Astrobiology Magazine website today (Tuesday) the astronomer, Francesco Pepe, said that not only can they not confirm the existence of planet ‘g’, but also the ‘f’ planet.

In 2009, the Geneva team announced the discovery of planet ‘e’ in the Gliese 581 solar system. At approximately 1.9 Earth masses, this ‘e’ planet is the lowest mass extrasolar planet found at that time, and has a 3.15-day orbital period around the star.

Pepe said they have studied this planet-rich system frequently, gathering a total of 180 data points in 6.5 years (with about 60 of those data points since 2009) and they can only see evidence of the 4 previously announced planets b, c, d, and e.

There is a signal which could possibly be f, but the signal amplitude of this potential fifth planet is very low and basically at the level of the measurement noise, said Pepe.

The planets in the Gliese 581 system were discovered using spectroscopic radial velocity measurements. Planets ‘tug’ on the star they orbit, causing it to shift in position (stars and planets actually orbit a common center of mass). By measuring the star’s movement in the sky, astronomers can figure out what sort of planets are orbiting it. Multi-planet systems create a complicated signal, and astronomers must tease out the spectral lines to figure out what represents a planet, and what is just “noise” – shifts in the star light not caused by an orbiting planet. Astronomers have developed various ways to reduce such noise in their telescopic observations, but it still creates a level of uncertainty in detecting extrasolar planets.

The Geneva team plugged the HARPS data on Gliese 581 into computer models, and the models show “the probability that such a signal is just produced ‘by chance’ out of the noise is not negligible, of the order of several percents,” Pepe said. “Under these conditions we cannot confirm the presence of the announced planet Gliese 581 g.”

While this doesn’t definitively mean Gliese 581g doesn’t exist, it certainly casts doubt on it. More teams will be looking at the Gliese 581 star to try and determine what is really out there. This story is not over yet.

As for the alien signal, this news has met some pretty harsh criticism — even from Dr. Frank Drake, a leader in SETI community. Astronomer Ragbir Bhathal, a scientist at the University of Western Sydney, said he detected an unusual pulse of light nearly two years ago from the same region at Gliese 581, and with the news of the potential habitable world there, his claims came up again. In an article in Space.com Drake said is suspicious because Bhathal would not share his data with anyone.

You can read an article published in 2009 in the Australian about Bhathal’s claimed discovery.