Tag Archives: exoplanets

Radial velocities of the Sun as an exoplanet host star

The main way of measuring the mass of an extra-solar planet is to record the motion of the host star, caused by the gravitational tug of the planet as it orbits. One can do that by measuring the Doppler shift (radial velocity or RV) of the spectrum of the host star.

However, as a planet gets smaller or further from its star, the tug gets smaller, and so the radial-velocity signal decreases. At some point it gets smaller than the intrinsic variations in spectral lines caused by the magnetic activity of the star. Whether one can account for this will limit our ability to prove the existence of small planets in wide orbits.

Radial velocity of the Sun, bounced off the asteroid Vesta

A team lead by Raphaëlle Haywood, of the University of St. Andrews, and now at Harvard, had the idea of treating our own Sun as a star, by looking at the RV signal in sunlight bounced off the asteroid Vesta. They could then compare the RV signal to images of the magnetic activity on our Sun.

Magnetic activity on the Sun.

Magnetic activity across the Sun’s disc

The spectral lines from each region of the Sun’s disc will depend on the local magnetic activity, but the RV measurement bounced off Vesta would be from light averaged over the whole disc of the Sun, just as we’d record from a star.

The results are shown in the plot below. The top panel shows the variations in the measured RV signal, in metres per second. The second panel shows the magnetic flux aggregated across the Sun’s disc, in Gauss. The third panel shows the fraction of the Sun’s disc filled by magnetic activity (Sun spots).

Radial velocity variations of our Sun

Thus a Sun-like star can show intrinsic RV variability at a level of metres per second, and this will cause a problem for detecting the small RV signals of low-mass planets in wide orbits. For example our Earth produces motion in our Sun of only 0.1 metre per second. Unless there are stars much less magnetically active than our Sun, it is going to be hard to obtain an accuracy sufficient to detect the RV signal of an Earth-like planet in an Earth-like orbit.

The authors note, though, a strong correlation between the RV signal and the total magnetic activity. Thus it might be possible to decorrelate against magnetic activity to provide a way of correcting RV signals for this effect, and so dig out smaller signals caused by planets.

15-yr-old work-experience schoolboy discovers a new planet

Press release:

A 15-yr-old schoolboy has discovered a new planet orbiting a star 1000 light years away in our galaxy. Tom Wagg was doing work-experience at Keele University when he spotted the planet by finding a tiny dip in the light of a star as a planet passed in front of it.

“I’m hugely excited to have found a new planet, and I’m very impressed that we can find them so far away”, says Tom, now aged 17. It has taken two years of further observations to prove that Tom’s discovery really is a planet.

Tom found the planet by looking at data collected by the WASP project, which surveys the night skies monitoring millions of stars to look for the tell-tale tiny dips (transits) caused by planets passing in front of their host star.

Tom’s planet has been given the catalogue number WASP-142b, being the 142nd discovery by the WASP collaboration. It is in the Southern constellation of Hydra. While astronomers worldwide have now found over 1000 extra-solar planets, Tom is possibly the youngest ever to have done so.

“The WASP software was impressive, enabling me to search through hundreds of different stars, looking for ones that have a planet”, says Tom. The planet is the same size as Jupiter, but orbits its star in only 2 days. With such a short orbital period the transits occur frequently, making such planets much easier to find.

While the planet is much too far away to see directly, an artist’s impression shows how it might look. The hemisphere facing the star is hot, blasted by the irradiation from the star, while the other hemisphere is much cooler.

Tom Wagg at Keele Observatory 3.

Tom Wagg at Keele Observatory    (Click for high-res version; 3MB)

Tom, a pupil at Newcastle-under-Lyme School who has always been keen on science, asked for the work-experience week after learning that Keele University had a research group studying extra-solar planets.

“Tom is keen to learn about science, so it was easy to train him to look for planets”, says Professor Coel Hellier, who leads the WASP project at Keele. Tom has since achieved 12 GCSEs, all at A*, and wants to study physics at university.

The planet is one of a class of “hot Jupiter” planets, which — unlike the planets in our own Solar System — have very tight orbits close to their stars. They are thought to have migrated inwards through interactions with another planet. Thus it is likely that Tom’s planet is not the only planet orbiting that star.

Artist's impression of Tom's planet, WASP-142b, orbiting its star, WASP-142. The  planet is depicted as seen from a hypothetical moon. A second, dimmer star is seen in the background. Being 1000 light years away, the planet is too distant to  obtain a direct image.

An artist’s impression of the planet WASP-142b, depicted as seen from a hypothetical moon.
(Credit: David A. Hardy. http://www.astroart.org/)    Click for high-res version (1.5MB)

For more information email waspplanets@gmail.com

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[Err, Wow!, successful press release! Now on about 650 news websites worldwide, about 205 in English, 59 in German, 48 in Spanish, 106 in Russian, 30 in Chinese, 23 in French, 20 in Italian, 40 in Turkish, 26 in Portuguese, 23 in Indonesian, 14 in Greek, 12 in Bulgarian, 11 in Hungarian, 8 in Polish, 4 in Slovakian, and others including Tamil, Vietnamese, Thai and Malayalam!]

2014: A bumper year for WASP planets

2014 is proving to be the WASP project’s most successful year yet for the publication of transiting exoplanets. With two months to go before the end of the year, there are already 17 new planets published in 2014 in refereed journals. 12 more planets have been announced on the arXiv preprint server, though many of those will likely appear with a 2015 publication date.

We are currently finding transiting exoplanets at a rate of about 30 a year (WASP-117 is the highest number published, though we have currently got as far as WASP-134). This results from improvements in data quality owing to adding multiple years of observation. Further, the combination of WASP-South with the TRAPPIST photometer and the Euler/CORALIE spectrograph is proving to be a highly effective team. The process involves a lot of telescope time and hard work — only 1 in 10 of candidates followed up proves to be a planet — but the reward is the strong worldwide interest in studying WASP planets.

Kepler-91b: a hot-Jupiter planet orbiting a red-giant star

When we at WASP search for candidate transit dips we usually ignore any star that we think is a red giant star. Such stars have evolved off the Main Sequence and the outer layers are becoming bloated and puffed-up to many times their original size. Because of this there is no room for a hot-Jupiter planet in a typical orbit of 2 to 4 days, it would be inside the star. Thus if we see a short-period dip apparently on a red giant, it usually means that the system is blended, and that the dip is caused by a fainter, unseen binary.

However, a team led by Jorge Lillo-Box at the Centro de Astrobiología in Madrid have reported that the Kepler “object of interest” KOI-2133 — now promoted to being called Kepler-91 — is a red giant orbited by a Jupiter-mass planet on an orbit of 6.2 days.

This star is the biggest known to host a planet, with a bloated radius of 6.3 times that of the Sun. The planet orbits just outside, and, since the red giant star is still expanding, the authors estimate that it will be engulfed within 50 million years, a very short time compared to usual planet lifetimes. Viewed from the planet the star would fill nearly half the sky.

Artists conception of Kepler-91 and its planet

So puffed up and tenuous are the star’s outer layers, and so close are they to the planet, that the planet’s presence distorts them into an ellipsoidal shape. This produces a modulation of the star’s light at the planet’s orbital period, a modulation that had previously led to Kepler-91 being considered to be a binary star, not a planet host.

Lightcurve of Kepler-91

The Kepler lightcurve of Kepler-91b showing the transits and the “ellipsoidal modulation” caused by the planet distorting the star’s outer layers

The WASP cameras have also been watching Kepler-91 over the years, accumulating 20,000 data points, but could never detect the dip caused by Kepler-91b. Not only is Kepler-91 relatively faint, at V = 12.9, but the transit dip is very shallow.

A typical hot-Jupiter planet transiting a typical star produces a dip of about 1% in the light, which is detectable by WASP. However with Kepler-91 being bloated to 6.3 solar radii its surface area will be 40 times bigger, and hence the planet occults only 1/40th as much of the light, producing a vastly shallower dip. The transit depth in Kepler-91 is only 0.04%, detectable by the superb Kepler photometry from the stability of space, but not detectable from the ground, another reason why WASP ignores stars that are giants.

Companions to WASP planets

When the first “hot Jupiter” planets were found they were a big surprise — no-one had expected to find massive Jupiter-sized planets very close to stars, in orbits of only a few days. Most planet-formation theory says that they can’t have formed there, and must have formed much further out, beyond the “snow line” where it is much colder.

Much investigation has gone into discovering what moves the planets inwards to become hot Jupiters. One favourite explanation is the long-term effect of gravitational perturbations to the planet’s orbit, caused by another massive planet or low-mass companion star much further out.

If this is right we should be able to find these outer companions, and one method is to monitor the radial-velocity motion of the host star, looking for the gravitational pull caused by the outer companion. Hence one would expect the stars’ radial velocity to show a short-term cycle with the period of hot Jupiter, plus a much longer term trend.

Keck Telescopes at dusk

An important paper just announced by Heather Knutson and colleagues announces the results of monitoring 51 hot-Jupiter systems — including 18 WASP planets — using the HIRES spectrograph on the 10-m Keck telescopes on top of Mauna Kea in Hawaii. They confirm long-term radial-velocity trends previously suspected in 9 systems and report newly found trends in 7 other systems.

Four WASP systems (WASP-8, WASP-10, WASP-22 and WASP-34) are found to have radial-velocity trends indicating a massive outer companion. The plot has the radial-velocity on the y-axis (units of metres per sec) plotted against time (years since 2000).

RV trends for WASP planets

In WASP-8 and WASP-34 the orbit of the companion is beginning to be constrained, while for WASP-10 and WASP-22 the timescale of the orbit appears to be longer. Further monitoring of these systems and other hot Jupiters (the plot also shows planets from the HAT and XO projects) might help to answer the question of whether these outer companions are the cause of hot Jupiters.

Three more planets from WASP-South, Euler and TRAPPIST

The latest three WASP-South planets appeared on the preprint server arXiv today, announcing WASP-68b, WASP-73b and WASP-88b.

TRAPPIST by night

The paper is led by the University of Liège, who operate the TRAPPIST robotic 0.6-m photometer, sited at La Silla in Chile. TRAPPIST plays a crucial role in WASP-South planet discovery. Since the WASP photometry, from 200mm, f/1.8 lenses, is relatively crude, a TRAPPIST lightcurve of a candidate gives a better idea of whether the candidate is worth pursuing. Further, the large pixels of WASP data mean that apparent transits are often caused by deeply eclipsing fainter stars within the WASP photometry aperture. TRAPPIST photometry shows up such blends, and thus avoids wasting valuable radial-velocity observations.

Blend image

As an example, the candidate 1SWASPJ113725.66–261925.6 showed a shallow 1% dip recurring with a period of 1.33 days, possibly caused by a planet transit. However, close by are two fainter stars (left, the yellow circle is the extraction aperture used for the WASP photometry; WASP pixels are much bigger than in this Sky Survey image, and so the candidate and the two close stars are merged).

blend

TRAPPIST is able to resolve the stars, and followup photometry showed that the dip is caused by the closest faint star, which is an eclipsing binary with a 20% deep eclipse (the image is reversed left–right). In photometry centered on the bright candidate this eclipse is diluted to a shallow planet-like dip.

NASA finds water on three WASP planets

A team using NASA’s Hubble Space Telescope has detected water in the atmosphere of five exoplanets. Three of these are WASP planets, WASP-12b, WASP-17b and WASP-19b. They were chosen because they orbit relatively bright stars and because they are close-in “hot Jupiter” planets with bloated and puffed-up atmospheres, the best targets for the highly demanding task of discerning molecules in those atmospheres. This study demonstrates how valuable WASP planets are for exoplanet research.

WASP-12b illustration.

An artist’s conception of WASP-12b, a hot-Jupiter planet orbiting so closely that its atmosphere is blasted by irradiation from its star

The NASA press release has been reported by websites and newspapers worldwide. It reads:

Hubble Traces Subtle Signals of Water on Hazy Worlds      Dec. 3, 2013

Using the powerful­ eye of NASA’s Hubble Space Telescope, two teams of scientists have found faint signatures of water in the atmospheres of five distant planets.

The presence of atmospheric water was reported previously on a few exoplanets orbiting stars beyond our solar system, but this is the first study to conclusively measure and compare the profiles and intensities of these signatures on multiple worlds.

The five planets — WASP-17b, HD209458b, WASP-12b, WASP-19b and XO-1b — orbit nearby stars. The strengths of their water signatures varied. WASP-17b, a planet with an especially puffed-up atmosphere, and HD209458b had the strongest signals. The signatures for the other three planets, WASP-12b, WASP-19b and XO-1b, also are consistent with water.

“We’re very confident that we see a water signature for multiple planets,” said Avi Mandell, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md., and lead author of an Astrophysical Journal paper, published today, describing the findings for WASP-12b, WASP-17b and WASP-19b. “This work really opens the door for comparing how much water is present in atmospheres on different kinds of exoplanets, for example hotter versus cooler ones.”

The studies were part of a census of exoplanet atmospheres led by L. Drake Deming of the University of Maryland in College Park. Both teams used Hubble’s Wide Field Camera 3 to explore the details of absorption of light through the planets’ atmospheres. The observations were made in a range of infrared wavelengths where the water signature, if present, would appear. The teams compared the shapes and intensities of the absorption profiles, and the consistency of the signatures gave them confidence they saw water. The observations demonstrate Hubble’s continuing exemplary performance in exoplanet research.

“To actually detect the atmosphere of an exoplanet is extraordinarily difficult. But we were able to pull out a very clear signal, and it is water,” said Deming, whose team reported results for HD209458b and XO-1b in a Sept. 10 paper in the same journal. Deming’s team employed a new technique with longer exposure times, which increased the sensitivity of their measurements.

The water signals were all less pronounced than expected, and the scientists suspect this is because a layer of haze or dust blankets each of the five planets. This haze can reduce the intensity of all signals from the atmosphere in the same way fog can make colors in a photograph appear muted. At the same time, haze alters the profiles of water signals and other important molecules in a distinctive way.

The five planets are hot Jupiters, massive worlds that orbit close to their host stars. The researchers were initially surprised that all five appeared to be hazy. But Deming and Mandell noted that other researchers are finding evidence of haze around exoplanets.

“These studies, combined with other Hubble observations, are showing us that there are a surprisingly large number of systems for which the signal of water is either attenuated or completely absent,” said Heather Knutson of the California Institute of Technology, a co-author on Deming’s paper. “This suggests that cloudy or hazy atmospheres may in fact be rather common for hot Jupiters.”Hubble’s high-performance Wide Field Camera 3 is one of few capable of peering into the atmospheres of exoplanets many trillions of miles away. These exceptionally challenging studies can be done only if the planets are spotted while they are passing in front of their stars. Researchers can identify the gases in a planet’s atmosphere by determining which wavelengths of the star’s light are transmitted and which are partially absorbed.