Measuring Non-Transiting Worlds

by Paul Gilster on June 28, 2012

Although I want to move on this morning to some interesting exoplanet news, I’m not through with fusion propulsion, not by a long shot. I want to respond to some of the questions that came in about the British ZETA experiment, and also discuss some of Rod Hyde’s starship ideas as developed at Lawrence Livermore Laboratory in the 1970s. Also on the table is Al Jackson’s work with Daniel Whitmire on a modified Bussard ramjet design augmented by lasers. But I need to put all that off for about a week as I wait for some recently requested research materials to arrive, and also because next week I’m taking a short break, about which more on Monday.

For today, then, let’s talk about an advance in the way we study distant solar systems, for we’re finding ever more ingenious ways of teasing out information about exoplanets we can’t even see. The latest news comes from the study of Tau Boötis b, a ‘hot Jupiter’ circling its primary — a yellow-white dwarf about 20 percent more massive than the Sun — with an orbital period of 3.3 days. An international team has been able to measure the mass of this planet even though it is not a transiting world. The work, published in Nature and augmented with another paper in Astrophysical Journal Letters, opens up a new way to study not just the mass of exoplanets but also their atmospheres, whose signature is a key part of the work.

Image: This image of the sky around the star Tau Boötis was created from the Digitized Sky Survey 2 images. The star itself, which is bright enough to be seen with the unaided eye, is at the centre. The spikes and coloured circles around it are artifacts of the telescope and photographic plate used and are not real. The exoplanet Tau Boötis b orbits very close to the star and is completely invisible in this picture. The planet has only just been detected directly from its own light using ESO’s VLT. Credit: ESO/Digitized Sky Survey 2.

51 light-years away in the constellation of Boötes (the Herdsman), Tau Boötis b was discovered by Geoff Marcy and Paul Butler in 1996 through radial velocity methods, which measure the gravitational tug of the planet on its host star. Picking up the stellar ‘wobble’ flags the presence of a planet but leaves us with a wide range of mass possibilities because the angle of the planet’s orbit around the star is unknown. Thus a gas giant at a high angle to the line of sight could show the same signature as a smaller planet at a lower angle. For that reason, we’ve been limited with radial velocity to setting just a lower limit to a planet’s mass.

Transits work better for determining mass because we can measure the lightcurve of the planet as the star’s light dips during the transit, adding that to the radial velocity findings to work out both mass and radius. We’ve also been able to study exoplanet atmospheres in transiting worlds, looking at the star’s light and subtracting out the atmospheric signature. But transiting worlds are uncommon, dependent on the chance alignment of the system with our line of sight. What this new work provides is a way to study non-transiting worlds at a higher level of detail, down to investigating the composition of their atmosphere. Future telescopes should be able to apply these techniques far beyond the realm of hot Jupiters like Tau Boötis b.

What Matteo Brogi (Leiden University) and team did was to use the high-resolution CRIRES spectrograph on the Very Large Telescope at the European Southern Observatory’s Paranal Observatory in Chile to study the light from this system and determine the exquisitely fine changes in wavelength caused by the motion of the planet around the star. The observations at near infrared wavelength (2.3 microns) worked with the signature of carbon monoxide in the atmosphere, which allowed the researchers to work out the angle (44 degrees) that Tau Boötis b orbits the primary. Brogi, lead author of the paper on this work, comments:

“Thanks to the high quality observations provided by the VLT and CRIRES we were able to study the spectrum of the system in much more detail than has been possible before. Only about 0.01% of the light we see comes from the planet, and the rest from the star, so this was not easy.”

The hope is that the technique can be used in future studies to look for molecules associated with life. We’re a long way from that result, but this first step tell us how exciting the era of the new giant observatories — think the European Extremely Large Telescope and its ilk — is going to be. Tau Boötis b turns out to be about six times as massive as Jupiter, as determined through the displacement of the spectral lines of the detected carbon monoxide, and the team intends to look for other molecules in its atmosphere as the studies continue. Larger instruments will allow us to move far beyond the limited range of transiting worlds to tighten our knowledge of distant exoplanet systems, a step forward in the refinement of radial velocity techniques.

The paper is Brogi et al., “The signature of orbital motion from the dayside of the planet τ Boötis b,” Nature 486 (28 June 2012), pp. 502-504 (abstract). See also Rodler et al., “Weighing the Non-Transiting Hot Jupiter τ Boo b,” Astrophysical Journal Letters Vol. 753, No. 1 (2012), L25 (abstract). A news release from the European Southern Observatory is also available, as is this release from the Carnegie Institution for Science. Thanks to Antonio Tavani for the pointer to this work.

tzf_img_post

Alex Tolley June 28, 2012 at 10:56

“The hope is that the technique can be used in future studies to look for molecules associated with life. ”

Which molecules are we looking for, and what assumptions about life is being made with the examples?

Joy June 28, 2012 at 19:20

For the past several decades physics has over promised but under delivered, while astronomy has repeatedly found new ways to exceed all expectations. So the trend continues. Astronomy with CCD imaging and computing is a huge beneficiary of Moore’s Law.

Of note, the Higgs discovery announcement scheduled for next week is not particularly good news. While the icing on the cake for the Standard Model, it does not point the way towards any new physics. Indeed, LHC energies might be inadequate to reveal any new physics, in which case, the end of experimental high energy physics may be at hand. We might just have to muddle on with our various flawed and incompatible models for the indefinite future.

philw1776 June 28, 2012 at 20:54

Joy’s points are well taken. The return/investment in particle physics has dropped substantively and if the Standard Model is confirmed as expected, it drops exponentially. No new science will be found by the LHC. NASA became an engineers jobs program and the LHC is a physics post doc jobs program.

Yet the short sighted fools who ran the Decadal Survey refused to recommend ANY substantive follow on to Kepler in extrasolar planet science for this entire decade.

coolstar June 28, 2012 at 22:48

philw1776: Couldn’t agree with you more about the last Decadel Survey! and Kepler is CHEAP and follow-ups to do bright exoplanet host stars (and thus better studied) would actually be LESS expensive….

Joy June 28, 2012 at 23:50

Exactly Phil. Suppose one is world science czar. One has to choose between a scaled up LHC follow on (to look for what exactly?) or a scaled up Hubble follow on (which the Webb is not). I could make that decision in a heartbeat.

If spacex continues to build success, the path forward will be clear. The proposed wide bodied Falcon X would be broad and hefty enough to loft a new optical telescope based on one of J. Roger P. Angel’s famous 8.4 meter mirrors equipped with a vortex coronagraph. No particular technological risk, merely a bigger Hubble with new instruments. For better or worse we would be guaranteed a census of all the HZ planets (if any) in the solar neighborhood, with atmospheric spectra. Why the decadal survey decided to eschew instruments to study extrasolar planets eludes me.

bigdan201 June 29, 2012 at 11:08

I always thought that the transiting issue was a major bottleneck for exoplanet discovery. There must be many more planets of all different types that don’t have a favorable chance alignment with our line of sight. Hopefully this technique will work with smaller, more distant terrestrial planets too.

HughP June 29, 2012 at 13:08

“Thus a gas giant at a high angle to the line of sight could show the same signature as a smaller planet at a lower angle. For that reason, we’ve been limited with radial velocity to setting just a lower limit to a planet’s mass.”

To what extent could a star’s sunspot rotation pattern disclose the star’s equator, axis of rotation, and the most probable plane for its most massive planet? If the star’s most massive planet is in that plane, could we get more information about the lower limit to that planet’s mass?

Holger June 30, 2012 at 12:35

HughP, that’s a good idea. The more the star’s rotation axis is inclined from our line of sight, the more the star’s brightness would change due to more sunspots moving in and out of sight during rotation. And if a planet’s orbital plane were known, radial velocity would give the exact mass of the planet, not just the “minimal mass”.
But I don’t know if such miniscule variations of brightness can be detected for faraway stars…

HughP June 30, 2012 at 23:41

Perhaps the variations of sun spot transit times are more susceptible to detection than variations in brightness are.

If observers look from the direction of Alpha Centauri to measure the transit times for the Sun’s dark spots, I believe they could determine that transit times are different for typical spots that are (1) 30 degrees from the northern pole, (2) near the equator, and (3) 30 degrees from the southern pole. The point is not to use the data to determine which is north and which is south, but rather to determine which Sun spots are most likely to be moving in equatorial latitudes. (I do not know whether the equatorial determination would be feasible; it is just an idea.) Once the equator has been identified, at least approximately, one could assume that the most massive planet was in the equatorial plane and, I suspect, get more information about that planet’s minimum mass.

andy July 2, 2012 at 4:02

The main problem with the idea of using the star’s rotation to determine the orbital plane of the planet is that hot Jupiters often have orbits that are strongly misaligned with the stellar equator. There is also evidence from orbital eccentricities that planet-planet scattering is a significant factor in planetary system architectures, and this would naturally lead to objects being thrown into inclined orbits: Upsilon Andromedae anyone?

And then you can still have fun with apparently well-ordered systems as well if they are in binary star systems: it would certainly be interesting to have measurements of the Rossiter-McLaughlin effect for 55 Cancri e…

Comments on this entry are closed.