The discovery and confirmation of the exoplanet HD 219134b give us a useful touchstone relatively close to the Solar System. At 21 light years away in the constellation Cassiopeia, HD 219134b distinguishes itself by being the closest exoplanet to Earth to be detected using the transit method. That’s useful indeed, because we’ll be able to use future instruments like the James Webb Space Telescope to learn about the composition of any atmosphere there.
Image: This sky map shows the location of the star HD 219134 (circle), host to the nearest confirmed rocky planet found to date outside of our solar system. The star lies just off the “W” shape of the constellation Cassiopeia and can be seen with the naked eye in dark skies. It actually has multiple planets, none of which are habitable. Credit: NASA/JPL-Caltech/DSS.
Too close to its star to be considered a candidate for life, the new world is a ‘super-Earth,’ sighted by the HARPS-North instrument using radial velocity techniques, which measure the pull of the planet on its host star. HARPS-N was built by researchers at the University of Geneva and installed at the 3.6-meter Telescopio Nazionale Galileo on La Palma, in the Canary Islands. Ati Motalebi (UNIGE), lead author of the paper on this work, knew the significance of a potential transit in a world this close, a transit that radial velocity methods couldn’t uncover:
“When the first HARPS-N radial-velocity measurements indicated the presence of a 3-day planet around HD 219134, we immediately asked NASA for Spitzer space telescope time,” said Motalebi. “The idea was to check for a potential transit of the planet in front of the star, a mini eclipse, that would allow us to measure the size of the planet. To do this, we needed to go to space to reach the required precision.”
Initial readings were of a planet with a mass 4.5 times that of the Earth, orbiting the star — a K-dwarf a bit cooler and less massive than the Sun — in three days. Data from the Spitzer instrument then revealed the transit, its measurements indicating the planet to be about 1.6 times larger than the Earth. Combined with the earlier mass calculation, this radius yields a density of six grams per cubic centimeter, pointing to the possibility that this is a rocky planet. Michael Gillon (University of Liege), who led the Spitzer work, calls HD 219134b “a kind of Rosetta Stone for the study of super-Earths.”
HD 219134 turned out to host other worlds as well, with three additional longer-term planets discovered from the radial velocity work. The first of these is 2.7 times as massive as Earth, orbiting in a 6.8 day orbit. A second has 8.7 times Earth’s mass and orbits in 46.8 days. There is also a Saturn-class gas giant at 2.1 AU, orbiting the star in about 3 years.
The possibility exists that the two other inner planets are also transiting worlds, a prospect that has driven planning for future observations. Stéphane Udry (also at UNIGE) considers the potential:
“In particular, the future CHEOPS satellite of the European Space Agency (ESA), developed under Swiss leadership with a strong involvement of UNIGE and of the University of Bern, will provide the perfect tool for such observations. Being able to characterise three transiting super-Earths in a single bright and close system would provide incomparable constraints for planet formation and composition models, in particular for super-Earths.”
Image: Lightcurve of HD 219134b. Credit: UNIGE/NASA/JPL-Caltech.
So HD 219134b, and perhaps its two inner-orbit cousins, could turn out to be a goldmine for future study as we turn CHEOPS as well as the JWST instrument toward them. JWST should be able to use transmission spectroscopy techniques to analyze starlight passing through any planetary atmosphere, while ground-based high-resolution spectroscopy should tell us still more, and there is the prospect of direct imaging of the outer planet in a system this close with the planned giant telescopes in the planning stage, including the European Extremely Large Telescope, the Giant Magellan Telescope, and the Thirty Meter Telescope.
From the paper:
The quality of the measurements of the radius, mass and then mean density actually foreshadows what can be expected from the future transit missions in preparation that will target bright stars (CHEOPS, TESS, PLATO). We also know from Kepler results that multi-transiting systems of small-size planets are numerous. It is then now highly suitable to search for traces of transits of the other planets in the systems. Finally, even if a potential atmosphere around the planet is expected a priori to be tiny, the brightness of the system makes it worth trying to detect features of this atmosphere in the UV, visible and NIR, from space and from the ground, especially in preparation for future measurements with larger facilities…
The paper is Motalebi et al., “The HARPS-N Rocky Planet Search I. HD219134b: A transiting rocky planet in a multi-planet system at 6.5 pc from the Sun,” accepted at Astronomy & Astrophysics (preprint). Both JPL and UNIGE offer news releases.
While HD 219134b is certainly the closest confirmed rocky transiting exoplanet, there is an unconfirmed one that is even closer. While searching for transits to confirm the existence of Alpha Centauri Bb using the Hubble Space Telescope, an international team of astronomers found a single transit-like signature suggesting another closely orbiting Earth-size planet instead.
http://www.drewexmachina.com/2015/03/28/has-another-planet-been-found-orbiting-alpha-centauri-b/
Although the observation needs to be independently confirmed, at a distance of 4.3 light years and with a size comparable to Earth (virtually guaranteeing it is a rocky planet and not a mini-Neptune) this could end up being the closest rocky transiting exoplanet.
Make no mistake, it’s an exciting system and we will learn a lot from it with coming new toys. But imagine how much more excitement there will be if it’s in HZ. 21lys… :-)
I see a lot of space in this system for small planets in the Hz. Its unlikely to transit, so lets see what emerges from the RV noise over the coming years. If any instrument can do it its HARPS-N.
P
The “Exoplanet Hunters’ Club” really needs to collaborate with the Moon base advocates. A lunar observatory equipped with telescopes larger than typical satellite-based space telescopes (which need not be as large as the Mount Wilson and Palomar Mountain reflectors, although such a lunar base could host ‘scopes that big later on) could observe the transits of–and likely directly image–all of the planets in nearby star systems like these.
Although this appears to be a rocky planet NOW, it probably wasn’t one for it’s first billion years of existance. I guess, that after it migrated from its original orbit, its radius was 1.7-1.8 Earth radii, and then it lost most of its hydrogen envelope, a la Gliese 436b. This would mean that it STILL has a SUBSTANTIAL (i.e. considerally denser than Venus’s atmosphere, but considerably less dense than Gliese 436b’s) atmosphere primarily composed of HELIUM! It is imperative that this be CONFIRMED by Hubble STIS measurements, because, if I am wrong, and this IS a bonafied rocky planet, the consensus that 1.6Re is the CUTOFF POINT for rocky planets, and the TRANSITION POINT into gassy planets would come under question. A REVISION of the cutoff point UPWARD would THEN bring planets like Kepler 452b into play.
@J. Jason Wentworth August 4, 2015 at 4:29
Despite fanciful claims made by some Moon base advocates, the surface of the Moon is not a good place to base an astronomical telescope especially one that operates in or near optical wavelengths. Measurements made from the lunar surface by the Soviet Lunokhod 2 rover in early 1973 demonstrated that the Moon’s daytime sky is over an order of magnitude brighter than Earth’s nighttime sky with a full Moon present due to sunlight scattering off of lunar dust above the surface.
http://www.drewexmachina.com/2014/08/21/the-original-lunar-observatories/
More recent observations like those made by NASA’s recent LADEE mission tend to support this view. For this reason as well as a host of logistical and engineering issues, large off-world telescopes operating in IR, visible and near-UV wavelengths to observe extrasolar planets or other astronomical targets are best based in the weightlessness of an orbital environment (e.g. LEO, the Lagrange points, solar orbit) than on the surface of the Moon.
@Jason – why is a lunar based telescope a better location than a space based one? The moon has gravity, requiring more mass for the scope, it only has 1/2 the sky in view at any one moment, the target must be tracked, there is lunar dust that can get at the moving parts and potentially contaminate the mirror and it is subject to noise from meteor impacts. It also seems more complicated if the astronomers want to deploy a star shade for exo-planet imaging. Lastly it is farther away and more expensive to reach for any repairs.
If you go to the end of the arXiv version of the paper, there is also the following:
So we’ll probably be seeing an update on this system in the near future.
How about a radio observatory on the lunar farside:
http://www.space.com/30084-moon-far-side-rovers-radio-telescope.html
@Andrew LePage August 4, 2015 at 10:49
‘Despite fanciful claims made by some Moon base advocates, the surface of the Moon is not a good place to base an astronomical telescope especially one that operates in or near optical wavelengths. Measurements made from the lunar surface by the Soviet Lunokhod 2 rover in early 1973 demonstrated that the Moon’s daytime sky is over an order of magnitude brighter than Earth’s nighttime sky with a full Moon present due to sunlight scattering off of lunar dust above the surface.’
The best location for a mirror would be the very cold poles, the mirrors could be made where there is more access to light and then moved. They could even be made elsewhere on the moon if buried with a long tube sticking out to prevent dust and light contamination. Although there is less sky to look at the views would staggering, we could see to the very edges of the universe in those directions.
@Alex
‘The moon has gravity, requiring more mass for the scope, it only has 1/2 the sky in view at any one moment, the target must be tracked, there is lunar dust that can get at the moving parts and potentially contaminate the mirror and it is subject to noise from meteor impacts.’
Gravity is very useful in shaping the mirror and mass is less of a problem in low gravity as the rigidity gravity ratio is much better and there is plenty of materials to use on the moon.
@ all, we must stop looking at these one shot programs. A base on the moon would be a huge stepping stone into space for all sorts of activities and start resolving the great Space infrastructure problem.
RE: Andrew LePage’s, Alex Tolley’s, and Michael’s comments:
The Sun only shines on any part of the Moon during half of its -much- longer day (the diffuse “dustosphere” wouldn’t be a problem during the lunar night), and in addition to polar crater-located optical telescopes, such instruments could also be co-located at or near the center lunar farside radio astronomy/SETI observatory that has been on astronomers’ “wish lists” for decades. Fabrication and repair services would be local, provided by the lunar base personnel.
@Michael August 4, 2015 at 15:33
” They could even be made elsewhere on the moon if buried with a long tube sticking out to prevent dust and light contamination. ”
You do realize that the lunar dust extends kilometers above the surface making this “long tube” solution completely impractical.
@J. Jason Wentworth August 5, 2015 at 23:19
Why base an optical telescope on the Moon that can only be used for two weeks each month to view less than half the sky (with the Moon blocking the other half) when you can base a telescope in a distant orbit and use it to view most of the sky (save for exclusion zones near the Sun or other bright nearby bodies) all the time? Until some sort of lunar infrastructure is actually established to provide support services, etc. to tip the equation somewhat towards Moon-based optical telescopes, a lunar based optical telescope simply makes no sense.
Andrew LePage wrote:
“Why base an optical telescope on the Moon that can only be used for two weeks each month to view less than half the sky (with the Moon blocking the other half) when you can base a telescope in a distant orbit and use it to view most of the sky (save for exclusion zones near the Sun or other bright nearby bodies) all the time? Until some sort of lunar infrastructure is actually established to provide support services, etc. to tip the equation somewhat towards Moon-based optical telescopes, a lunar based optical telescope simply makes no sense.”
A lunar base will require as many rationales as can be mustered in order to obtain the needed funding, and astronomy (optical as well as radio, including exoplanet hunting and SETI searches) is one of the rationales to solve this “chicken and egg” problem…
@Andrew LePage August 6, 2015 at 13:28
@Michael August 4, 2015 at 15:33
” They could even be made elsewhere on the moon if buried with a long tube sticking out to prevent dust and light contamination. ”
‘You do realize that the lunar dust extends kilometers above the surface making this “long tube” solution completely impractical.’
The dust problem can be mitigated by using microwave glassification of the surrounding area, easy I have tried this in my microwave, and coating it with a reflective aluminium (conductive) layer and applying an appropriate charge to the layer would allow us to control the dust issue better.
‘Why base an optical telescope on the Moon that can only be used for two weeks each month to view less than half the sky (with the Moon blocking the other half) when you can base a telescope in a distant orbit and use it to view most of the sky (save for exclusion zones near the Sun or other bright nearby bodies) all the time? ‘
The telescopes with tubes can be used during daylight as well as there is no atmosphere to diffuse the light into the tube, you just can’t look near the sun. As for the space based telescope you will run out of fuel eventually.
@J. Jason Wentworth August 5, 2015 at 23:19
‘The Sun only shines on any part of the Moon during half of its -much- longer day (the diffuse “dustosphere” wouldn’t be a problem during the lunar night),’
The dust is worse at night which surprised me as well but will not be reflective causing Sun reflectance issues in the scope.
A moon based infrastructure will give us the foundation of our Empire upon which Sun’s will never set. We need not just telescopes to see into the expanses of Space but men, women and machines to do great works.
@J. Jason Wentworth August 7, 2015 at 2:56
I am sorry but your logic simply escapes me. If I were an astronomer wishing to build a space-based optical telescope to perform cutting-edge research looking for exoplanets, etc., what possible motivation would I have to base my very expensive telescope that I have fought to get approved and funded for years at an inferior site (i.e the surface of the Moon with only half the sky visible at any given time and severe light pollution issues for half the time) with the added expense and risk of being a “guinea pig” of having it be the first of its kind to be built and maintained on the lunar surface?
There are plenty of good reasons to exploit lunar resources and there is certainly some good astronomy that could be performed from the lunar surface (e.g. radio, X-ray, gamma ray) but I do not think you will find any optical astronomers choosing to base high-performance space-based telescopes on the lunar surface. I am afraid you will have to find some other justification for a lunar base because this one simply impractical.
@Andrew LePage August 7, 2015 at 13:19
‘I am sorry but your logic simply escapes me. If I were an astronomer wishing to build a space-based optical telescope to perform cutting-edge research looking for exoplanets, etc..’
The problem is that there are lot less Astronomers who want to do cutting edge exoplanet research than there are people who want to get out there into space by a considerable margin. I would rather wait an extra 5 to 10 years for a state of the art telescope and have a base on the moon from which we never look back.