Proximity Hunt: Exoplanets Around Nearby Stars

by Paul Gilster on July 19, 2012

Finding new worlds with Kepler is an absorbing occupation, but the one thing missing from most exoplanet news is proximity. While we continue to search for planets around the Alpha Centauri stars, the closest candidate I know about is the gas giant thought to orbit Epsilon Eridani, some 10.5 light years out. If you’re looking for potential habitability, you have to extend all the way out to Gliese 581 (almost twice the distance), where planets are plentiful and there is at least the chance (GL 581d) that one skirts the edge of the habitable zone. There are probably many planets closer than 20 light years, but we don’t have the tools in space to find them easily.

Kepler, you’ll recall, studies a field of stars in Cygnus, Lyra and Draco, the goal being to develop a statistical approximation of the prevalence of Earth-sized planets in the galaxy. Looking out along the Orion arm, Kepler is watching stars anywhere from 600 to 3000 light years away. In fact, fewer than 1 percent of the stars Kepler sees are closer than 600 light years. We have missions on the drawing board like TESS — Transiting Exoplanet Survey Satellite — which are designed to study 2.5 million of the brightest stars in the sky, looking for nearby transiting planets, and the European Space Agency’s PLATO — PLAnetary Transits and Oscillations of stars — would likewise go to work on relatively bright stars near our Solar System.

Will these missions ever fly? While we wait for the inexorable review process to grind its way forward, it’s gratifying to see the discovery of another planet that’s not, in astronomical terms, all that far away. The candidate exoplanet, UCF-1.01, is 33 light years from us orbiting the red dwarf GJ 436, already known to be home to a hot Neptune designated GJ 436b. Working at the University of Central Florida, Kevin Stevenson and colleagues went to work on what appeared to be the signature of a small planet, going through observations from Spitzer as well as the Deep Impact spacecraft, the Very Large Telescope and the Canada-France-Hawaii Telescope on Mauna Kea. The find marks the first time Spitzer has been used in a transit discovery.

Image: GJ 436, a red dwarf in the constellation of Leo with a ‘hot Neptune’ and two new exoplanet candidates. Credit: ESO Online Digitized Sky Survey.

It’s an interesting little world, this UCF-1.01, with a diameter estimated at 8400 kilometers, or about two-thirds that of the Earth, quite a catch for Spitzer, which some project scientists believe may be able to discover exoplanets no larger than Mars. UCF-1.01 would orbit its star in a snappy 1.4 days, with surface temperatures reaching close to 600 degrees Celsius. Due to its size and proximity to the host star, the planet is unlikely to have retained its original atmosphere, though the paper does note that impacts or tidal heating could create a transient atmosphere.

Most likely, this is a dead, cratered world, conceivably a place covered in magma, according to Joseph Harrington (UCF), a co-author on the paper. If confirmed, the new world will be called GJ 436c, but the UCF team is not done yet. There is also evidence for a third world in this system, dubbed UCF-1.02. Confirming the new worlds will involve additional work. From the paper:

To definitively establish UCF-1.01 as a planet (to be called GJ 436c), we require only a few hours of additional observations, preferably from another telescope or at least at a different wavelength. Establishing UCF-1.02 as a planet (to be called GJ 436d) would likely require an extended observing campaign to constrain its period then successfully predict a transit.

We only have one Kepler world smaller than the two Spitzer candidates, though there will surely be more as the data analysis continues. Meanwhile, we’re still waiting for information about flight possibilities for PLATO, which will have a larger field of view than Kepler, while TESS is involved in a concept study that could lead to selection as a NASA Explorer-class mission. One way or another, through space-based resources or ground observation, the list of nearby exoplanets is going to grow, though we can only speculate on when — and where — we’ll find the first true Earth analog. Interesting, conceivably habitable M-dwarf planets are probably more common.

The paper is Stevenson et al., “Two nearby sub-Earth-sized exoplanet candidates in the GJ 436 system,” accepted for publication in The Astrophysical Journal (abstract).

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{ 15 comments }

Harry Ray July 19, 2012 at 10:59

Spitzer data ALONE may be able to confirm UCF1.o1 as Gliese 436c! There appears to be a near 2 to 1 resonance between UCF1.01 and Gliese 436b, AND, since they pass almost as close to each other as the two planets in the KEPLER 36 system, Gliese 436b SHOULD cause considerable TTV’s in UCF1.o1 which should appear in future Spitzer observations!

Paul July 19, 2012 at 12:08

The one thing missing is proximity?

How about an earth sized planet (at ANY distance) orbiting it’s G star host in that star’s habitable zone?

This is not to fail to take the author’s good point, which is that we need to search nearby. I hope to somehow still be looking at this site if/when a beautiful planet is found in the Centauri system. What a celebration that would be! What a vindication for the author!

andy July 19, 2012 at 13:41

That is a bit of a surprise, especially given the eccentric orbit of the hot Neptune. Maybe there are still more planets to be found in the system: looks like both of these candidates are far too small to prevent the circularisation of Gliese 436b’s orbit.

Thomas Tarrants July 19, 2012 at 18:24

“…quite a catch for Spitzer, which some project scientists believe may be able to discover exoplanets no larger than Mars.”
We need more telescopes that are sensitive only to sub-Mars-sized exoplanets. :P

andy July 20, 2012 at 1:35

Talking of Gliese 581, the latest round in the ongoing debate over the existence of planets f and g has appeared on the arXiv, this time from Vogt et al. again arguing for the presence of planet g. (Though interestingly enough, not planet f which seemed to be the slightly more secure of the two in the previous analyses…)

At this rate Gliese 581 is going to be the exoplanetary equivalent of Capella, where the stars that are actually members of the system are designated Aa, Ab, H and L because of the use of the intermediate letters for unrelated objects…

Holger July 20, 2012 at 11:24

@Andy: That’s interesting. I have no idea which side will turn out to be right, but I had naively hoped that the two conflicting positions on Gliese 581 g could be reconciled by assuming g to be substantially smaller than Vogt’s value of 3.1 Earth masses minimum, which would be better news than either of the two original positions. Now Vogt is doing just that: he revises the minimum mass of g down to 2.2 Earth masses. That’s still probably too large for a rocky planet in the habitable zone, but we’re coming closer to a terrestrial g…

Antonio July 20, 2012 at 11:56

Questo nuovo pianeta scoperto(UCF-1.01) potremmo chiamarlo “Mustafar” come quello presente in Guerre Stellari”…

(E’ una battuta: spero che nessuno dei lettori del “blog” se ne abbia a male).

Saluti da Antonio Tavani

Via Google Translate:

This new planet found (UCF-1.01), we might call “Mustafar” like the one in Star Wars “…

(It’s a joke: I hope that none of the readers of “blog” take offense).

Greetings from Antonio Tavani

Daniel July 21, 2012 at 9:30

Talk About TESS and Hunt for planets around our nearby stars,I think in my opinion that ELEKTRA mission would be much better and interest that TESS.

Because work on the Infrared Spectrum,and our nearby most of then are cool Red Dwarf (and many nearby Brown Dwarf) ,and because of the star small size and shot period Habitable zone small Rock planet in HZ would much easy to detect and interesting targets for future interstellar travel

I don’t know why nobody talk about this interest project (??)

http://nexsci.caltech.edu/conferences/Flagstaff/posters/missions.05_beichman.pdf

http://nexsci.caltech.edu/conferences/Flagstaff/posters.shtml

Mike July 22, 2012 at 12:47

I think for the next 20 years at least the search for nearby exoplanets will by relying on ground based radial velocity searches like HARPs and the recently commissioned Automated Planet Finder. And a few transit search programs like MEarth. Ofcourse funding is the reason.
This is not to despair however. Long term RV observing programs of nearby stars with dedicated instrumentation can potentialy get down to near Earth mass detections if the parent stars are quiet enough and given many repeated observations. It just will take some years to begin to produce a more comprehensive data base but that’s all we got for now.
Projects like TESS and ELECTRA would provide much new data and also would bird-dog reasonably bright transiting exoplanet targets for the James Webb space telescope for to analyse exo-atmospheres. Especially for the numerous late-sequence K and M stars. That by itself is a very good reason to fund TESS or ELECTRA.
Unforturnatly these relativly cheap all-sky transit missions are of little use for a comprehensive exoplanet search of all the ( 20 or so light years) closest stars. The ground based RV observing programs are entirely better suited for this even though it will take many years to build results.
SIM would have been ideal for that purpose. GAIA will likely find some of the larger exoplanets orbiting nearby stars. But it’s Earth mass worlds in HZ orbits that really peak our interest. Do they not?

Eniac July 23, 2012 at 1:41

When it comes to planets in proximity to us, necessarily we will have to use a method that can pick up planets with orbits in any orientation. Transits are definitely out, and radial velocity will also be insufficient. The methods that I can think of are interferometric astrometry (as with SIM) and direct optical detection (as with a vortex coronagraph or an occulter).

Direct optical detection has the advantage that it works well for large orbits without requiring several revolutions of observing time. This would serve to counteract the current extreme bias towards close-in “hot” planets. Those are likely all exceptions, but seem common because we cannot see the others as of now.

coolstar July 23, 2012 at 14:54

Thanks Andy for the link to the Vogt et al. paper as it seems like very good work. Interesting, they withdrew it from the Ap. J. and it’s set for publication in a much less prestigious journal since one of the papers it was meant to refute STILL hasn’t been accepted for publication in Astronomy & Astrophysics (an European journal).

andy July 24, 2012 at 17:13

@coolstar: yes Astronomy & Astrophysics seems to have quite a long time between submission, acceptance and final publication.

Though in this case, it is quite amusing if you take a look at who the Editor-in-Chief of A&A is…

Ronald July 25, 2012 at 9:57

(From the campsite in the Ardeche in southern France, darn, those French key boards are so different, e.g. you only get the period with Shift!)

Hello everybody, sorry if I say or ask anything stupid, but it,s warm here and my brain is in vacation mode;
Eniac, could both methods you mention, interferometric astrometry (as with SIM) and direct optical detection, possibly be combined in one and the same (groundbased) telescope, an interferometer?
And: as far as I know there is still plenty of space for small planetary orbits outside the 3 discovered planets near 82 Eridani.

Daniel July 30, 2012 at 14:18

Eniac Said:
“When it comes to planets in proximity to us, necessarily we will have to use a method that can pick up planets with orbits in any orientation. Transits are definitely out, and radial velocity will also be insufficient. The methods that I can think of are interferometric astrometry (as with SIM) and direct optical detection (as with a vortex coronagraph or an occulter).”

I totally agree with you,talk about Nearby planetary system Here is a Interesting proposal submitted to ESA for its 2010 call for
M-size mission within the Cosmic Vision 2015-2025 plan.

NEAT: a space born astrometric mission for the detection and characterization of nearby habitable planetary systems”

http://arxiv.org/pdf/1207.6511v1.pdf

Let’s hope that NEAT mission get approval

ljk October 9, 2012 at 10:56

Future exoplanet missions: NASA and the world (part 1)

The search for extrasolar planets, particularly those similar in size and orbit to the Earth, has become one of the hottest fields in astronomy.

In the first of a two-part article, Philip Horzempa examines some of the planned and proposed missions that can support those searches.

Monday, October 8, 2012

http://www.thespacereview.com/article/2167/1

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