Smallest Known Exoplanet Found

by Paul Gilster on June 2, 2008

Smaller and smaller planets keep coming into view. A prime goal, of course, is to find something around the size of the Earth, implying as it would the existence of a world that might be like ours in other ways. My suspicion is that one day soon a transit study is going to come up with an exoplanet that’s closer to the size of Mars (definitely possible with today’s technologies), and we’ll skip right past the ‘Earth twin’ point before finding a planet that really is close to the same diameter.

But so far we’re still looking at worlds larger than Earth, like the tongue-twisting MOA-2007-BLG-192Lb, now thought to be the lowest mass planet ever found around another star. Announced today at the American Astronomical Society’s meeting in St. Louis, the new planet orbits a brown dwarf. At six percent of the mass of the Sun (and thus unable to sustain nuclear reactions in its core), the host is the lowest mass star to have a companion with a planetary mass ratio. But the fudge factor in the data does allow for the possibility of it being an extremely low mass hydrogen-burning star rather than a brown dwarf.

Smallest planet yet found

The news conference making these announcements is just winding down, a major point being that before this discovery, no planets had been found around stars less than twenty percent as massive as the Sun. We’re finding out that low mass stars can indeed host planets in the Earth-mass range, making nearby red dwarfs more and more interesting as systems with astrobiological possibilities. And with hundreds of brown dwarfs within 100 light years of Earth (fifteen percent of them in binary systems), we now must factor in the kind of planets we can expect to find around them.

Image: Artist’s conception of the newly discovered planet MOA-2007-BLG-192Lb orbiting a brown dwarf “star” with a mass of only 6% of that of the Sun. Theory suggests that the 3-earth-mass planet is made primarily of rock and ice. Observational and theoretical studies of brown dwarfs reveal that they have a magenta color due to absorption by elements such as sodium and potassium in their atmospheres. Credit: David Bennett.

This particular world has an orbital radius similar to that of Venus, but in a system dominated by such a faint central star, temperatures at the top of the atmosphere would be frigid. Even so, the possibility is strong that the planet has a thick atmosphere, one that would allow warmer temperatures on the surface, and there is some speculation that interior heating by radioactive decay could keep that surface warmer still. We may be dealing with a vast, deep ocean under all that atmosphere. But let me quote the paper on this:

…it is possible that MOA-2007-BLG-192Lb could have a habitable surface temperature itself, despite the fact that its host star or brown dwarf provides extremely feeble radiative heating. Stevenson (1999) has speculated that even a free floating Earth-mass planet could have a surface temperature that would allow liquid water even though the heating from internal radioactive decays provides a factor of ∼ 104 times less energy than the Earth receives from the Sun. The key point of Stevenson’s argument was that such a free floating planet might retain a molecular Hydrogen atmosphere that could provide very strong insulation that would allow the surface temperature to remain above the melting point of water ice. If it was possible to detect nearby analogs to MOA-2007-BLG-192Lb, it would be worthwhile to attempt to study their spectra to see if they do have H2 atmospheres that might allow warm surface temperatures.

Congratulations to two microlensing teams — the Microlensing Observations in Astrophysics (MOA) and Optical Gravitational Lensing Experiment (OGLE) collaborations — which used lensing to detect the planet. In this method, light from a background star is magnified by an intervening star (in this case, the planetary host), with the planet being spotted by additional perturbations to that light. I’m now looking at a press release on this work and noting a statement by David Bennett (University of Notre Dame), who led the team: “I’ll hazard a prediction that the first extra-solar Earth-mass planet will be found by microlensing. But we’ll have to be very quick to beat the radial velocity programs and NASA’s Kepler mission, which will be launched in early 2009.”

Yes, better hurry, for each planet detection method is improving rapidly, but my money is still on the transit method to make that first Earth-mass discovery. The paper is Bennett et al. (46 co-authors!), “A Low-Mass Planet with a Possible Sub-Stellar-Mass Host in Microlensing Event MOA-2007-BLG-192,” accepted by the Astrophysical Journal. I’ll have the arXiv link up as soon as it becomes available.

devicerandom June 2, 2008 at 14:07

Let me be annoyingly pedantic: It is the smallest only if you don’t take into account pulsar planets (which are of course of little interest for exobiology research).

However, I would like to know how has pulsar planet detection improved and if and how it can help us understand general statistical features of stellar systems. Since pulsar timing can detect extremly small planets (down to almost-asteroidal size, at least according to Wikipedia), it seems a currently unsurpassed tool for this kind of research.

Daniel Fischer June 2, 2008 at 15:06

Another nitpick: The mass of this planet is only poorly defined as it says in the paper (but not in the news conference): 3 (-2/+5) Earth masses. And we have had several exoplanets – detected by more direct means – in the 5 Earth-mass range in recent years; this one could easily exceed that.

tacitus June 2, 2008 at 15:28

Exciting news indeed, but let’s nor forget the COROT mission. They still have an outside chance of being first to detect an Earth twin too. If you remember, they announced that their instruments were operating at well above spec after the post-launch checkout, and that there was a possibility of detecting Earth-sized planets. Now, if that assertion is holding up, I am sure the COROT team is busting a gut to be the first to bring home the bacon.

Administrator June 2, 2008 at 16:47

devicerandom, how useful pulsar planetary detection would be in characterizing main sequence stellar systems is unclear to me, but maybe some of those here who are more in the know on pulsar work will have some ideas.

And tacitus, I share your COROT enthusiasm. Had also been wondering whether today’s exoplanet announcement was going to turn out to be Drake Deming with a GJ 436 planet via EPOCh, thus settling the current controversy over same, but the one we’ve gotten is still pretty interesting!

ljk June 3, 2008 at 0:29

A Low-Mass Planet with a Possible Sub-Stellar-Mass Host in Microlensing Event MOA-2007-BLG-192

Authors: D.P. Bennett, I.A. Bond, A. Udalski, T. Sumi, F. Abe, A. Fukui, K. Furusawa, J.B. Hearnshaw, S. Holderness, Y. Itow, K. Kamiya, A.V. Korpela, P.M. Kilmartin, W. Lin, C.H. Ling, K. Masuda, Y. Matsubara, N. Miyake, Y. Muraki, M. Nagaya, T. Okumura, K. Ohnishi, Y.C. Perrott, N.J. Rattenbury, T. Sako, To. Saito, S. Sato, L. Skuljan, D.J. Sullivan, W.L. Sweatman, P.J. Tristram, P.C.M. Yock, M. Kubiak, M.K. Szymanski, G. Pietrzynski, I. Soszynski, O. Szewczyk, L. Wyrzykowski, K. Ulaczyk, V. Batista, J.P. Beaulieu, S. Brillant, A. Cassan, P. Fouque, P. Kervella, D. Kubas, J.B. Marquette

(Submitted on 30 May 2008)

Abstract: We report the detection of an extrasolar planet of mass ratio q ~ 2 x 10^(-4) in microlensing event MOA-2007-BLG-192. The best fit microlensing model shows both the microlensing parallax and finite source effects, and these can be combined to obtain the lens masses of M = 0.060 (+0.028 -0.021) M_sun for the primary and m = 3.3 (+4.9 -1.6) M_earth for the planet.

However, the observational coverage of the planetary deviation is sparse and incomplete, and the radius of the source was estimated without the benefit of a source star color measurement.

As a result, the 2-sigma limits on the mass ratio and finite source measurements are weak. Nevertheless, the microlensing parallax signal clearly favors a sub-stellar mass planetary host, and the measurement of finite source effects in the light curve supports this conclusion. Adaptive optics images taken with the Very Large Telescope (VLT) NACO instrument are consistent with a lens star that is either a brown dwarf or a star at the bottom of the main sequence. Follow-up VLT and/or Hubble Space Telescope (HST) observations will either confirm that the primary is a brown dwarf or detect the low-mass lens star and enable a precise determination of its mass.

In either case, the lens star, MOA-2007-BLG-192L, is the lowest mass primary known to have a companion with a planetary mass ratio, and the planet, MOA-2007-BLG-192Lb, is probably the lowest mass exoplanet found to date, aside from the lowest mass pulsar planet.

Comments: Accepted for publication in the Astrophysical Journal. Scheduled for the Sept. 1, 2008 issue

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0806.0025v1 [astro-ph]

Submission history

From: David Bennett [view email]

[v1] Fri, 30 May 2008 20:25:23 GMT (1865kb,D)

http://arxiv.org/abs/0806.0025

MaDeR June 3, 2008 at 2:50

COROT is overhyped. Anyone remember this craze when COROT was launched? And over year later what we have: 4 hot jupiters and one strange thingy. Even with latter, not very impressive.

dad2059 June 3, 2008 at 4:26

Brown dwarf stars and their solar systems may be the most common type in this galaxy, next to the M-type stars. Detecting them might be the sticking point because of the brown dwarfs’ low mass and luminousity.

As for the various methods of searching for and finding Earth-type worlds, I’m not sure which one would be the first, but I think if one method finds an inkling of one, the other methods will be used to help verify the find.

devicerandom June 3, 2008 at 5:48

What I mean is: pulsars were once main sequence stars (IIRC). Their planetary systems therefore are probably remnants of planetary systems of main sequence stars. In understanding planetary formation, I (naively) can’t see why those data shouldn’t be counted in statistics.

andy June 3, 2008 at 9:04

devicerandom: The main problem with the idea that the pulsar planets are remnants of the pre-supernova system is that the supernova explosion results in a significant and sudden mass loss from the system. This has serious implications for the stability of the planets after the supernova.

Take a system of two masses on circular orbits and let one of them suddenly lose mass, and assume the mass loss is spherically symmetrical, so the velocity of the exploding star is unchanged. If you do the mathematics, it turns out that the system becomes unbound if the mass lost during the supernova is more than half the system’s total mass. In a real supernova, the progenitor mass is somewhere upwards of around 10 solar masses, while the neutron star remnant is about 1.4 solar masses, which means a supernova easily meets this condition (the contribution of the planets to the total mass of the system is negligible): this suggests any orbiting planets go flying off into interstellar space.

Of course there is evidence for non spherical symmetry of the supernova explosion, which gives the pulsar a “kick” – if it is of the right magnitude and in the right direction you might save one of the planets, but it is likely to end up on a highly eccentric orbit, and in any case the kick would likely be in the wrong direction to save the other planets: ending up with multiple planets on nice stable circular orbits (like the terrestrial-mass planets around PSR B1257+12) after the supernova is extremely implausible.

Thus the pulsar planets are pretty much certainly going to have formed post-supernova.

ljk June 11, 2008 at 23:22

How to Find Faraway Moons

It’s possible to detect a moon orbiting a known exoplanet,
says astrophysicist.

http://www.space.com/scienceastronomy/080609-mm-extrasolar-moons.html

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