New planet discovery by Bennett et al, about three Earth masses, adds to the variety of exoplanets.
Prospects for lots of Earth mass planets being out there are improved.
Bennett et al report today the discovery of a 3.3 Earth mass planet at the AAS meeting in St Louis.
Discovery is through microlensing by the MOA and OGLE collaborations.
It is a very, very interesting result.
Planet is 3.3+4.9-1.6 Earth masses,
that is it could have a mass as low as 1.7 Earth masses or as high as 8.2 Earth masses.
The host star of the planet is 0.06+0.028-0.021 Solar masses - that is, it is a high mass brown dwarf or very low mass red dwarf.
Orbital radius of the planet is uncertain, but probably about 0.6 AU - or little over half the distance of the Earth from the Sun.
Distance of the planet host star from the sun is about 1.1 kpc, or little over 3000 light years, but is very uncertain.
The planet would likely be the second lowest mass detected yet, with the lowest mass pulsar planet around PSR1257+12 being significantly lower mass, and it is comparable to the two higher mass planets around PSR1257+12.
It is a cold planet, the host star is much less luminous than the Sun, and the planet would have formed outside the "snow line" in the disk around the star (despite the young star having been substantially more luminous than it is currently).
So it is likely to be an ice ball - a planet with a rocky core and a substantial ice mantle.
Formally, what they have is a microlensing signal, which was unfortunately a bit undersampled, with a low mass lens and a secondary feature, which is poorly resolved but clearly present.
The mass ratio of the primary to secondary lens is about 5000:1, and the measurement of the mass of the primary lens and the mass ratio leads to the conclusion that there is very likely a low mass planet orbiting the lensing star.
That is, they estimate that the other object in the system inferred from the shape of the microlensing curve is 5000 times lower mass than the star doing the lensing.
The paper has an excellent discussion of the statistical and systematic uncertainties and some alternative models.
It is very hard to draw a robust conclusion from a single data point, the formal uncertainties are infinite; but, this is a small corner of the observing parameters space, low mass stars have low cross-sections for microlensing, we only see them because there are so many of them.
That we already see evidence for a few Earth mass planet from microlensing observations, very strongly suggests that there are a lot of Earth mass planets out there, and that they are found all over the place.
It also tells us that the observational capability really is here. The microlensing groups, with enough stars to observe frequently enough, really can detect Earth mass planets around distant stars right now.
I should note that sampling microlensing light curves is something amateur astronomers can make significant contributions to, and that their contributions are rewarded with co-authorship on discovery papers.
More data is needed.
I finally note that a lot of the degeneracies in their models are removed by the measurement of lensing parallax. If the source exhibits xallarap instead, then the results are not as reported, but, as the authors argue, source xallarap is unlikely.
xallarap is, of course, the parallax of the telescope due to source motion...
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I think Bennett withdrew an abstract in the winter meeting--was this the result, I wonder?
I went to the ExoPTF meeting in Pasadena last week, and I was very impressed with Scott Gaudi's presentation on microlensing. He made a very strong case for funding a wide field imager in South Africa to give microlensing continuous coverage of the bulge, so that they could look for the very short timescale spikes of planetary lensing events.
Given the geometry of the bulge and typical microlensing, they're sensitive to 1 to 5 AU and down to Mars mass planets! This makes a very strong case for a funding agency, as you can then just measure eta_Earth and work out how many Earths you'll be seeing with a space astrometric mission.
Years and years ago, I predicted in a Usenet posting that doppler detections of three-earth-mass planets were possible with a small improvement in the (then, mid-nineties) doppler sensitivity, when combined with low star mass and close orbit.
And can I now find that Usenet posting!? No, dammit.
The HARPS team reputedly has a 4 Earth mass planet or so, rumour is over at Oklo,
they flashed it up at the recent IAU symposium and reputedly will announce this summer.
It is clear that the current generation of radial velocity observations are going to get to Earth mass for low mass red dwarfs. Real Soon Now.
Gaudi's much desired imager is in an interesting position - it is "NASA science" but on the ground - the NSF can't really afford it and NASA probably wouldn't fund it.
Needs a sugar daddy. Anyone?
It is possible Las Cumbres Observatory will eat everyone's lunch on this, if they decide it is something they want to do.
JohnD: The paper I withdrew from the winter meeting was on the discovery of the Jupiter+Saturn analog system ( http://arxiv.org/abs/0802.1920 ). I had hoped that it would be published in time for the meeting, but it was published a month later in Science.
are the error bars on the mass 68% limits?
It is a cold planet, the host star is much less luminous than the Sun, and the planet would have formed outside the "snow line" in the disk around the star (despite the young star having been substantially more luminous than it is currently).
So it is likely to be an ice ball - a planet with a rocky core and a substantial ice mantle.
We've discovered Hoth. Time to send in the probe droids.
two sigma error bars, as I recall
The paper mentions the Stevenson (1999) hypothesis that a cold planet with a hydrogen atmosphere could maintain surface liquid water, but I thought hydrogen accretion wasn't supposed to turn on until about 6-10 Earth masses... is this really a likely hypothesis for this planet?
David, thanks for clarifying that!
As for accreting hydrogen, runaway accretion occurs at that mass range, but you still get some hydrogen at any mass. Typically, planets don't hold onto it for solar system analogues because the central star eventually burns it off. This should be less of a problem around a lower mass star. Although I guess you'd have to worry about wind ablation and flaring, so someone would have to think about it carefully