# A Planet Orbiting HD 156668 with a Minimum Mass of Four Earth Masses

## New Super-Earth: HD156668b !

Announced at the 215th AAS meeting by Eta-Earth Survey: m sin(i) = 4.15 Earth masses

http://www.universetoday.com/2010/01/07/second-smallest-exoplanet-found/#more-49920

exoplanet
Planetesimal

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## Re: A Planet Orbiting HD 156668 with a Minimum Mass of Four Earth Masses

Second smallest exoplanet... let's see.

 Planet m sin(i)/mEarth True mass/mEarth HD 156668 b 4.15 ? PSR B1257+12 B 3.4 4.3 PSR B1257+12 C 2.9 3.9 MOA-2007-BLG-192Lb ? 3.3 Gliese 581 e 1.93 ? PSR B1257+12 A 0.015 0.020?
Also it would be worth considering exoplanets which have m sin(i) slightly above that of HD 156668 b, which may well turn out to be less massive, e.g. HD 40307 b which has m sin(i) = 4.2 Earth masses.

Hypothesis 1: astronomers can't count.
Hypothesis 2: astronomers have redefined the ordering of the set of real numbers.

Any suggestions?

Lazarus
dF star

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## Re: A Planet Orbiting HD 156668 with a Minimum Mass of Four Earth Masses

If I may suggest other hypotheses

3) The news media is uninformed misinformed.
4) The scientists don't want to confuse people by adding terms such as "radial-velocity". This leads to (3).
5) They may have said "2nd lowest radial velocity amplitude planet-based signal", half of which would at best be a grazing transit across someone's brain. So it gets simplified (dumbed down) so that others can understand.

There's little excuse for it though.
You know how many times planets have been imaged for the first time?
Or how many times water has been discovered on Mars?

Pretty cool discovery though. Has Eta-Earth found anything other than gas giants? Would be more than awkward if they kept turning up exclusively super-Earths.

Edit, looks like my third hypothesis is incorrect. They're pretty suggestive that this is indeed the 2nd lowest mass exoplanet.

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Sirius_Alpha

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## A Planet Orbiting HD 156668 with a Minimum Mass of Four Earth Masses

http://xxx.lanl.gov/abs/1003.3444We report the discovery of HD 156668b, an extrasolar planet with a minimum mass of M_P sin i = 4.15 M_Earth. This planet was discovered through Keplerian modeling of precise radial velocities from Keck-HIRES and is the second super-Earth to emerge from the NASA-UC Eta-Earth Survey. The best-fit orbit is consistent with circular and has a period of P = 4.6455 d. The Doppler semi-amplitude of this planet, K = 1.89 m/s, is among the lowest ever detected, on par with the detection of GJ 581e using HARPS. A longer period (P ~ 2.3 yr), low-amplitude signal of unknown origin was also detected in the radial velocities and was filtered out of the data while fitting the short-period planet. Additional data are required to determine if the long-period signal is due to a second planet, stellar activity, or another source. Photometric observations using the Automated Photometric Telescopes at Fairborn Observatory show that HD 156668 (an old, quiet K3 dwarf) is photometrically constant over the radial velocity period to 0.1 mmag, supporting the existence of the planet. No transits were detected down to a photometric limit of ~3 mmag, ruling out transiting planets dominated by extremely bloated atmospheres, but not precluding a transiting solid/liquid planet with a modest atmosphere.

Mongo
Asteroid

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## Re: A Planet Orbiting HD 156668 with a Minimum Mass of Four Earth Masses

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Sirius_Alpha

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## Re: A Planet Orbiting HD 156668 with a Minimum Mass of Four Earth Masses

Further, many exoplanets are announced with a model that includes a linear or second order RV trend, presumably due to a long period orbital companion. The source of the long-period signal remains unknown and motivates additional measurements. If it is due to a planet, the body has minimum mass ∼45 M ⊕ and orbits with P(c)=2.31 yr and a(c)=1.6 AU, a cold super-Neptune near the ice line.

Borislav
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