Three Super-Earths Orbiting HD 7924

Artist’s impression of a view from the HD 7924 planetary system looking back toward our sun, which would be easily visible to the naked eye. Since HD 7924 is in our northern sky, an observer looking back at the sun would see objects like the Southern Cross and the Magellanic Clouds close to our sun in their sky.

The following are data quoted, with little modification, from the paper that describes the discovery:

The Eta-Earth Survey at Keck Observatory played a important role in the discovery that super-Earths are abundant. Using the HIRES spectrometer, our team searched for planets in a volume-limited sample of 166 nearby G and K dwarf stars. Our search yielded new planets and detection limits for each star.

The first low-mass planet discovered in the Eta-Earth Survey was HD 7924b, a super-Earth with a mass of 8.7 M_⊕ and an orbital period of 5.4 days.

HD 7924, also known as HIP 6379 or GJ 56.5, is a nearby (16.82 pc) and bright K0.5V dwarf star. It is slightly metal poor relative to the Sun. It is relatively inactive, but we do detect some low-level chromospheric activity nonetheless. We computed the stellar mass and radius from Teff, log g, and [Fe/H] and found 0.81 ± 0.02 M_⊙ and 0.75 ± 0.03 R_⊙.

The two new planets have minimum masses of M_c sin i_c = 7.8 M_⊕ and M_d sin i_d = 6.3 M_⊕, and orbit HD 7924 with semi-major axis of ac = 0.113 AU and ad = 0.155 AU. Their orbital periods are of 15.3 and 24.5 days. Both planets receive far too much radiation from HD 7924 to be within the habitable zone with incident stellar irradiation values 114, 28, and 15 times that received from the Sun by Earth for planets b, c, and d respectively. Assuming that these planets have bond albebos similar to the mean total albedos of super-Earths (At = 0.32) their equilibrium temperatures are 826, 584, and 499 K.

Three Almost Earth-Size Planets Found Orbiting Nearby Star

This whimsical cartoon shows three recently discovered extrasolar planets (right) casting shadows on their host star that can been seen as eclipses, or transits, at Earth (left). Earth can be detected by the same effect, but only in the plane of Earth’s orbit (the ecliptic). During the K2 mission, many of the extrasolar planets discovered by the Kepler telescope will have this lucky double cosmic alignment that would allow for mutual discovery—if there is anyone on those planets to discover Earth. The three planets, orbiting a nearby star named EPIC 201367065, are just out of alignment; while they are visible from Earth, our solar system is tilted just out of their view.

The three planets, each just larger than Earth, are 1.5-2 times the size of Earth. The outermost planet orbits on the edge of the so-called “habitable zone,” where the temperature may be just right for liquid water, believed necessary to support life, on the planet’s surface.

“The compositions of these planets are unknown, but, there is a very real possibility the outer planet is rocky like Earth,” said Erik Petigura, a University of California, Berkeley graduate student. “If so, this planet could have the right temperature to support liquid water oceans.”

The planets were confirmed by the NASA Infrared Telescope Facility (IRTF) and the W. M. Keck Observatory in Hawaii as well as telescopes in California and Chile.

The astronomers next hope to determine what elements are in the planets’ atmospheres. If these warm, nearly Earth-size planets have thick, hydrogen-rich atmospheres, there is not much chance for life.

“A thin atmosphere made of nitrogen and oxygen has allowed life to thrive on Earth. But nature is full of surprises. Many extrasolar planets discovered by the Kepler Mission are enveloped by thick, hydrogen-rich atmospheres that are probably incompatible with life as we know it,” said Ian Crossfield, the University of Arizona astronomer who led the study.

The discovery is all the more remarkable because Kepler is now hobbled by the loss of two reaction wheels that kept it pointing at a fixed spot in space. Kepler, launched in 2009, was reborn in 2014 as “K2” with a clever strategy of pointing the telescope in the plane of the Earth’s orbit to stabilize the spacecraft. Kepler is back to mining the cosmos for planets by searching for eclipses, or transits, as planets orbit in front of their host stars and periodically block some of the starlight.