Object 31: GJ 3470

Podcast release date: 05 October 2020

Right ascension: 07:59:05.8


Epoch: ICRS

Constellation: Cancer

Corresponding Earth location: About 13 km off the coast near Palauig in the Philippines

GJ 3470 itself is a seemingly ordinary red dwarf star. It's located in the constellation Cancer at a distance of 96.05 light years (29.45 pc) from Earth [1,2], it has a mass slightly more than half the mass of the Sun [3], and it has a diameter slightly more than half the size of the Sun [3]. What makes it interesting is that it has at least one exoplanet orbiting it and maybe a second.

The first exoplanet was discovered during a set of observations between 2008 and 2012 with an instrument called the High Accuracy Radial velocity Planet Searcher (HARPS) mounted on the European Southern Observatory 3.6 Metre Telescope in Chile [4]. As indicated by the name, the instrument is designed to detect planets, and it does this by measuring the Doppler shifting of light from the slight movement of individual stars caused by the gravitational pull of planets in orbit around those stars. After HARPS detected a potential planet orbiting GJ 3470, several other observatories confirmed the exoplanet's presence [4].

This exoplanet (which has the designation GJ 3470b) is frequently referred to as either a hot Uranus or a warm Neptune. The mass was measured to be about 14 times the mass of the Earth [3,4], which means that it is close to the same mass as Uranus and Neptune, and GJ 3470b is probably similar to these planets in that it has a very thick atmosphere of hydrogen with a small rocky core at its center. The planet is located at a distance of only about 0.035 AU from the red dwarf [4]. For reference, 1 AU is the distance from the Earth to the Sun, and 0.387 AU is the distance from Mercury to the Sun, so GJ 3470b is actually located more than 10 times closer to its host star than Mercury is to our Sun. The exoplanet's close proximity to its host star would imply that it should be very hot, but on the other hand, the host star is a red dwarf, which is technically described by most astronomers as wimpy. The exoplanet's surface atmosphere is estimated to be somewhere between 160 and 340 degrees Celsius [4], which is indeed hot compared to the Earth's surface but actually cooler than the surface of Venus. Partly because of its relatively high surface temperature, the planet has a density lower than water [3], which means that it would float in a bathtub full of water if you could find one large enough. However, you've also probably heard that weird joke about Saturn, so it's probably not that funny or not that new. The planet also orbits its host star once every 3.33 Earth days, which is extremely fast.

What really makes GJ 3470b interesting is that it orbits its star in such a way that it passes directly between the star and the Earth. When this happens, the brightness of the red dwarf drops slightly. However, some of the star's light will pass through the outer atmosphere of the planet, and astronomers looking at this light can make some interesting measurements of the planet. So, to understand how astronomers are working, imagine being in a movie theater and eating a bag of jelly beans or, if you just stream movies at home, imagine sitting in your living room with the lights off while watching your TV and eating jelly beans. If you're like a lot of people, including me, you're probably going to hold up each and every jelly bean between you and the screen to figure out what flavor it is before eating it. Using related techniques, astronomers can effectively analyze the atmosphere of the exoplanet orbiting GJ 3470 in the same way when the planet passes in front of the star, and this has yielded a lot of information.

So, to begin with, several groups of astronomers have identified a phenomenon called Rayleigh scattering in the atmosphere of GJ 3470b (and Rayleigh is spelled R-a-y-l-e-i-g-h) [5,6,7,8]. This phenomenon is actually the same type of phenomenon that makes the Earth's sky blue, and in fact a press release indicated that astronomers expect the sky to be blue on GJ 3470b as well [9]. Several other measurements have tried to examine the weather, and they have found that the atmosphere seems to have no clouds, although it may be covered in haze [3,5,7,10,11]. Various other measurements have probed the chemical composition of the atmosphere and have identified the presence of hydrogen and helium [12,13], although the planet's atmosphere is expected to contain gases made up of heavier elements as well, including potentially methane and carbon monoxide [14]. It also looks like the outer layer of the exoplanet's atmosphere is being blown away by the star's stellar winds, which means that it could shrink in mass slowly over time [12,13].

So, GJ 3470b is a very well-studied exoplanet with a huge amount of evidence demonstrating its existence. However, I also found a paper based on observations by a large network of amateur astronomers claiming that a second planet may have been found orbiting GJ 3470 [15]. This planet, if it exists, would be called GJ 3470c. It is estimated to be the size of Saturn, and it would orbit its host star once every 66 days at a distance of 0.25 AU from the star. The problem is that the paper on GJ 3470c has not been published in a scientific journal, so it has not yet gone through the same type of scientific review as the multiple papers that have been published on GJ 3470b. This by itself means that the results from the paper should be treated extremely cautiously. To be fair to the amateur astronomers, though, they themselves also indicate that their results should be treated with a lot of caution and that their results need to be confirmed by the broader astronomical community, and they even posted their data online. It will be interesting to see whether the existence of GJ 3470c is confirmed, and if it is, I think the amateur astronomers deserve a lot of credit for their work.


[1] Gaia Collaboration et al., The Gaia mission, 2016, Astronomy & Astrophysics, 595, A1

[2] Gaia Collaboration et al., Gaia Data Release 2. Summary of the contents and survey properties, 2018, Astronomy & Astrophysics, 616, A1

[3] Demory, Brice-Olivier et al., Spitzer Observations of GJ 3470 b: A Very Low-density Neptune-size Planet Orbiting a Metal-rich M Dwarf, 2013, Astrophysical Journal, 768, 154

[4] Bonfils, X. et al., A hot Uranus transiting the nearby M dwarf GJ 3470. Detected with HARPS velocimetry. Captured in transit with TRAPPIST photometry, 2012, Astronomy & Astrophysics, 546, A27

[5] Nascimbeni, V. et al., The blue sky of GJ3470b: the atmosphere of a low-mass planet unveiled by ground-based photometry, 2013, Astronomy & Astrophysics, 559, A32

[6] Ehrenreich, D. et al., Near-infrared transmission spectrum of the warm-Uranus GJ 3470b with the Wide Field Camera-3 on the Hubble Space Telescope, 2014, Astronomy & Astrophysics, 570, A89

[7] Dragomir, Diana et al., Rayleigh Scattering in the Atmosphere of the Warm Exo-Neptune GJ 3470b, 2015, Astrophysical Journal, 814, 102

[8] Chen, G. et al., The GTC exoplanet transit spectroscopy survey. V. A spectrally-resolved Rayleigh scattering slope in GJ 3470b, 2017, Astronomy & Astrophysics, 600, A138

[9] Astronomy Now, A blue, Neptune-sized exoplanet around a red dwarf star, 2015, Astronomy Now

[10] Crossfield, Ian J. M. et al., Warm ice giant GJ 3470b. I. A flat transmission spectrum indicates a hazy, low-methane, and/or metal-rich atmosphere, 2013, Astronomy & Astrophysics, 559, A33

[11] Awiphan, S. et al., Transit timing variation and transmission spectroscopy analyses of the hot Neptune GJ3470b, 2016, Monthly Notices of the Royal Astronomical Society, 463, 2574

[12] Bourrier, V. et al., Hubble PanCET: an extended upper atmosphere of neutral hydrogen around the warm Neptune GJ 3470b, 2018, Astronomy & Astrophysics, 620, A147

[13] Palle, E. et al., A He I upper atmosphere around the warm Neptune GJ 3470 b, 2020, Astronomy & Astrophysics, 638, A61

[14] Venot, Olivia et al., The atmospheric chemistry of the warm Neptune GJ 3470b: Influence of metallicity and temperature on the CH4/CO ratio, 2014, Astronomy & Astrophysics, 562, A51

[15] Scott, Phillip et al., GJ 3470 c: A Saturn-like Exoplanet Candidate in the Habitable Zone of GJ 3470, 2020, arXiv e-prints, arXiv:2007.07373


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

Sound Effects: Anthousai, Dan099, dronemachine, ivolipa, jameswrowles, sarena6487528, skymary, smokum, and Xulie at The Freesound Project

Image Viewer: Aladin Sky Atlas (developed at CDS, Strasbourg Observatory, France)