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How Common Are Planets Around Red Dwarf Stars?

We’re beginning to learn how common planets are around stars of various types, but M-dwarfs get special attention given their role in future astrobiological studies. As I’ve just been talking about CARMENES, the Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs program, I’ll fold in today’s news about their release of 20,000 observations covering more than 300 stars, for we can mine some data here about planet occurrence rates.

59 new planets turn up in the spectroscopic data gathered at the Calar Alto Observatory in Span, with about 12 thought to be in the habitable zone of their star. I’ll await with interest our friend Andrew LePage’s assessment. His habitable zone examinations serve as a highly useful reality check.

I mentioned spectrographic data above. The CARMENES instruments are built for optical as well as near-infrared studies, and have been used to explore nearby red dwarfs and their possible planets since 2015. The original project ended at the end of 2020, covering the data released here, although later observations are continuing, so we can expect further releases. Remember, this is radial velocity rather than transit work. CARMENES offers an accuracy of about 1 meter per second, meaning astronomers can measure the tiny motions induced by an orbiting planet with that degree of sensitivity. At this level, low-mass stars yield up Earth-sized planets.

The new CARMENES planets appear in the graphic below, drawn from publication of the project’s first large dataset as published in Astronomy & Astrophysics. 19,633 spectra for a sample of 362 targets were collected. The current paper covers only the visible-light data, with the infrared data to be made public when processing is complete. CARMENES has now observed about half of all nearby red dwarfs, limited of course by the fact that its location in Spain precludes southern hemisphere observations. A bit more on the original 362 target stars:

The sample was designed to be as complete as possible by including M dwarfs observable from the Calar Alto Observatory with no selection criteria other than brightness limits and visual binarity restrictions. To best exploit the capabilities of the instrument, variable brightness cuts were applied as a function of spectral type to increase the presence of late-type targets. This effectively leads to a sample that does not deviate significantly from a volume-limited one for each spectral type. The global completeness of the sample is 15% of all known M dwarfs out to a distance of 20 pc and 48% at 10 pc.

Image: An animated illustration of the CARMENES planets. All planets discovered with the same method as CARMENES, but with other instruments, are shown as grey dots. With the data collected in the period 2016-2020, CARMENES has discovered and confirmed 6 Jupiter-like planets (with masses more than 50 times that of the Earth), 10 Neptunes (10 to 50 Earth masses) and 43 Earths and super-Earths (up to 10 Earth masses). The vertical axis indicates what star type the planets orbit around, from the coolest and smallest red dwarfs to brighter and hotter stars (the Sun would correspond to the second from the top). The horizontal axis depicts the distance from the planet to the star by showing the time it takes to complete the orbit. Planets in the habitable zone (blue-shaded area) can harbor liquid water on their surface. Credit: © Institut d’Estudis Espacials de Catalunya (IEEC).

As to the title of today’s post, the authors have calculated new planet occurrence rates from a subset of the overall sample, with low-mass planets tagged at 1.06 planets per star in periods of 1 day to 100 days, “and an overabundance of short-period planets around the lowest-mass stars of our sample compared to stars with higher masses.” Nearly every M-dwarf, in other words, appears to host at least one planet. The long-period giant planet occurrence rate comes in at 3%.

CARMENES Legacy Plus, which continues the original project, began in 2021 and continues to observe the same stars at least until the end of this year. Juan Carlos Morales is a researcher at IEEC (Institut d’Estudis Espacials de Catalunya):

“In order to determine the existence of planets around a star, we observe it a minimum of 50 times. Although the first round of data have already been published to grant access to the scientific community, the observations are still ongoing.”

The paper is Ribas et al., “The CARMENES search for exoplanets around M dwarfs Guaranteed time observations Data Release 1 (2016-2020),” Astronomy & Astrophysics Vol. 670, A139 (22 February 2023). Full text.

{ 7 comments… add one }
  • Alex Tolley February 22, 2023, 13:25

    Also, CARMENES has discovered six Saturn- and Jupiter-mass planets, some of them around very low-mass primaries, thus defying canonical planet formation models (Morales et al. 2019)

    One of three differences was remarked on in the paper between observed vs computational models. Observation should always take precedence and should inform modeling. Models are interesting, but should always be considered as “iffy” until confirmed by data.

  • Daniel February 22, 2023, 19:34

    The Occurrence Rate of Terrestrial Planets Orbiting Nearby Mid-to-late M Dwarfs from TESS Sectors 1-42

    • Andrew LePage February 23, 2023, 2:26

      One should be careful comparing the results of this particular TESS survey with these results from CARMENES. It is sort of an “apples and oranges” situation because each survey covers a different part of parameter space. The TESS survey is focused on the smallest red dwarfs (CARMENES include much large red dwarfs in its sample). The TESS survey covers planets with orbital periods in the 1 to 7 day range (CARMENES covers planets with orbital periods as great as 100 days). The exoplanets size range in the TESS-based estimate are as small as 0.5 Earth-radii (equivalent of ~0.1 Earth masses for a rocky planet compared to the minimum of about an Earth mass for the CARMENES result). Still, the results all suggest that roughly Earth-size exoplanets are common around red dwarfs.

  • Andrew LePage February 23, 2023, 0:51

    Thanks for the shout out, Paul. As it turns out I’ve already written about a handful of the stars observed by CARMENES and mentioned in this paper (potentially habitable and otherwise). Some of these reviews are several years old but the new data from CARMENES will certainly help in updates:

    Teegarden’s Star: https://www.drewexmachina.com/2019/06/19/habitable-planet-reality-check-the-earth-size-planets-of-teegardens-star/

    Barnard’s Star (although this may be a false positive): https://www.drewexmachina.com/2018/11/16/our-new-neighbor-orbiting-barnards-star-details-historical-background/

    Lalande 21185: https://www.drewexmachina.com/2017/02/18/our-new-neighbor-orbiting-lalande-21185/

    GJ 581: https://www.drewexmachina.com/2015/03/09/habitable-planet-reality-check-gj-581d/

    • Paul Gilster February 23, 2023, 8:40

      Thanks for the pointers, Drew. Fine work indeed!

  • wdk February 25, 2023, 0:03

    Since we are lucky to detect more than one planet per star, there remains an open question: Is a terrestrial planet simply the same basic object as a gas giant except that in its case only a core like structure remains and the solar abundant hydrogen blew away? Were we to see a jovian planet in an orbit inside of terrestrial planet ( and perhaps there are such in the data base), then that would argue toward a fundamental distinction. On the other hand, Trappist-1 has a string of terrestrial planets crowded in or near its putative HZ. And then here and there a jovian planet can be found. Coincidentally, saw a report of such just today. TOI-5205b. Its transit depth appears to exceed 5%.


  • Edwin February 25, 2023, 1:25

    While on the subject of planets around red dwarfs this was an interesting paper.

    The interesting thing to think about are any orbiting Moons?

    TOI-5205b A Short-period Jovian Planet Transiting a Mid-M Dwarf

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