From astrobites: “TESS reveals HD118203 b transits after 13 years”

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From astrobites

Dec 26, 2019
Emma Foxell

Title:TESS Reveals HD 118203 b to be a Transiting Planet
https://arxiv.org/abs/1911.05150
Authors: Joshua Pepper, Stephen R. Kane, Joseph E. Rodriguez et al
First Author’s Institution: Department of Physics, Lehigh University, Bethlehem, USA

Status: Submitted

There are multiple ways to discover an exoplanet. The first exoplanet around a solar type star was discovered by radial velocity measurements, and earned the discoverers’ this year’s Nobel Prize.

Radial Velocity Method-Las Cumbres Observatory


Radial velocity Image via SuperWasp http http://www.superwasp.org-exoplanets.htm

After the advent of wide field exoplanet surveys, from SuperWASP starting in 2006, to NGTS and TESS, most exoplanets have been discovered using the transit method and confirmed by radial velocity.

Planet transit. NASA/Ames

However, the exoplanet in today’s article, HD118203 b, was detected by radial velocity back in 2006 and has been found to transit 13 years after its discovery.

Radial velocity discovery

HD118203 b was found in 2006 by using the radial velocity technique: measuring the amount the star’s spectrum ‘wobbles’ tugged by its orbiting planet. Spectra are red shifted as the planet tugs its star away from us and blue shifted as the star is tugged towards us over one orbit of the planet. Radial velocity measurements give us the planet’s orbital period as well as its eccentricity and the minimum mass of the planet. The true planet mass depends on the relative inclination between star and planet. 43 radial velocity measurements from ELODIE* revealed HD118203 b as an eccentric planet with an orbital period of ~6.13 days, and a minimum mass of about 2 Jupiters (see Figure 1).

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Figure 1: 43 radial velocity measurements by ELODIE showed the existence of HD118203 b (da Silva 2006). The top plot shows the radial velocity measurements over time. The bottom plot is phase folded with the period found by EXOFASTv2, and more clearly shows the periodic change in the star’s radial velocity by 100s m/s it is orbited by an object with minimum mass twice that of Jupiter. Figure 3 of today’s paper.

While most orbit orientations will produce radial velocity signatures, only a small percentage happen to line up so that we can see the planet pass in front of its star, or transit. Transiting exoplanets block out a small fraction of light giving us the relative radii of planet and star. If a planet transits, this constrains the planet’s inclination and means that the minimum mass from radial velocity is very close to the true mass.

A number of exoplanets discovered using radial velocities have since been found to transit but this process is time consuming for two reasons. First, only a small fraction of exoplanets will actually transit. Second, transits only last a very small fraction of the orbit (usually a couple of hours for an orbit of less than 10 days), so telescopes must stare at a star for long periods of time to discover when the transit actually occurs.

TESS spies a transit (or five)

NASA/MIT TESS replaced Kepler in search for exoplanets

Transits of HD118203 b were discovered thanks to the ongoing TESS mission. TESS is a space mission which will observe most of the sky, staring at each sector for 28 days, looking for transiting exoplanets. Five transits were automatically identified using the Science Processing Operations Center (SPOC), see Figure 2, and following vetting to check for false positives, it was identified as a promising candidate.

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Figure 2: TESS photometry of HD118203 b. The top plot shows the light curve as processed by SPOC and the bottom plot shows the flattened lightcurve as used in EXOFASTv2. Figure 1 of today’s paper.

*ELODIE was an echelle type spectrograph installed at the Observatoire de Haute-Provence 1.93m reflector in south-eastern France for the Northern Extrasolar Planet Search.

L’Observatoire de Haute-Provence, 1.93 meter telescope in the southeast of France. Altitude 650 m (2,130 ft)

Its optical instrumentation was developed by André Baranne from the Marseille Observatory. The purpose of this instrument was extrasolar planet detection by the radial velocity method. This instrument was also used for the M-Dwarf Programmes.

ELODIE first light was achieved in 1993. ELODIE was decommissioned in August 2006 and replaced in September 2006 by SOPHIE, a new instrument of the same type but with improved features.

The SOPHIE (Spectrographe pour l’Observation des Phénomènes des Intérieurs stellaires et des Exoplanètes, literally meaning “spectrograph for the observation of the phenomena of the stellar interiors and of the exoplanets”) échelle spectrograph is a high-resolution echelle spectrograph installed on the 1.93m reflector telescope at the Haute-Provence Observatory located in south-eastern France. The purpose of this instrument is asteroseismology and extrasolar planet detection by the radial velocity method. It builds upon and replaces the older ELODIE spectrograph. This instrument was made available for use by the general astronomical community October 2006

See the full article here .


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