astronomy

Gliese 876 d

Gliese 876 d is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Gliese 876 d rather than just read about it. In short: Gliese 876 d is an exoplanet 15.2 light-years (4.7 parsecs) away in the constellation of Aquarius. The planet was the third planet discovered orbiting the red dwarf Gliese 876, and is the innermost planet in the system.

Key takeaways

Gliese 876 d belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Gliese 876 d to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Gliese 876 d from memory before moving on to harder problems.

Reference excerpt

Gliese 876 d is an exoplanet 15.2 light-years (4.7 parsecs) away in the constellation of Aquarius. The planet was the third planet discovered orbiting the red dwarf Gliese 876, and is the innermost planet in the system. It was the lowest-mass known exoplanet apart from the pulsar planets orbiting PSR B1257+12 at the time of its discovery. Due to its mass, it can be categorized as a super-Earth.

Characteristics

Mass, radius, and temperature The mass of any exoplanet from radial velocity has one problem, in that only a lower limit on the mass can be obtained. This is because the measured mass value also depends on the orbital inclination, which in general is unknown. However, in the case of Gliese 876, models incorporating the gravitational interactions between the resonant outer planets enables the inclination of the orbits to be determined. This reveals that the outer planets are nearly coplanar with an inclination of around 59° with respect to the plane of the sky. Assuming that Gliese 876 d orbits in the same plane as the other planets, the true mass of the planet is revealed to be 6.83 times the mass of Earth. The low mass of the planet has led to suggestions that it may be a terrestrial planet. This type of massive terrestrial planet could be formed in the inner part of the Gliese 876 system from material pushed towards the star by the inward migration of the gas giants. Alternatively the planet could have formed further from Gliese 876, as a gas giant, and migrated inwards with the other gas giants. This would result in a composition richer in volatile substances, such as water. As it arrived in range, the star would have blown off the planet's hydrogen layer via coronal mass ejection. In this model, the planet would have a pressurised ocean of water (in the form of a supercritical fluid) separated from the silicate core by a layer of ice kept frozen by the high pressures in the planetary interior. Such a planet would have an atmosphere containing water vapor and free oxygen produced by the breakdown of water by ultraviolet radiation. Distinguishing between these two models would require more information about the planet's radius or composition. The planet does not transit its star, which makes obtaining this information impossible with current observational capabilities. The equilibrium temperature of Gliese 876 d is estimated to be around 614 K (341 °C; 646 °F).

Host star

The planet orbits a (M-type) star named Gliese 876. The star has a mass of 0.33 M☉ and a radius of around 0.36 R☉. It has a surface temperature of 3350 K and is 2.55 billion years old. In comparison, the Sun is about 4.6 billion years old and has a surface temperature of 5778 K.

Orbit Gliese 876 d is located in an orbit with a semimajor axis of only 0.0208 AU (3.11 million km). At this distance from the star, tidal interactions should in theory circularize the orbit; however, measurements reveal that it has a high eccentricity of 0.207, comparable to that of Mercury in the Solar System. Models predict that, if its non-Keplerian orbit could be averaged to a Keplerian eccentricity of 0.28, then tidal heating would play a significant role in the planet's geology to the point of keeping it completely molten. Predicted total heat flux is approximately 104–5 W/m2 at the planet's surface; for comparison the surface heat flux for Io is around 3 W/m2. This is similar to the radiative energy it receives from its parent star of about 40,000 W/m2.

Discovery Gliese 876 d was discovered by analysing changes in its star's radial velocity as a result of the planet's gravity. The radial velocity measurements were made by observing the Doppler shift in the star's spectral lines. At the time of discovery, Gliese 876 was known to host two extrasolar planets, designated Gliese 876 b and c, in a 2:1 orbital resonance. After the two planets were taken into account, the radial velocity still showed another period, at around two days. The planet, designated Gliese 876 d, was announced on June 13, 2005 by a team led by Eugenio Rivera and was estimated to have a mass approximately 7.5 times that of Earth.

Notes

References

External links "GJ 876 d". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2008-06-21. "Smallest extrasolar planet found". BBC News. 2005-06-13. Archived from the original on 2015-11-06. Retrieved 2008-06-21. GJ 876 d Archived 2015-11-07 at the Wayback Machine Catalog

Illustrations

Worked examples

Example 1 — a first encounter with Gliese 876 d

Start with the simplest possible case. Write down what Gliese 876 d claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Gliese 876 d before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Gliese 876 d ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Gliese 876 d

In research: Gliese 876 d appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Gliese 876 d in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Gliese 876 d is common in secondary-school and first-year university syllabi. It links to neighbouring topics Aquarius (constellation), Exoplanets detected by radial velocity, Exoplanets discovered in 2005, so understanding it makes those chapters shorter.
In everyday life: Look for Gliese 876 d outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Gliese 876 d in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Gliese 876 d means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Gliese 876 d out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Gliese 876 d in simple terms?

Why does Gliese 876 d matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Gliese 876 d?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Gliese 876 d.