# Space and science in Esperanto: planets, stars, sciences

Source: https://esperantopedia.org/phrases/space-science/
Language: en
Updated: 2026-10-08

> The Sun is suno, a planet planedo, a star stelo: planets, space travel, school sciences and lab words in Esperanto, with pronunciation.

In Esperanto the Sun is ***suno***, the Moon ***luno***, a star ***stelo*** and a planet ***planedo***. Planet names are international and end in *-o* (*Marso*, *Jupitero*, *Plutono*), school subjects end in *-io* or *-iko* (*astronomio*, *biologio*, *fiziko*), and the people who work in them are *-isto* words or shorter roots (*fizikisto*, *astronomo*). Everyday sky words are on the [nature](https://esperantopedia.org/phrases/nature/) page. Pronunciation guides show the stressed syllable in capitals; it is always the second-to-last (see the [alphabet](https://esperantopedia.org/learn/alphabet/)).

## Planets of the Solar System

Counted from the Sun, the planets are *Merkuro, Venuso, Tero, Marso, Jupitero, Saturno, Urano* and *Neptuno*.

| English | Esperanto | Pronunciation | Note |
|---|---|---|---|
| Mercury | *Merkuro* | mer-KU-ro | nearest the Sun |
| Venus | *Venuso* | ve-NU-so | second; the goddess is *Venero* |
| Earth | *Tero* | TE-ro | third |
| Mars | *Marso* | MAR-so | fourth; adjective *marsa*; a science-fiction Martian is a *marsano* |
| Jupiter | *Jupitero* | ju-pi-TE-ro | fifth and largest |
| Saturn | *Saturno* | sa-TUR-no | sixth, second largest, known for its rings |
| Uranus | *Urano* | u-RA-no | seventh |
| Neptune | *Neptuno* | nep-TU-no | eighth |
| Pluto | *Plutono* | plu-TO-no | a dwarf planet (*nanplanedo*); counted as the ninth planet until 2006 |

The order comes from ReVo, which defines *Marso* as *la kvara planedo* "the fourth planet" of the solar system. Most planet names double as names of gods: ReVo lists *Marso* as the Roman god of war and *Urano* as the Greek god of the sky. The weekdays *lundo, mardo, merkredo* are separate words, not built from planet names, and take a small letter; see [days and months](https://esperantopedia.org/phrases/days-months/).

## Capital letters: suno or Suno?

Planet names are normal proper names: capital letter, no *la* (*Marso*, *sur Marso*). *Suno, luno* and *tero* are ordinary words. PMEG notes that people often write *la Suno* with a capital when they mean our Sun as a name, and such names keep *la*, like *la Granda Ursino*. Capitals are not compulsory, so both spellings occur.

| Written | What it means |
|---|---|
| *La suno brilas.* | The sun is shining (PMEG's example) |
| *la Suno*, *la Luno*, *la Tero* | our Sun, our Moon, the planet Earth as names; ReVo writes *Suno-Tero-distanco* |
| *sunoj* | suns, that is stars with planets of their own |
| *la lunoj de Jupitero* | the moons of Jupiter (any natural satellite is a *luno*) |
| *tero* | soil or ground; in plain text also the planet |

## The Solar System and beyond

| English | Esperanto | Pronunciation | Note |
|---|---|---|---|
| solar system | *sunsistemo* | sun-sis-TE-mo | the Sun with the bodies around it |
| planet | *planedo* | pla-NE-do | circles a star and sweeps its orbit clear |
| dwarf planet | *nanplanedo* | nan-pla-NE-do | *nana* "tiny" + *planedo* |
| asteroid | *asteroido* | as-te-ro-I-do | metres to kilometres across |
| comet | *kometo* | ko-ME-to | grows a tail near the Sun |
| meteor | *meteoro* | me-te-O-ro | the light of a body entering the atmosphere |
| shooting star | *falstelo* | fal-STE-lo | "falling star", a bright meteor |
| galaxy | *galaksio* | ga-lak-SI-o | one of the great star masses of the universe |
| Milky Way | *Lakta Vojo*, *Laktovojo* | LAK-ta VO-jo, lak-to-VO-jo | ReVo advises against *galaksio* for our own galaxy |
| universe | *universo* | u-ni-VER-so | everything that exists |
| eclipse | *eklipso* | ek-LIP-so | *luneklipso* lunar, *suneklipso* solar |
| new moon, full moon | *novluno*, *plenluno* | nov-LU-no, plen-LU-no | |

## Astronomy and space travel

| English | Esperanto | Pronunciation | Note |
|---|---|---|---|
| star | *stelo* | STE-lo | self-luminous; ReVo counts our Sun as one |
| constellation | *konstelacio* | kon-ste-la-CI-o | one of 88 official regions of the sky |
| star pattern | *stelaro* | ste-LA-ro | the figure it draws is a *stelfiguro* |
| Ursa Major | *la Granda Ursino* | GRAN-da ur-SI-no | the large dipper-shaped figure |
| Orion | *Oriono* | o-ri-O-no | no *la* (PMEG) |
| telescope | *teleskopo* | te-le-SKO-po | for observing distant objects |
| observatory | *observejo*, *stelobservejo* | ob-ser-VE-jo, ste-lob-ser-VE-jo | ReVo also has *observatorio* |
| orbit | *orbito* | or-BI-to | the curve one body follows around another |
| satellite | *satelito* | sa-te-LI-to | a moon, or a device orbiting a planet |
| rocket | *raketo* | ra-KE-to | also a firework rocket |
| spaceship | *kosmoŝipo* | kos-mo-ŜI-po | ReVo: same as *kosmoveturilo* |
| astronaut | *astronaŭto*, *kosmonaŭto* | as-tro-NAŬ-to, kos-mo-NAŬ-to | ReVo defines *astronaŭto* as *kosmonaŭto* |
| outer space | *kosmo* | KOS-mo | the universe, esp. space beyond the Earth |
| space (room, gap) | *spaco* | SPA-co | ReVo labels *spaco* for outer space *evitinde* "to be avoided" |
| alien | *eksterterano* | ek-ster-te-RA-no | one from outside the Earth |

Verbs: *orbiti* "to orbit", *rondiri* "to circle" (ReVo: the Moon *rondiras ĉirkaŭ* the Earth), *turniĝi* "to turn, rotate" (the Earth turns around its axis, *akso*), *ekflugi* "to take off" (ReVo: a *kosmodromo* is the place for the *ekflugo* of spaceships), *observi* "to observe".

## How are space words built?

| Word part | Meaning | Examples |
|---|---|---|
| *-ar-* | a group of many | *stelo* → *stelaro* |
| *-et-* | small | *stelo* → *steleto* small star |
| *-ej-* | place | *observejo* observatory |
| *-il-* | instrument | *mezuri* "measure" → *mezurilo* |
| *-ig-* | make, cause | *en·orbit·ig·i* put into orbit |
| *ekster-* | outside | *eksterterano* alien, *ekstersunsistema* outside the solar system |
| compound | first word describes the second | *sunsistemo, suneklipso, plenluno, kosmoŝipo* |

More in [suffixes](https://esperantopedia.org/grammar/suffixes/) and [word building](https://esperantopedia.org/grammar/word-building/).

## School sciences and scientists

| English | Esperanto | Pronunciation | Note |
|---|---|---|---|
| science | *scienco* | sci-EN-co | knowledge built on research |
| physics | *fiziko* | fi-ZI-ko | movement of bodies and their interactions |
| chemistry | *ĥemio*, *kemio* | ĥe-MI-o, ke-MI-o | ReVo lists both forms |
| biology | *biologio* | bi-o-lo-GI-o | |
| mathematics | *matematiko* | ma-te-ma-TI-ko | |
| geography | *geografio* | ge-o-gra-FI-o | |
| geology | *geologio* | ge-o-lo-GI-o | study of the Earth's crust |
| astronomy | *astronomio* | as-tro-no-MI-o | the science of the heavenly bodies |
| meteorology | *meteorologio* | me-te-o-ro-lo-GI-o | weather science, also *meteologio*; see [weather and seasons](https://esperantopedia.org/phrases/weather-seasons/) |
| computer science | *komputiko* | kom-pu-TI-ko | ReVo: same as *informatiko*, *informadiko*; see [technology](https://esperantopedia.org/phrases/technology-internet/) |

| English | Esperanto | Pronunciation | Note |
|---|---|---|---|
| scientist | *sciencisto*, *scienculo* | sci-en-CIS-to, sci-en-CU-lo | *sciencisto* is a professional; *scienculo* anyone who researches a field |
| physicist | *fizikisto* | fi-zi-KIS-to | *fiziko* + *-ist-* |
| chemist | *ĥemiisto*, *kemiisto* | ĥe-mi-IS-to, ke-mi-IS-to | *ĥemio* + *-ist-* |
| mathematician | *matematikisto* | ma-te-ma-ti-KIS-to | *matematiko* + *-ist-* |
| biologist | *biologo* | bi-o-LO-go | also *biologiisto* |
| geographer | *geografo* | ge-o-GRA-fo | also *geografiisto* |
| geologist | *geologo* | ge-o-LO-go | also *geologiisto* |
| astronomer | *astronomo* | as-tro-NO-mo | not *astronomisto* |

Two patterns explain the list. When the person has no root of its own, add *-ist-* to the science: *fiziko* → *fizikisto*. PMEG points out that *fiziko* cannot also mean the physicist, because it is the name of the science. Many names in *-io* have a parallel root without the *i*: *biologio* → *biologo*. The *-iisto* forms are possible, but the double *i* is clumsy, so people prefer the short root, and PMEG says not to coin *astronomisto*. PMEG calls the *-olog-* of *biologo* an ex-suffix: an international ending, not a regular Esperanto suffix. Subjects and classroom words are on the [school](https://esperantopedia.org/phrases/school-education/) page.

## Laboratory and experiment

| English | Esperanto | Pronunciation | Note |
|---|---|---|---|
| laboratory | *laboratorio* | la-bo-ra-to-RI-o | a workplace for research and experiments |
| experiment | *eksperimento* | eks-pe-ri-MEN-to | a methodical test of a hypothesis |
| to experiment | *eksperimenti* | eks-pe-ri-MEN-ti | usually *eksperimenti pri* + topic |
| hypothesis | *hipotezo* | hi-po-TE-zo | an unproven assumption, to be tested |
| theory | *teorio* | te-o-RI-o | an organised explanation that predicts phenomena |
| to measure | *mezuri* | me-ZU-ri | compare with a unit; *mezuro* measurement |
| to prove | *pruvi* | PRU-vi | show by reasoning or fact |
| microscope | *mikroskopo* | mi-kro-SKO-po | shows what the naked eye cannot |
| atom | *atomo* | a-TO-mo | nucleus plus electrons |
| molecule | *molekulo* | mo-le-KU-lo | smallest unit of a chemical substance |
| energy | *energio* | e-ner-GI-o | in physics, the capacity to do work |
| gravity | *gravito* | gra-VI-to | the force that pulls masses together |

## Space and science in sentences

| Esperanto | English |
|---|---|
| *La Tero turniĝas ĉirkaŭ sia akso.* | The Earth turns around its axis. |
| *La Luno rondiras ĉirkaŭ la Tero.* | The Moon goes around the Earth. |
| *Marso estas la kvara planedo de la sunsistemo.* | Mars is the fourth planet of the solar system. |
| *Saturno havas multajn lunojn.* | Saturn has many moons. |
| *Ĉu estas vivo sur Marso?* | Is there life on Mars? |
| *Nokte ni observas la stelojn per teleskopo.* | At night we watch the stars through a telescope. |
| *Mia fratino volas esti astronaŭto.* | My sister wants to be an astronaut. |
| *La kosmoŝipo ekflugis hieraŭ.* | The spaceship took off yesterday. |
| *La sciencistoj eksperimentas pri nova metodo.* | The scientists are experimenting with a new method. |

## Common questions

### How do you say "planet" in Esperanto?

*Planedo*. Pluto, *Plutono*, is a *nanplanedo* "dwarf planet"; ReVo says it was counted as the ninth planet until 2006.

### Are planet names capitalised in Esperanto?

Yes. They are proper names, so *Marso* and *Jupitero* take a capital and no *la*. A capital on *la Suno*, and likewise *la Luno* and *la Tero*, is optional; PMEG says *la Suno* is often written that way.

### How do you say "space" in Esperanto?

*Kosmo* for outer space, with *kosma* "of space" and *kosmoŝipo* "spaceship". ReVo labels *spaco* for outer space *evitinde* "to be avoided".

### What is the difference between *astronaŭto* and *kosmonaŭto*?

None in Esperanto: ReVo defines *astronaŭto* as *kosmonaŭto*, a person who travels into space.

### How do you say "scientist" or "physicist" in Esperanto?

*Sciencisto* or *scienculo* for a scientist and *fizikisto* for a physicist; a biologist is a *biologo* and an astronomer an *astronomo*.

## Sources

- [ReVo: suno, sunsistemo](https://reta-vortaro.de/revo/art/sun.html), [luno, plenluno, novluno](https://reta-vortaro.de/revo/art/lun.html), [tero, eksterterano](https://reta-vortaro.de/revo/art/ter.html), [planedo, nanplanedo](https://reta-vortaro.de/revo/art/planed.html), [kometo](https://reta-vortaro.de/revo/art/komet.html), [asteroido](https://reta-vortaro.de/revo/art/astero.html), [galaksio](https://reta-vortaro.de/revo/art/galaksi.html), [Lakta Vojo](https://reta-vortaro.de/revo/art/lakt.html), [universo](https://reta-vortaro.de/revo/art/univer.html), [nana](https://reta-vortaro.de/revo/art/nan.html)
- [ReVo: Merkuro](https://reta-vortaro.de/revo/art/merkur1.html), [Venuso](https://reta-vortaro.de/revo/art/venus.html), [Marso, marsano](https://reta-vortaro.de/revo/art/mars.html), [Jupitero](https://reta-vortaro.de/revo/art/jupiter.html), [Saturno](https://reta-vortaro.de/revo/art/saturn.html), [Urano](https://reta-vortaro.de/revo/art/uran.html), [Neptuno](https://reta-vortaro.de/revo/art/neptun.html), [Plutono](https://reta-vortaro.de/revo/art/pluton.html)
- [ReVo: stelo, stelaro, falstelo](https://reta-vortaro.de/revo/art/stel.html), [konstelacio](https://reta-vortaro.de/revo/art/konste.html), [Oriono](https://reta-vortaro.de/revo/art/orion.html), [Granda Ursino](https://reta-vortaro.de/revo/art/urs.html), [meteoro](https://reta-vortaro.de/revo/art/meteor.html), [eklipso, luneklipso, suneklipso](https://reta-vortaro.de/revo/art/eklips.html), [orbito](https://reta-vortaro.de/revo/art/orbit.html), [akso](https://reta-vortaro.de/revo/art/aks.html)
- [ReVo: raketo](https://reta-vortaro.de/revo/art/raket.html), [satelito](https://reta-vortaro.de/revo/art/sateli.html), [teleskopo](https://reta-vortaro.de/revo/art/telesk.html), [observi, observejo, stelobservejo](https://reta-vortaro.de/revo/art/observ.html), [observatorio](https://reta-vortaro.de/revo/art/observ1.html), [kosmo](https://reta-vortaro.de/revo/art/kosm.html), [spaco](https://reta-vortaro.de/revo/art/spac.html), [kosmonaŭto](https://reta-vortaro.de/revo/art/kosmonauxt.html), [astronaŭto](https://reta-vortaro.de/revo/art/astronauxt.html), [kosmoŝipo](https://reta-vortaro.de/revo/art/sxip.html), [kosmodromo](https://reta-vortaro.de/revo/art/kosmodrom.html)
- [ReVo: astronomo](https://reta-vortaro.de/revo/art/astron.html), [astronomio](https://reta-vortaro.de/revo/art/astron1.html), [fiziko, fizikisto](https://reta-vortaro.de/revo/art/fizik.html), [ĥemio, kemio, ĥemiisto](https://reta-vortaro.de/revo/art/hxemi.html), [biologio](https://reta-vortaro.de/revo/art/biolog1.html), [biologo](https://reta-vortaro.de/revo/art/biolog.html), [matematiko, matematikisto](https://reta-vortaro.de/revo/art/matema.html), [geografio](https://reta-vortaro.de/revo/art/geogra1.html), [geografo](https://reta-vortaro.de/revo/art/geogra.html), [geologio](https://reta-vortaro.de/revo/art/geolog1.html), [geologo](https://reta-vortaro.de/revo/art/geolog.html), [meteorologio](https://reta-vortaro.de/revo/art/meteor2.html), [informatiko](https://reta-vortaro.de/revo/art/informatik.html), [scienco, sciencisto, scienculo](https://reta-vortaro.de/revo/art/scienc.html)
- [ReVo: laboratorio](https://reta-vortaro.de/revo/art/labora.html), [eksperimento](https://reta-vortaro.de/revo/art/eksper.html), [hipotezo](https://reta-vortaro.de/revo/art/hipote2.html), [teorio](https://reta-vortaro.de/revo/art/teori.html), [mezuri](https://reta-vortaro.de/revo/art/mezur.html), [pruvi](https://reta-vortaro.de/revo/art/pruv.html), [mikroskopo](https://reta-vortaro.de/revo/art/mikros.html), [atomo](https://reta-vortaro.de/revo/art/atom.html), [molekulo](https://reta-vortaro.de/revo/art/moleku.html), [energio](https://reta-vortaro.de/revo/art/energi.html), [gravito](https://reta-vortaro.de/revo/art/gravit.html)
- [PMEG: majuskloj ĉe propraj nomoj](https://bertilow.com/pmeg/gramatiko/propraj_nomoj/majuskloj.html), [la ĉe propraj nomoj](https://bertilow.com/pmeg/gramatiko/difiniloj/la/propraj_nomoj.html), [la kaj konata afero (la suno)](https://bertilow.com/pmeg/gramatiko/difiniloj/la/konateco.html)
- [PMEG: IST](https://bertilow.com/pmeg/vortfarado/afiksoj/sufiksoj/ist.html), [I (geografio → geografo)](https://bertilow.com/pmeg/vortfarado/neoficialaj_afiksoj/sufiksoj/i.html), [OLOG](https://bertilow.com/pmeg/vortfarado/neoficialaj_afiksoj/sufiksoj/olog.html), [IĜ (La tero turniĝas ĉirkaŭ sia akso)](https://bertilow.com/pmeg/vortfarado/afiksoj/sufiksoj/igh.html)