xplanet: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/xplanet - from package: xplanet

You may install the required package with command apt-get install {package}

Error in other languages:
xplanet: bevel nie gevind nie
xplanet: comandă negăsită
xplanet: comando não encontrado
xplanet: commande introuvable
xplanet: command not found
xplanet: käsku ei ole
xplanet: không tìm thấy lệnh
xplanet: komanda nerasta
xplanet: Kommando nicht gefunden.
xplanet: kommandot finns inte
xplanet: komut yok
xplanet: nie znaleziono polecenia
xplanet: níor aimsíodh an t-ordú
xplanet: no se encontró la orden
xplanet: no s'ha trobat l'ordre
xplanet: opdracht niet gevonden
xplanet: parancs nem található
xplanet: perintah tidak ditemukan
xplanet: príkaz nenájdený
xplanet: příkaz nenalezen
xplanet: команда не найдена
xplanet: командата не е открита
xplanet: 命令找不到
xplanet: コマンドが見つかりません